• It has been assumed that life has been originated about 5-6 billion years ago.
• Two hypotheses has been proposed for the origin of Earth:
• 1. Planetesimal hypothesis:
  • It proposes that origin of earth took place as a part broken off from the molten mass of sun.
• 2. Nebular hypothesis:
  • It is most commonly accepted hypothesis of origin of earth.
  • It proposes that the earth is originated by gradual condensation of interstellar dust or cosmic dust termed as nebula.
  • About 10000-20000 million years ago, a highly condensed mass of cosmic material was present termed as ylem that included of neutron, proton, and electron like particles.
  • The explosion of cosmic material led to numerous pieces called nebula.
  • This explosion of cosmic material was termed as big-bang, so called big bang theory.
  • According to nebular hypothesis in the beginning, it was a spinning ball of hot gases and vapours of elements.
  • However, because of the gradual cooling, the condensation of  gases into solid form and stratification according to their density occured.
  • The elements such as heavy metals Nickel, Iron, Zinc, etc, went to the centre to form the  core.
  • Lighter elements as aluminium, silicon, sulphur etc, constituted mantle and crust of earth.
  • The lightest and gaseous substances like hydrogen, helium, carbon, nitrogen, oxygen, formed atmosphere.

Theories of origin of life:

• Various theories are proposed by various biologists in several period, some of the important theories are listed as below:

1. Theory of special creation:

• This theory states that entire universe was formed by super natural power, god.
• It was proposed by Hebrew et. al. and was largely supported by Father Suarez.
• God created the entire universe in six days, according to Christianity. The earth and the heaven were created on the first day,  sky on second day, land and plants were formed on third day, the sun, the moon and the stars on fourth day, fish and fowls on the fifth day and animals including human beings on the sixth day.
• Adam was the first man and the first woman was Eve.
• God Brahma created life in a single stroke, according to Hindu mythology.
• The first man was Manu and Shradha was the first woman.
• Due to lack of scientific explanations, this theory was rejected.

2. Theory of spontaneous Generation (Abiogenesis):

• This theory proposes that the living form arose spontaneously from non-living materials such as dung, mud, earth etc.
•  VonHelmont proposed this theory and was supported by Anaximenes and Aristotle.
• According to this theory, the origin of insects took place from dew, frogs and toads from muddy bottom of ponds, maggots from decaying meat, tapeworms from excreta of animals and micro-organisms from air or water.
• However, Francisco Redi (1668), Louis Pasteur (1864) and Spallanzani (1765) rejected the abiogenesis concept experimentally.

3. Biogenesis theory:

• It proposed that life arises from pre-existing life only.
• This concept is supported by following experiments.
• i) Redi’s Experiments:
  • Francisco Redi(1668), an Italian physician, placed a piece of boiled meat in each of three separate jars.
  • One jar was covered with parchment paper, the other was covered with muslin cloth, and one was left open.
  • He found that only in an opened jar maggots grew.
  • Only in the opened jar the flies joined and laid eggs that had grown into maggots.
  • Redi concluded, on the basis of the aforementioned experiment, that life can only derive from pre-existing life, not from non-living substances.
• ii) Spallanzani’s experiments:
  • In eight bottles, Lazaaro Spallanzani (Italy) put hay infusion and then it was boiled to make them sterile.
  • He kept four bottles airtight and four of them loosley corked.
  • After few days, dense growth of micro-organisms was found in loosely corked bottles whereas no organisms were found to develop in airtight bottles.
  • It was concluded that air consisted of micro-organism and was the source of contamination.
• iii) Pasteur’s Experiments:
• In 1864, an experiment was performed by French biochemist Louis Pasteur in favour of ideal of biogenesis.
• He used a flask whose neck was bent in form of ‘S’ via heat and filled nearly half of the flask with nutrient solution.
• Then it was boiled for several hours to kill all the micro-organisms.
• He sealed the tube and left the apparatus undisturbed for various days.
• No sign of life was seen in flask.
• However, when the neck of the flask was broken, micro-organisms appeared.
• Hence, Louis Pasteur concluded that the life can arise only from pre-existing life.

4. Cosmozoic theory:

• This theory states that life, in the form of spores or seeds called panspermia, originated from some other planet on earth.
• Richter(1865) suggested this idea and Arrhenius supported it.
• But this hypothesis is unable to understand why panspermia, including high temperatures and many harmful radiations, could survive in adverse conditions on earth at that time.

5. Modern or Chemosynthetic Theory of Origin of life (Scientific hypothesis):

• Scientists now accept that it is not possible to create life spontaneously.
• Specific requirements for life’s appearance are essential.
• T.H. Huxley and John Tyndall asserted that inorganic chemicals could produce life.
• But as the knowledge of biochemistry was not available that time, their ideas were vagued.
• This idea was proposed by the Russian biochemist A.I. Oparin(1923) and with J.B.S. Haldane’s assistance.
• In his book “The Origin of Life on Earth” in1936, they offered an extensive description of the origin of life by evolution or chemicals.
• It is, thus, often called the Oparin and Haldane theory.
• According to this theory, through a series of chemical reactions around 4.2 billion years ago, life emerged in water on primitive earth from chemicals, thus called the biochemical theory of origin of life.
• It is often referred to as the modern synthetic theory of origin of life.
• In three main stages, this theory can be described: Chemogeny, Biogenesis, and Cognogeny.

i. Chemogeny:

• It includes the creation of complex organic molecules from basic chemicals, such as polysaccharides, fats, polypeptides, nucleic acids, etc.
• The different chemogenic phases are as follows:
• a. Primitive atmosphere formation:
  • The earth’s primitive atmosphere was composed of elements such as N, H, O, C, etc.
  • Earth was a fiery spinning ball of hot gases at that time and produced dicarbon, cynogen, metal carbide, etc.
  • Oxygen was not present in the free state, but as oxides of aluminium, boron, hydrogen, etc.
• b. Formation of inorganic compounds:
  • The chemical evolution was favored  when the temperature of the primitive earth fell below.
  • The most common were atoms of hydrogen.
  • They associated with all available oxygen to form water.
  • They all combined with nitrogen and carbon atoms to form ammonia and methane respectively.
    • 2H + O → H₂O (water)
    • 3H+ N → NH₃ (ammonia)
    • 4H + C → CH₄ (methane)
• c. Formation of simple organic compound:
  • As the earth cooled down it established a solid crust, which later formed depressions and elevations.
  • Meanwhile, the vapors of atmospheric water accumulated and eventually arrived as rain on the surface of the earth.
  • In the depressions, the water that accumulated dissolved the minerals and eventually found the large sized bodies of water called oceans.
  • If the earth’s surface cools to 50^oC-60^oC, the inorganic molecules combine to form simple organic compounds such as acetylene, ethylene, ethane, methane, etc. in different forms.
    • CH+CH→ HC=CH (Acetylene)
    • CH₂+CH₂ → H₂C=CH₂ (ethylene)
    • CH₂ + CH₂ → C + CH₄ (methane)
• d. Formation of Complex Organic Compounds
  • As previously formed, the saturated and unsaturated hydrocarbons mixed and recombined in different forms to form complex organic compounds such as acetaldehyde, aldol, ethyl acetate, acetic acid, ethyl alcohol, amino acids, glycol, etc. through the process of condensation, polymerization, and oxide reduction.
  • Condensation reactions:
    • HC≡ CH + H₂O → CH₃CHO (acetaldehyde)
    • CH₃CHO + CH₃CHO → CH₃CHOHCH₂CHO (aldol)
  • Oxidoreduction:
    • CH₃CHO + H₂O → CH₃COOH + C₂H₅OH
  • Polymerization:
    • CH₃COOH + C₂H₅OH → CH₃COOCH₃CH₂ + H₂O
    •  CH₂OHCOOH + NH₃ → CH₂NH₂COOH + H₂O
  • For the above reactions, the sources of energy  were ultraviolet rays, volcanic eruptions, electric energy produced during lightening.
  • The complex organic compounds that were formed in ocean gradually got collected in primitive ocean.
• e. Formation of carbohydrates, Proteins, and Fat:
  • Eventually, the primitive ocean polymerized amino acids, sugars, glycerol, and fatty acids, etc., producing massive macromolecules such as proteins, carbohydrates, and fats.
  • Since these complex compounds are the key protoplasm constituents of living cells, the possibilities for the origin of life in the primitive ocean were established by their synthesis.
  • In the meantime, the hydrocarbon reacted to form nitrogen bases, purines and pyrimidines with the hot water vapor hydrocyanic acid and ammonia.
  • The oceanic water becomes a rich blend of organic compound called hot dilute soup or primordial soup or broths by Haldane as a result of the above chemical reaction.

ii. Biogeny:

• Following chemogeny, the next step is biogeny.
• Biogeny refers to the formation of primitive life.
• Biogeny consists of following events:
• a. Formation of nucleic acids and nucleoproteins:
  • Organic compounds reacted and aggregated to create new molecules of greater size and complexity in the primordial water, organic soup.
  • Nitrogen-bases combined with sugar and phosphate to form nucleotides at high temperatures in primitive soil during chemical reactions.
  • A large number of nucleotides in various combinations are joined together to form very complex molecules called nucleic acids.
  • Nucleic acids had tendency to replicate.
• b. Formation of Coacervates:
  • Due to intermolecular attraction, the complex organic compounds of primordial soup are collected to form large colloidal cells such as aggregates called coacervates or microsphere.
  • As they had the power of growth and break, such coacervates were efficient and multiplied.
  • Coacervate formation is referred to as coacervation.
• c. Formation of Primary organism:
  • In approximately 3.8 billion years ago, according to Oparincoacervates, which acquired nucleoproteins either from seawater or by synthesizing themselves, first cellular organisms called eobionts or pre-cell or protobionts were created.
  • The outer limiting membranes were created by certain fatty acids, having strong affinity ,to water.
  • For both positive and destructive reactions, some forms of proteins of eobionts started acting as enzymes.
  • The eobionts were identical to protovirus, a present-day virus.
  • There were anaerobic and prokaryotic chemoheterotrophs.

iii. Cognogeny:

• Diversification of primary species into various modes of life is involved.
• a. Origin of autotrophs:
  • As the number of chemoheterotrophs steadily increased, they absorbed organic nutrients, contributing to a reduction in the ocean’s natural food supplies.
  • Therefore, primitive organisms began to synthesize organic compounds abundantly present in the sea from inorganic molecules.
  • The anaerobic breakdown of chemicals due to the absence of chlorophyll has provided the energy needed for the synthesis of organic food.
  • Therefore, they were chemoautotrophs, such as nitrifying bacteria, sulphur bacteria, iron bacteria, etc.
    • 6CO₂  + 12H₂S → C₆H₁₂O₆ + 6H₂O +12S
  • Later, a green substance from the sea’s magnesium porphyrin called bacteriochlorophyll was formed by some autotrophic prokaryotes and photosynthesis began.
  • This led to the creation of photoautotrophs, such as today’s marine planktonic bacteria.
  • As they did not use water to photosynthesize, they were anoxygenic.
    • 6CO₂+ 12H₂S → solar energy  C₆H₁₂O₆ + 6H₂O + 12 Bacteriochlorophyll
  • Upto this time, free oxygen was unavailable in the atmosphere.
  • At that duration, bacteria chlorophyll went through molecular changes to form true chlorophyll and hence, true photoautotrophs were evolved.
  • These phototrophs synthesized their food by photosynthesis utilizing water as raw material.
  • Cyanobacteria was the first oxygenic and aerobic photoautotrophs that evolved about 2700 million years ago.
    • 6CO₂+ 6H₂O →  C₆H₁₂O₆ + 6O₂
• b. Origin of eukaryotes:
  • True photosynthetic prokaryotes have shifted to aerobic respiration.
  • Then cyano bacteria developed a true nucleus like prokaryotes and transformed into eukaryotes.
  • It were like today’s unicellular species.
  • Multicellular species have evolved from unicellular organisms via the colonization process.

Evidences in support of origin of life: Miller-urey experiment

• In 1953, Stanley Miller and Harold Urey conducted an experiment to test the biochemical origin of life hypothesis offered by Oparin and Haldane.
• Miller- Urey Experiment:
  • Miller constructed an apparatus of glass tube and flask termed as the spark discharge apparatus.
  • The apparatus depicted primitive earth conditions, including a reducing atmosphere and an ocean.
  • A mixture of gases methane, ammonia and hydrogen was maintained in the gas chamber in the ratio 2:2:1 and water in another chamber.
  • The gas mixture was pumped through the apparatus and the energy was supplied through the use of electrodes in the gas chamber by boiling water and electric sparks.
  •  The experiment initiated with switching on the electric source and boiling the water and is continued for a week.
• Observation:
  • They noticed a condensed liquid with a dark colour.
  • It was gathered and chromatographically analyzed, and the liquid was found to be a mixture of sugars, amino acids (glycine, analine, etc.) and fatty acids.
• Conclusion:
  • The experimental results support the Oparin-Haldane theory of the origin of life that organic molecules are created from inorganic molecules during the course of the origin of life.

The post Theories of origin of life on Earth appeared first on Online Biology Notes.

• A representation of the genetic differences that occur within a population is termed as genetic variation.
• The genetic variation of whole the species is usually termed as genetic diversity.
• The differences in genes or DNA segments are genetic variations and each variation of a gene is termed as an allele.
• For instance, there is a high amount of genetic variation in a population of several different alleles at a single chromosome locus.
• For natural selection, genetic variation is important, because natural selection can only increase or decrease the frequency of alleles that already exist in the population.
• Since conditions are unpredictable, genetically diverse populations may be able to adapt faster than those that do not involve genetic variation to evolving circumstances.
• All examples of genetic variations that can occur in a human population are the skin tone, hair color, dimples, freckles, and type of blood of an individual.
• The modified leaves of carnivorous plants and the development of flowers that mimic insects to attract plant pollinators are examples of genetic variation in plants.
• Albinism, cheetahs with markings, snakes flying, animals that play dead, and animals that resemble leaves are examples of genetic variation in animals.
• These variations allow the animals in their habitats to better adapt to conditions.

What are the causes of genetic variation?

• The main causes of genetic variation are:
  • Mutation
  • Gene flow
  • Sexual reproduction
  • Genetic drift
• However, there are other causes of genetic variation:
  • Random mating
  • Crossing over
  • Random fertilization
  • Non-random mating
  • Environmental variance

i. Mutation:

• In general, mutation is the primary source of genetic variation, which is the raw material for evolution through natural selection.
• A change in the DNA sequence is a mutation.
• Often these differences in gene sequences may be beneficial for an organism.
• By modifying genes and alleles in a population, mutations result in genetic variation.
• They may affect an individual gene or a whole chromosome.
• Though mutations alter the genotype of an organism (genetic makeup), they do not necessarily change the phenotype of an organism.
• They may affect an individual gene or a whole chromosome.
• Though mutations alter the genotype of an organism (genetic makeup), they do not necessarily change the phenotype of an organism.
• During replication, mutations occur in the DNA molecule, such that the daughter cells vary in sequence or DNA quantity from the parent cells.
• A mutation initially appears in a single cell of the body, but it is transferred on to all cells descended from the first.
• Mainly two types of mutation are discussed they are gene mutations and chromosomal mutations.

ii. Gene mutations:

• In the case where the nucleotide sequence is modified and is passed on to the offspring, gene mutation occurs.
• The alteration might be either insertion or deletion or substitution of one or a few nucleotides.
• The addition or deletion of nucleotides within the DNA sequence in the coded protein shifts the reading frame and hence results in highly altered sequence of amino acids.
• Substitutions of nucleotides, however, may or may not severely affect the biological function of the protein.
•  Those substitution that results in terminator codon are supposed to be harmful.
• Gene mutation take place spontaneously without being purposedly induced by humans.
• Ultraviolet light, X-rays, and other high-frequency electromagnetic radiation, as well as exposure to such mutagenic substances, such as mustard gas, can also trigger them.
• The effects of gene mutations can vary from negligible to lethal.
• Mutations that modify one or even many amino acids can have a minor or undetectable impact on the ability of the organism to live and replicate if the coded protein’s basic biological role is not hampered.
• But when an amino acid substitution influences an enzyme’s active site or changes a protein’s essential function in some other way, the effect can be severe.

iii. Chromosomal mutations:

• The mutation process produces several new genetic variants for each generation. The nucleus of each cell contains chromosomes that hold the inherited material, or DNA.
• Chromosomes occur in pairs and each of them is inherited from each parent.
• In the pair, the two members are termed as homologous chromosomes.
• As a rule, every cell of an organism and all people of the same species have the same number of chromosomes.
• However, the number, size, and organization of chromosomes differs between species.
• Alterations in the number, size, or organization of chromosomes within a species are referred as chromosomal mutations, chromosomal abnormalities, or chromosomal aberrations.
• By fusion of two chromosomes into one, by fission of one chromosome into two, or by addition or subtraction of one or more whole chromosomes or sets of chromosomes, changes in number can occur.
• Polyploidy is the condition in which one or more additional sets of chromosomes are acquired by an organism.
• Changes in the arrangement of chromosomes may occur by inversion when a chromosomal segment rotates 180 degrees in the same place; by duplication when a segment is added; by deletion when a segment is lost; or by translocation when a segment changes from one place to another.
• These are the mechanisms by which chromosomes evolve.
• The amount of DNA is not changed by inversions, translocations, fusions, and fissions.
• The significance of these mutations in evolution is that they alter the linkage relationships among genes.
• Genes that are closely related to each other are isolated and vice versa; this may impact their expression because genes are mostly sequentially transcribed, two or more at a time.

iv. Gene flow:

• It is also termed as gene migration.
• New genes are introduced into a population as organisms migrate into a new environment.
• The availability of new alleles in the gene pool makes it possible for new gene combinations.
•  The emigration, i.e., moving out of organisms from a population also alters the gene frequencies.
• The emigration results in the lack of genetic diversity.
• Immigration of new organisms into a population may favor organisms to adapt in changing environmental conditions.
• Sustained gene flow between two populations can also result to a combination of the two gene pools, decreasing the genetic variation between the two groups.
• Gene flow strongly plays role against speciation.
• It does so by the recombination of the gene pools of the groups, and hence, repairment of the developing differences in genetic variation that would have resulted to full speciation and formation of daughter species.
• For instance, if a species of grass grows on both sides of a road, pollen is likely to be migrated from one side to the other and vice versa.
• If this pollen is capable of fertilizing the plant where it ends up and producing viable offspring, then the pollen alleles have effectively connected the population to the other on one side of the road.

v. Sexual reproduction:

• Sexual reproduction enhances genetic variation by producing various gene combinations.
• The process by which gametes or sex cells are formed is known as meiosis.
• As alleles in the gametes are separated and haphazardly united upon fertilization, genetic variation occurs.
• During meiosis, the crossing over or swapping of homologous chromosomes also results in the genetic recombination of genes.

vi. Genetic drift:

• Genetic drift is the alteration of a population’s allele frequencies owing to occurrences of random chance, such as natural disasters.
• The converse of natural selection is genetic drift.
• The theory of natural selection maintains that certain individuals in a population have characteristics that allow more offspring to survive and produce, whereas other people have characteristics that are harmful and can cause them to die before reproducing.
• These selection pressures will alter the gene pool and the traits within the population over successive generations.
• A huge, strong male gorilla, for instance, will mate with more females than a small, weak male, so more of his genes will be passed on to the next generation.
• His descendants will continue to control the troops and also pass on their genes.
• Over time, in the gorilla population, the selection pressure would cause the allele frequencies to change towards big powerful males.
• Unlike natural selection, genetic drift explains the impact of chance on populations in the absence of positive or negative selection pressure.
• Allele frequencies within a population may change through random sampling, or the survival or reproduction of a random sample of individuals within a population.
• Instead of a male gorilla generating more offspring since he is stronger, he may be the only male accessible when a female is ready to mate.
• His genes are transferred on to future generation because of chance, not as he was the biggest or the strongest.
• Genetic drift is the transition of alleles within a population due to chance events that cause random samples of the population to produce offspring or not.
• Small populations are more prone to the powers of genetic drift.
• On the other hand, large populations are buffered against the consequences of chance.
• If one individual in a population of 10 happens to die at a young age before leaving any offspring to the next generation, all of their genes (1/10 of the gene pool of the population) will instantly be lost.
• The individual comprises only 1 percent of the total gene pool in a population of100; thus, genetic drift has much less effect on the genetic structure of the larger population.
• The Bottleneck effect:
  • Natural disasters, such as a natural catastrophe that destroys a large portion of the population at random, may also magnify genetic drift.
  • When only a few individuals survive, the bottleneck effect arises and reduces diversity in a population’s gene pool.
  • The survivors’ genetic structure becomes the whole population’s genetic structure, which could be very different from the pre-disaster population.
• The founder effect:
  • If any part of the population leaves to start a new population in a new location or if a population gets separated by a physical barrier of some sort, another scenario arises in which populations may experience a strong impact of genetic drift.
  • In this scenario, it is not likely that those individuals are representative of the total population, which leads to the founder effect.
  • When the genetic structure changes to match that of the founding fathers and mothers of the new population, the founder effect occurs.
  • The underlying effect is believed to have been a key factor in the genetic history of Dutch settlers in South Africa’s Afrikaner population, as evidenced by mutations that are prevalent in Afrikaners, but rare in most other populations.
  • This was likely due to the fact that these mutations were carried by a higher-than-normal proportion of the founding colonists.
  • As a result, Huntington’s disease (HD) and Fanconi anemia (FA), a genetic condition known to cause blood marrow and congenital defects, including cancer, have exceptionally high incidences in the population.

vii. Non-random mating:

• If individuals nonrandomly mate with the other individuals within a population, i.e., they select their mate, selections can drive evolution in a population.
• There are several explanations for the phenomenon of nonrandom mating.
• Simple mate choice or sexual selection is one reason; female peahens, for instance, may prefer peacocks with larger, brighter tails.
• Traits that contribute to more matches for an individual lead to more offspring and ultimately lead to a higher prevalence of that feature in the population through natural selection.
• An individual’s desire to mate with partners that are phenotypically similar to themselves is one common type of mate choice, called positive assortative mating.
• Physical location is another cause of nonrandom mating.
• This is particularly true in large populations distributed over large geographical distances where not all individuals would have equal access to each other.
• These may be miles apart across forests or over rugged terrain, while others may live nearby immediately.

viii. Environmental variance:

• Genes are not the only players involved in population variance determination.
• Other factors, such as the environment, also affect phenotypes.
• For instance, due to daily exposure to the sun, an environmental factor, a beachgoer is likely to have darker skin than a city dweller.
• For certain animals, some major features, such as gender, are determined by the environment.
• For instance, some turtles and other reptiles have temperature-dependent sex determination (TSD).
• TSD implies that individuals grow into males if their eggs are incubated over a certain temperature range, or females at a distinct temperature range.
• Geographic separation among populations can contribute to variation in the phenotypic variation between certain populations.
• In most populations, this geographical variation is seen and can be important.
• One sort of geographic variation, named a cline, could be seen as populations of a given species differ progressively across an ecological gradient.

Gene pool:

• The term gene pool in evolutionary science refers to the collection of all available genes that can be passed down in the population of a single species from parents to offspring.
• The more variation there is in the population, the greater the gene pool.
• The gene pool decides the phenotypes (visible features) are present at any given time in the population.

How do gene pools change?

• Due to the movement of individuals into or out of a population, the gene pool can shift within a geographic region.
• When people with population-specific characteristics move abroad, the gene pool in that population shrinks and the characteristics are no longer available to be passed on to the offspring.
• In the other hand, as new people with new special features immigrate into the population, they expand the gene pool.
• A new form of diversity is added into the population when these new individuals interbreed with individuals already present.
• The size of the gene pool directly impacts the population’s evolutionary trajectory.
• The theory of evolution claims that natural selection works on a population to benefit the favorable features of that environment while weeding out the undesirable characteristics at the same time.
• The gene pool changes since natural selection operates on a population.
• Within the gene pool, beneficial adaptations become more frequent, and the less desirable characteristics become less prevalent or may even vanish entirely from the gene pool.
• Populations with larger gene pools are more likely than those with smaller gene pools to survive as the local climate changes.
• This is due to the fact that a broader range of traits are available to larger populations with wider diversity, which gives them an advantage as the environment changes and demands new adaptations.
• If there are few to no individuals with the genetic diversity needed to survive changes, a smaller and more homogeneous gene pool puts the population at risk of extinction.
• The more diverse the population, the more likely they are to survive significant changes in the environment.

Examples of gene pool in evolution:

• Individuals who are antibiotic-resistant are more likely to withstand any form of medical intervention in bacteria populations and live long enough to reproduce.
• The gene pool evolves over time (rather quickly in the case of rapidly reproducing organisms such as bacteria) to comprise only bacteria resistant to antibiotics.
• In this way, new strains of virulent bacteria are produced.
• A large number of plants that farmers classify as weeds are so tenacious because they have a diverse gene pool that helps them to adapt to a variety of environmental conditions.
• On the other hand, advanced hybrids also need very precise, even ideal conditions, because they have been bred to have a very small gene pool that prefers certain features, such as beautiful flowers or large fruits.
• It can be said that dandelions are superior to hybrid roses, on basis of size of gene pools.

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• Evolution is a change in the inherited traits of a population over time via the process of natural selection that might result in the creation of new species.
• The theory of evolution is a short form of the word “theory of evolution by natural selection,” which was introduced by Charles Darwin and Alfred Russel Wallace in the nineteenth century.
• In the theory of natural selection, species generate more offspring in their environment that are able to survive.
• Those who are better suited physically to live, they evolve and grow to maturity.
• In the other hand, those who lack such fitness either do not reach an age where they can reproduce or produce less offspring than their equivalents.
• Natural selection is often summed up as “survival of the fittest” since the “fittest” species are the ones that reproduce more efficiently and are more likely to pass on their characteristics to the next generation, the most appropriate for their environment.
• This implies that as an environment changes, the characteristics that boost survival will also gradually change or develop in that environment.
• In explaining the evolution of organisms, natural selection was such a strong concept that it was set up as a scientific theory.
• Numerous examples of natural selection shaping evolution have since been observed by biologists.
• It is understood today that it is only one of the pathways by which life evolves.
• A process known as genetic drift, for example, may also cause organisms to evolve.
• In genetic drift, some species produce more descendants than would be predicted, simply by chance.
• These organisms are not essentially the fittest of their species, but they are passed on to the coming generation by their genes.

History of Darwin’s theory of evolution:

• When Darwin was on his youthful voyage as naturalist on the survey ship Beagle, he was inspired by observations made during that time.
• On the Galapagos Island, he found out subtle variations that made tortoises recognizably distinct from other islands.
• The famous “Darwin’s finches,” which displayed minor variations from island to island, also observed a whole array of unique finches.
• Moreover, all of them seemed to resemble, but vary from the common finch on Ecuador’s mainland, 600 miles to the east.
• Patterns in the distribution and similarity of species had a significant impact in Darwin’s thinking.

Darwin’s theory of evolution:

• As Darwin started his study, the first three ideas were already under discussion among earlier and scholarly naturalists researching on the “species problem”.
• The mechanism of natural selection and copious quantities of evidence for evolutionary change from several sources were Darwin’s original contributions.
• For our understanding of the origin of life and contemporary biological diversity, he also offered thoughtful descriptions of the implications of evolution.
• The following basic principles are included in Darwin’s theory of evolution.
• Species are evolving over time and space (populations of interbreeding organisms).
  • Representatives of today’s organisms vary from those of the recent past, and communities of various geographical regions today differ marginally in shape or behavior.
  • These differences reach into the fossil record, which provides this argument with sufficient evidence.
• Each organism shares common ancestors with the other organisms.
  • Populations can split into separate species over time, which share a common ancestral population.
  • Any pair of species shares a common ancestor far enough back in time.
  • For instance, humans shared a common ancestor about eight million years ago with chimpanzees, about 60 million years ago with whales, and over 100 million years ago with kangaroos.
  • The similarities of species that are grouped together are explained by mutual ancestry: their similarities reflect the inheritance of traits from a common ancestor.
• In Darwin’s view, evolutionary changes are progressive and slow.
  • The long episodes of gradual change in organisms in the fossil record and the fact that no naturalist had witnessed the sudden rise of a new species in Darwin’s time supported this claim.
  • The long episodes of gradual change in organisms in the fossil record and the fact that no naturalist had witnessed the sudden rise of a new species in Darwin’s time supported this claim.

What is natural selection?

• Natural selection is a mechanism that will sustain and replicate species that are best suited to an environment.
• This suggests that this variant organism’s beneficial alleles are passed on to offspring.
• The mechanism of natural selection over several centuries contributes to evolution.

Examples of natural selection:

1. Peppered moths:

• Most peppered moths were of the pale variety till the industrial revolution in Britain in the early 1800s.
• This indicated that they were camouflaged against the pale birch trees where they rested.
• Moths with a mutant black coloring were quickly detected by birds and eaten.
• This gave an advantage to the white variety, and they were more likely to continue to reproduce.
• Airborne emissions in manufacturing areas blackened the birch tree bark with soot during the second half of the 1800s.
• This indicated that they were now camouflaged by the mutant black moths, while the white variety became more susceptible to predators.
• This offered a benefit for the black type, and they were more likely to survive and replicate.
• The dark moths passed on the black wing color alleles that led to offspring with the phenotype of black wing color.
• Over time, in urban areas, black peppered moths have become much more common than the pale type.
• It should be noted that the shift in phenotype was not due to the moths being darker by pollution.
• The dark variety still existed, but when the climate changed, it was the better suited version.
• It took several years until the moth population was predominantly black in colour.

2. Galapagos finches:

• It is popular example of natural selection from Darwin’s voyage.
• Every Galapagos island visited by Darwin had its own kind of finch (14 in all), found nowhere else in the world.
• Some had beaks adapted for feeding large seeds, some for small seeds, some for feeding on buds and fruits had parrot-like beaks, and some for feeding on small insects had slender beaks.
• As some woodpeckers do, one used a thorn to test for insect larvae in wood.
• Eight of them were tree finches and six of them were ground dwellers.
• It seemed that each was slightly altered from an original colonist, most likely the finch on South America’s mainland, some 600 miles to the east.
• Adaptive radiation is likely to contribute to the creation of so many species because other birds were few or absent, leaving empty niches to fill; and because there were enough opportunities for geographical isolation in the various Galapagos islands.

The process of natural selection:

• There are four components in the Darwin’s process of natural selection:
• Variation:
  • In appearance and behavior, species (within populations) exhibit individual variation.
  • Body size, hair color, facial markings, speech properties, or number of offspring can be included in these variants.
  • On the other hand, some characteristics show little or no difference among individuals, such as the number of vertebrate eyes.
• Inheritance:
  • Some characteristics are routinely passed on from parent to offspring.
  • These characteristics are heritable, while other traits are highly affected by environmental factors and demonstrate poor heritability.
• Increasing population growth rate:
  • Every year, most populations have more offspring than local resources can sustain, leading to a battle for resources.
  • Considerable mortality is faced by each generation.
• Variability in survival and reproduction:
  • Individuals with characteristics well suited to the fight for local resources will bring more offspring to the next generation.
• Evolution as a genetic function:

The concept of natural selection:

• With the presence of genetic variation, the core argument of Darwin’s theory of evolution begins.
• It was demonstrated to Darwin that the experiences with the plant and animal breeding led to variations that could be significant for man.
• Thus, he concluded that variations that are favorable or important for organism to survive should exist.
• These favorable variations enhanced the chances for the living and procreation.
• Those beneficial variations were conserved and passed on to generations.
• This process is actually known as natural selection.
• An organism that is well adapted to its environment is the outcome of the process, and evolution also occurs as a consequence.
• Natural selection can then be characterized as the differential reproduction of alternative hereditary variants, defined by the fact that certain variants increase the probability of survival and reproduction of the organisms more effectively than organisms that carry alternative variants.
• Selection may occur by differences in survival, fertility, rate of development, success in mating, or any other aspect of the life cycle.
• All of these differences can be integrated under the term differential reproduction since all result in natural selection to the point that they affect the number of progenies an organism leave.
• It is possible to see evolution as a two-step process.
• First, hereditary variation takes place; second, certain genetic variations are chosen that will be passed on to the subsequent generations most effectively.
• Hereditary variation also contains two mechanisms—the spontaneous mutation of one variant into another and the sexual method that recombines those varieties (see recombination) to form a wide range of variations.
• The variants that occur from mutation or recombination are not equally passed from one generation to another.
• Others will occur more often because they are beneficial to the organism; events of chance, called genetic drift, may decide the frequency of others.

Types of natural selection:

1. Stabilizing selection:

• Through studying its effects on changing gene frequencies, natural selection can be studied, but it can also be studied by examining its effects on the observable characteristics or phenotypes of individuals in a population.
• Distribution scales of phenotypic traits such as height, weight, progeny number, or longevity usually display higher numbers of people with intermediate values and the so-called natural distribution is less and less towards the extremes.
• The selection is said to be stabilizing when individuals with intermediate phenotypes are preferred and extreme phenotypes are chosen against.
• Then the range and dispersion of phenotypes remains roughly the same from one generation to another.
• Stabilizing selection is quite usual.
• Those who have intermediate phenotypic values are the ones that live and reproduce more effectively.
• For example, mortality among newborn infants is greatest when they are either very small or very large; intermediate-size infants are more likely to survive.
• Stabilizing selection is usually noticeable after artificial selection.
• As a consequence of stabilizing selection, populations frequently maintain a steady genetic constitution in relation to many traits.
• This characteristic of populations is called genetic homeostasis.

2. Directional selection:

• In a population, the distribution of phenotypes often systematically shifts in a specific direction.
• The physical and biological aspects of the environment are evolving constantly, and the changes can be important over long periods of time.
• The environment and even the structure of the land or waters differ continuously.
• In biotic environments, that is, in the other species present, whether predators, prey, parasites, or rivals, changes often take place.
• As a result, genetic changes occur as the genotypic fitnesses can shift so that different sets of alleles are preferred.
• When species colonize new habitats where the conditions are different from those of their original habitat, the potential for directional selection often occurs.
• Furthermore, as the new genetic constitution replaces the preexisting one the emergence of a new desirable allele or a new genetic combination may prompt directional changes.
• The procedure of directional selection occurs in spurts.
• The substitution of one genetic constitution with another alters the genotypic fitnesses at other sites, which then modify their allelic frequencies, stimulating further modifications, and so on in a cascade of consequences.
• Directional selection is only possible if genetic variation occurs with regard to the phenotypic characteristics under selection.
• There are vast stores of genetic variation in natural populations, and these are constantly replenished by additional new variations that emerge by mutation.
• Artificial selection’s almost universal success and natural populations’ rapid response to new environmental challenges show that the current variety provides the materials needed for directional selection.
• Directional selection contributes to significant changes in morphology and ways of life over geologic time.
• Evolutionary changes that remain in a more or less consistent fashion over extended periods of time are defined as evolutionary trends.
• From the tiny brain of Australopithecus, human ancestors three million years ago, which was less than 500 cc in volume, to a brain about three times as large in modern humans, lateral evolutionary improvements expanded the cranial ability of the human lineage.
• Another well-studied example of directional selection is the evolution of the horse from more than 50 million years ago to modern times.

3. Diversifying selection:

• Diversifying selection, much like directional selection, drives the population towards the extremes of the trait.
• This type of selection is also termed disruptive selection.
• Diversifying selection moves the trait both directions, in contrast to directional selection.
• This can happen in a number of ways, but since species can become so distinct, it also leads to speciation.
• However, if only diversified for short periods, the selection will lead to a variety of characteristics that can be shared by one species.
• By diversifying selection, two or more divergent phenotypes in an environment may be preferred simultaneously.
• No natural environment is homogeneous; instead that the environment of any plant or animal population is a mosaic comprised of more or less distinct sub-environments.
• Also, the heterogeneity could be temporal, with change occurring over time, along with spatial.
• Species cope in different ways with environmental heterogeneity.
• One of the strategies is the selection of a generalistic genotype i.e called genetic monomorphism, that is well suited to all of the species’ sub-environments.
• Genetic polymorphism, the selection of a diversified gene pool that yields various genotypes, each suited to a particular sub-environment, is another strategy.
• In conditions in which populations living a short distance apart have been genetically distinct, the efficiency of diversifying natural selection is very evident.
• In one example, on heaps of mining waste heavily polluted with metals such as lead and copper, populations of bent grass can be found growing.
• The soil has been so polluted that it is poisonous to most plants, but it has been shown that the thick stands of bent grass growing over these refuse heaps have genes that make them resistant to high lead and copper concentrations.
• But bent grass plants that are not resistant to these metals can be found only a few metres from the polluted soil.
• Bent grasses reproduce mainly by cross-pollination, so that wind-borne pollen from the neighboring non-resistant plants is collected by the resistant grass.
• Since non-resistant seedlings are unable to grow in the polluted soil and the non-resistant seedlings outgrow the resistant ones in the surrounding uncontaminated soil, they retain their genetic differentiation.
• The evolution of these resistant strains has occurred in the lesser than 400 years since the mines were first opened.

4. Sexual selection:

• A significant factor in reproduction is mutual attraction between the sexes.
• Except for the reproductive organs and secondary sexual features, such as the breasts of female mammals, the males and females of many animal species are identical in size and form.
• However, there are species in which striking dimorphism is displayed by the sexes.
• Males are often larger and heavier, more brightly colored, or endowed with conspicuous adornments, especially in birds and mammals.
• However, bright colors make animals more conspicuous to predators-in the best of situations, the long plumage of male peacocks and paradise birds and the large antlers of aged male deer are bulky tons.
• Darwin knew that natural selection could not be predicted to favour the evolution of undesirable traits, and he was capable of offering a solution to this problem.
• He indicated that such characteristics occur by “sexual selection,” which does not depend on a fight for existence in relation to other organic beings or external conditions.”
• Other things being equal, species with greater fitness are more proficient in securing partners.
• There are two general conditions that lead to sexual selection.
• One is the choice displayed one sex (often females) for individuals of the other sex that display certain traits.
• The other is enhanced strength (usually among males) that produces greater success in attracting mates. The existence of a specific attribute among members of one sex can make them more attractive to the opposite sex in some way.
• In all kinds of species, from vinegar flies to pigeons, rats, dogs, and rhesus monkeys, this form of “sex appeal” has been experimentally illustrated.
• For example, when Drosophila flies are put together, some with yellow bodies as a result of random mutation and others with regular yellowish gray pigmentation, normal males are preferred over yellow males by females with either body color.

5. Kin selection and reciprocal altruism:

• Like other examples of sexual selection, the apparent altruistic behavior of many species is a characteristic that initially appears incompatible with the theory of natural selection.
• Altruism is a type of behavior that favors other people at the cost of the one who performs the action; the altruist’s fitness is decreased by his behavior, whereas people who behave selfishly benefit from it at no cost to themselves.
• Accordingly, natural selection may be expected to encourage the production of selfish behaviour and eradicate altruism.
• This outcome is not so convincing when it is realised that the beneficiaries of altruistic behaviour are usually relatives.
• Many of them bear the same genes, including those that foster altruistic behaviour.
• Altruism can grow through the selection of kin, which is simply a form of natural selection in which relatives are taken into account when determining the fitness of an individual.
• Natural selection favors genes that increase their carriers’ reproductive success, but it is not mandatory for reproductive success to be greater for all individuals that share a given genotype.
• On average, it is necessary for carriers of the genotype to replicate more effectively than those with alternative genotypes.
• A parent shares half of its genes with each progeny, so if the cost of the behavior to the parent is less than half of its average benefits to the progeny, a gene that promotes parental altruism is preferred by selection.
• Over the generations, such a gene is more likely to increase in frequency than an alternative gene that does not support altruistic behavior.
• Therefore, parental care is a type of altruism readily explained by the selection of kin.
• As it promotes the reproductive success of the parent’s genes, the parent spends some energy caring for the progeny.
• Kin selection goes beyond the association between parents and their offspring.
• It promotes the development of altruistic behavior when an individual’s energy invested, or the risk incurred, is compensated in excess by the benefits that follow through relatives.
• The finer the relationship between the beneficiaries and the altruist, and the higher the number of beneficiaries, the greater is the altruist’s risks and efforts.
• Individuals who live together in a herd or troop are generally related and often act in this way towards each other.
• For instance, adult zebras, instead of fleeing to protect themselves will turn towards an attacking predator to protect the young in the herd.
• Altruism often happens when the action is reciprocal among unrelated people and the cost of the altruist is smaller than the gain to the recipient.
• This reciprocal altruism is noticed in the mutual grooming of chimpanzees and other primates as they scrub each other of lice and other pests.

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• In evolution, speciation is the process that results in the formation of new and distinct species that are isolated from one another.
• However, biologists have developed two different pathways for the speciation to occur.
• Allopatric speciation, referring to speciation in other homelands,” includes a spatial separation from a parent species and eventual evolution of populations.
• Sympatric speciation, referring to speciation in the ‘same homeland’, includes speciation taking place within a parent species while staying in a same location.

What are the causes of speciation?

• There are various factors that cause speciation. The two main causes for speciation are listed as:

i. Geographical isolation:

• Speciation results from a splitting event in which a parent species is separated into two separate species, often as a result of geographical isolation or some driving force involving population separation.
• Separation could occur either due to physical barriers such as huge ocean expanses, mountain ranges, glaciers, deep river valleys, large rivers or deserts, or a substantial distance due to wider geographical range.
• The free-flow of alleles is prevented when populations become geographically isolated.
• The two species are able to evolve into different trajectories when the separation continues for a period of time.
• Thus, their allele frequencies progressively become more and more different at various genetic loci as new alleles in each population independently emerge through mutation.
• Usually, environmental conditions for the two groups, such as climate, resources, predators, and competitors, will vary, causing natural selection in each group to favor divergent adaptations.

ii. Reproductive isolation:

• The reproductive isolation which is central to the process of speciation takes place because of reproductive barriers, that are formed as a result of genetic, behavorial or physical differences emerging between the new species.
• These are either pre-zygotic processes i.e. differences in courtship behaviors, non-compatible genitalia, or gametes that are unable to fertilize between species.
• On other way, they are post zygotic for instance zygote mortality or the production of sterile offspring.

Types of speciation:

• There are total of 4 types of speciation i.e. allopatric, parapatric, peripatric and sympatric speciation. However, artificial speciation is also included sometimes.
• Allopatric speciation occurs when speciation via geographical separation takes place whereas sympatric speciation occurs when speciation occurs without geographic isolation.
• Peripatric and parapatric speciation, however are similar to allopatric speciation as they occur when populations are isolated. 

1. Allopatric speciation (Speciation via Geographical separation)

• The process of speciation that take place when the members of the population are isolated geographically from each other, where they are unable to mate and hence genetic exchange is prevented or interfered is termed as allopatric speciation.
• There may be a number of ways of isolating populations leading to allopatric speciation: from a river forming a new branch, erosion forming a new valley, or a group of species migrating to a new location without the opportunity to return, such as seeds floating to an island across the ocean.
• The essence of the geographical separation needed to separate populations is entirely dependent on the organism’s biology and its capacity for dispersion.
• If two flying insect populations took up residence in different neighboring valleys, it is possible that individuals would fly back and forth from each population, continuing gene flow.
• If two rodent populations were split by the creation of a new lake, however, continued gene flow would be unlikely; speciation would therefore be more likely.
• Allopatric processes are classified into two groups.
  • If a few individuals of a species migrate to a new geographical area, this is called dispersal.
  • If a natural condition happens to physically separate species, this is called vicariance.

How does allopatric speciation occurs?

• Allopatric speciation occurs due to geographical separation of population.
• The geographical separation of population may be due to geological shifts, such as the formation of a mountain by a volcano, the formation of islands, the division of ecosystems by glaciers and rivers, or the destruction of habitats due to human activity.
• As a result of various selective pressures acting on populations, the separated populations then experience divergence in genotypic or phenotypic traits.
• When mutations occur within species, this allows natural selection to induce genetic drift.
• Over time, because of adaptation to their new environment, the separate populations can evolve morphologically different characteristics.
• The characteristics can become so markedly distinct that there is reproductive isolation, preventing the inbreeding of populations and thereby generating new species.
• If this is the case then it is suggested that allopatric speciation has taken place.
• If populations become sufficiently different to be classified as new species, but not sufficiently distinct for the occurrence of reproductive isolation, the species can return into contact and mate, creating hybrids.

Example of Allopatric speciation; Darwin’s Finches:

• In the Galapagos finches that Charles Darwin studied, a significant example of allopatric speciation occurred.
• On the islands of the Galapagos, there are about 15 different species of finches, each of which looks different and has specialized beaks for consuming various kinds of food, such as insects, seeds and flowers.
• All these finches originated from a single species of ancestor that must have emigrated to the several islands.
• When populations on the islands were created, they were isolated from each other and numerous mutations emerged.
• In their respective habitats, the mutations that caused the birds to be more powerful became more and more common, and several different species evolved over time.
• If several new organisms evolve in a relatively rapid geological period from one common ancestor, this is called adaptive radiation.

2. Peripatric speciation:

• It occurs when the individuals lying on the periphery, or border of a huge population split off from the main group and result to a new species in course of time.
• Differentiating it from allopatric speciation can be hard.
• When the population that branch off enters a distinct biological niche, like feeding on different food or surviving in a different environment, peripatric speciation occurs.
• Often these new populations that split away from the existing one are typically small, so this can have an effect on the proportion of some characteristics in the new population compared with the old one.
• Say for instance, that there is a bird population that is mostly blue, but some are red.
• A smaller group of birds splits out of the main group, and red is the majority of this smaller group.
• It is probable that their descendants will also be mainly red, which is different from the main group.
• This type of change in gene frequency is referred to as genetic drift.
• Many changes can take place over time, and these, combined with the effects of genetic drift, can cause new species to evolve.

Example of peripatric speciation; the London underground mosquitoes

• The London Underground mosquito is a type of mosquito found in the Underground area of London.
• Because of to its edacious biting, biologists called the London Underground mosquito Culex pipiens f. molestus.
• It eventually adapted to human-made underground structures, from being a local above-ground Culex pipiens.
• Recent evidence indicates it is a southern mosquito variety related to C. pipiens that has modified to the warm underground spaces of northern cities.
• The proof for this specific mosquito becoming a distinct species from C. pipiens comes from studies done by Kate Byrne and Richard Nichols.
• The species have very unique features and are particularly difficult to mate.
• More precisely, the C. pipiens f. molestus is cold intolerant and bites rodents,  and humans, and can breed all year round, while the above-ground species is also cold tolerant, but hibernates in the winter and targets only birds.
• The eggs were infertile when these two varieties were cross-bred, indicating reproductive isolation.

3. Parapatric speciation:

• Parapatric speciation occurs when subpopulations of the same species are largely isolated from each other however have a small region where their ranges overlap.
• This could be caused by a partial geographical barrier or an uneven distribution of members of subpopulation.
• It has very less chances to occur.
• It can occur between several neighboring subpopulations where all the neighboring populations can interbreed, but each subpopulation is so slightly different that it would not be possible for the members on the extreme ends to interbreed with each other. This is referred as ring species.
• That means within the group, the population does not mate randomly, but rather individuals mate with their nearest geographical neighbors more generally resulting in unequal gene flow.
• Non-random mating could increase the rate of dimorphism within populations, in which differed morphological aspects of the same species are exhibited.
• Parapatric speciation results in one or more distinct sub-populations (termed as ‘sister species’) that have small continuous overlaps in their biogeographic range and are genotypically dimorphic.

Example of parapatric speciation; Agrostis tenuis:

• In populations of the grass Agrostis tenuis that span mine tailings and natural soils, the best-known example of ongoing parapatric speciation occurs.
• Heavy metal tolerant individuals, a heritable trait, live well on polluted soil, but poorly on soil that is not contaminated.
• For intolerant populations, the reverse happens.
• Gene flow occurs between sub-populations on and off mine tailings, but small variations in flowering time between the two locations inhibit hybridization.

4. Sympatric speciation (Speciation without geographical separation):

• It is the evolutionary process by which organisms are created from a single ancestral species while occupying the same geographical area.
• The distribution ranges of organisms that evolve by sympatry may be similar, or they may only overlap, as contrasted to allopatric speciation.
• Instead of geographical distance causing a reduction in gene flow between populations,  sympatry occurs as members of one population make use of a new niche.
• For example, this could occur if a herbivorous insect starts feeding on a new or noble source of plants with which it is not ancestrally associated, or if a new plant species is introduced into the geographical range of the species.
• As insects normally reproduce or lay eggs within the type of fruit in which they were born, the individuals will specialize in feeding and mating on specific fruits over time.
• As a result, gene flow between populations that specialize in different fruits would be decreased, leading to populations being reproductively isolated.
• As new species emerge from populations living in highly overlapping or even similar environments, sympatric speciation is very distinct from the other forms.
• It may be more prevalent in bacteria than in multicellular organisms because when they split, bacteria may shift genes to each other as well as transfer genes to offspring.

How does sympatric speciation occurs?

• One type of sympatric speciation can start with a chromosomal defect during meiosis or the formation of a hybrid individual with large number of chromosomes.
• A condition in which there is an additional set of chromosomes, or sets, in a cell or organism is termed as polyploidy.
• Polyploidy results from a meiosis defect in which, instead of dividing, all the chromosomes pass into one cell.
• There are two major types of polyploidy that could result in reproductive isolation of an individual in the polyploid state.
• One is autopolyploidy where polyploid individuals will possess two or more complete set of chromosomes from its own species.
• For instance, if a plant species with 2n=6 results in autopolyploid gametes which are also diploid, the gametes now possess twice as many chromosome they should possess.
• These new gametes are incompatible with the usual gametes that this species of plant produces.
• However, they could either self-pollinate or reproduce with several other autopolyploid plants with gametes that have the same diploid number.
• In this way, sympatric speciation will occur rapidly by producing offspring with 4n called a tetraploid.
• Only those of this new kind and not those of the ancestral species will be able to reproduce immediately with these individuals.
• Allopolyploidy is the other form of polyploidy where individuals of two different species reproduce to yield a fertile offspring.
• The examples of allopolyploids are cultivated forms of wheat, cotton, and tobacco plants.
• Sympatric speciation can occur in ways other than polyploidy, as well.
• If we consider a species of fish residing in a lake.
• Competition for food increased as the population grew.
• Under pressure to find food, if we assume that the genetic versatility of a group of these fish was to discover and feed off another resource unused by the other fish.
• If the new food source was discovered at a different depth of the lake then, Those fed on the second food source would communicate more with each other over time than the other fish; they would therefore also breed together.
• The offspring of these fish are likely to act and live in the same area as their parents and feed, keeping them apart from the original population.
• If this group of fish continued to remain separate from the first population, as more genetic differences accumulated between them, sympatric speciation would eventually occur.

Example of sympatric speciation; Cichlid fish

• In Tanzania, cichlid fish that live in a small volcanic crater lake are seen as one such example of sympatric speciation.
• There are two very distinct ectomorphs or forms in the population: a yellow-green one that lives along the shore, and a blue-black one that lives at the bottom of the lake.
• By looking at the DNA of the fishes, researchers could see that the two ectomorphs were genetically very distinct.
• It is assumed that these two forms are in the gradual speciation phase at present.

5. Artificial speciation:

• The type of speciation that can be accomplished through the input of human intervention is termed as artificial speciation.
• Human beings may create new, distinct species by separating populations and thus preventing reproduction, or by purposely breeding individuals with desired morphological or genotypical traits.
• This is also known as ‘artificial selection’; artificial selection has been undertaken for most modern domesticated animals and plants.
• While evolution of our modern crops and livestock has taken thousands of years, it is possible to imagine the process of artificial selection in organisms that have short life cycles.

Example of artificial speciation; Drosophila melanogaster:

• In fruit fly (Drosophila melanogaster) species, artificial selection has been most effectively displayed.
• The changes that occur as flies adapt to each environment are shown by experiments in which flies are placed in environments that contain different resources or habitats.
• The flies are removed from the experimental zone after several generations and allowed to cohabit, although the populations are unable to mate due to the process of reproductive isolation that occurred while in isolation.

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• Human brain weighs about 1.5 kilograms (3.3 lbs) and is larger in relation to body size.
• It is the most complex part of the body in human that is responsible to control thoughts, memory and speech, legs and arm movements along with the many functions within the body.
• The brain is located inside the cranium.
• Cranium is the bony covering that protects the brain from external injury.
• The Nervous system:
  • The nervous system in human is divided into central nervous system and peripheral nervous system.
  • The central nervous system comprises of brain, its cranial nerves and the spinal cord.
  • The peripheral nervous system comprises of autonomous nervous system (divisible into the sympathetic and parasympathetic nervous system) and the spinal nerves that branch from the spinal cord.

Cells of the Human brain:

• The brain is composed of two types of cells i.e. nerve cells (neurons) and glial cells also termed as neuroglia or glia.

1. Nerve cells (Neurons):

• Neurons vary in shape and size, however they all consist of cell body, dendrites and axon.
• The neuron transmits the information via the electrical and chemical signals.
• The transmission of information is mediated by a gap called as synapse.
• Dendrites are the arms that plays role as antennae picking up messages from other nerve cells.
• The neurotransmitter after crossing the synapse, fit into special receptors on the receiving nerve cell, that stimulates that cell to convey the message.

2. Glial cells:

• These are about 10-50 times more glial cells than the neurons.
• Glial cells are responsible for nourishing the neurons. It also aids for the protection as well as structural support to neurons.

What are the types of glial cells?

• Astroglia or astrocytes: They are the nurse cells and aids in regulating the blood brain barrier, permitting the nutrients to make contact with neurons.
• They regulate homeostasis, and also have impact on electrical impulses.
• They are also engaged in defense and repair of neurons.
• Oligodendroglia cells forms myelin that aids in the fast transport of electrical impulses.
• Ependymal cells secrete cerebrospinal fluid (CSF) and line the ventricles.
• Microglia cells are the macrophages of brain that are responsible for invading and cleaning up debris.

How is the brain protected in the body?

• The brain is contained in the skull, where a layer of fluid called cerebrospinal fluid is suspended.
• CSF is a transparent watery substance and is produced within the channels in the brain called as ventricles.
• Major portion of CSF is produced by the choroid plexus.
• Choroid plexus is a specialized structure within each ventricle.
• It protects the brain from mechanical shocks and jolts that are mild.
• In addition, it also plays minor immunological functions and provides the brain’s necessary nutrients.

Ventricles in brain:

• There are 4 ventricles of brain that are connected to each other by foramen, and tubes.
• Lateral ventricles (first and second): These are two ventricles that are enclosed in the cerebral hemispheres.
• Third ventricle: It is located at the center of the brain and its wall are made up of the thalamus and hypothalamus. There is a pineal gland behind the third ventricle. It helps control the inner clock and circadian rhythms of the body through the secretion of melatonin. In sexual growth, it has some function.
• Fourth ventricle:
• It is located posterior or dorsal to the pons and medulla oblongata and is anterior to the cerebellum.
• The first and second ventricle connects with the third ventricle through a distinct opening termed as the Foramen of Munro.
• The communication between the third and the fourth ventricle takes place via the Aqueduct of Silvius that is a long tube.

How is blood supplied to the brain?

• Two paired arteries, the internal carotid arteries and the vertebral arteries, bring blood to the brain.
• Most of the cerebrum is supplied by the internal carotid arteries.
• The cerebellum, brainstem, and the cerebrum’s underside are supplied by the vertebral arteries.
• The right and left vertebral arteries join together after passing through the skull to form the basilar artery.
• At the base of the brain, called the Circle of Willis, the basilar artery and the internal carotid arteries interact with each other.
• A significant safety function of the brain is the communication between the internal carotid and vertebral-basilar structures.
• It is possible for collateral blood flow to come through the Circle of Willis and avoid brain damage if one of the main arteries is blocked.
• The brain’s venous circulation varies significantly from that of the rest of the body.
• As they supply and drain particular areas of the body, arteries and veins usually run together.
• Hence one would assume that a pair of vertebral veins and internal carotid veins would be there.
• This is not the case in the brain, however.
• The main vein collectors are integrated into the dura to form venous sinuses.
• Blood from the brain is gathered by the venous sinuses and transferred to the internal jugular veins.
• The superior and inferior sagittal sinuses drain the cerebrum and the anterior skull base is drained by the cavernous sinuses.
• Eventually, all sinuses drain to the sigmoid sinuses, which leave the skull and form the veins of the jugular.
• In reality, these two jugular veins are the sole drainage of the brain.

Meninges of the brain:

• The three layers of tissue called as meninges cover and secure the brain and spinal cord.
• They are the dura mater, arachnoid mater, and pia mater from the outermost layer to inwards.
• Dura mater:
• It is a strong, thick membrane that lines the inside of the skull closely.
• It has two layers i.e. the periosteal and meningeal dura that are fused. They separate only to form venous sinuses.
• Little folds or compartments are formed by the dura.
• Two unique dural folds, the falx and the tentorium, are present.
• The falx divides the brain’s right and left hemispheres and the cerebrum is separated from the cerebellum by the tentorium.
• Arachnoid mater:
• It is a thin, web-like membrane covering the whole brain.
• The arachnoid is made of tissue that is elastic.
• The space between the dura and the arachnoid membranes is termed as the subdural space.
• Pia mater:
• It embraces the surface of the brain following its fold and grooves.
• There are several blood vessels in pia mater that reach far into the brain.
• The subarachnoid space is termed as the space between the arachnoid and the pia.
• The cerebrospinal fluid bathes and cushions the brain in pia mater.

Describe the structure and function of different parts of human brain

• Brain are divisible into the fore brain, the mid-brain and the hind brain.
• The forebrain is further divided into cerebrum, hypothalamus and thalamus.
• The midbrain includes tectum and tegmentum.
• The hind brain is composed of cerebellum, medulla and pons.
• However, the three main components of the brain are:
  • Cerebrum
  • Brain stem
  • Cerebellum

1. Cerebrum:

• It is the largest part of the brain and is divisible into right and left hemispheres.
• The right and left hemispheres is connected by a bundle of nerve fibres called as corpus callosum.
• The outer layer of the cerebrum is termed as cerebral cortex.
• Cerebrum is responsible for many of functions such as learning, emotions, problem-solving, motor control, interpretation of sensory information etc.
• Cerebrum can be classified into four different lobes: the frontal, parietal, temporal and occipital lobes.
• Frontal lobe:
  • It is located in the frontal part of the brain just over the eyes where the largest section of the brain is located.
  • Human frontal lobe is larger and more developed in comparison to any other organism.
  • It is responsible for executive functions such as language, reasons, emotions, personality characteristics and movement.
  • In the frontal lobe, the Broca’s area, one of the areas in speech is located.
  • The area of the brain responsible for controlling the voluntary movement is also present in it.
  • It is because of the frontal lobe that we are able to communicate and form rational thoughts.
• Parietal lobe:
  • It lies just behind the frontal lobe and is separated from frontal lobe by the central sulcus.
  • The two major functions of parietal lobes are:
  • Somatosensation: touch sensations such as pressure, pain, temperature, heat, cold.
  • Proprioception: the sense of how the body parts are oriented in the space.
  • It also regulates the ability to taste.
• Temporal lobe:
  • It lies posterior to the frontal lobe and is separated by lateral fissure.
  • In appropriate terms, it lies in the base of the brain.
  • Primarily, its function is to process and interpret sounds.
  • It is also a center for the cognitive processes for the memory formation and recognition of language.
  • In this lobe, primary auditory cortex is present that receives the sensations related to hearing.
  • Along with it, the processing of complex visual information, such as environments or images having various elements and large variety of colors takes place in the temporal lobe.
  • The key function of temporal lobe is the long-term memory.
  • Hippocampus lies in this lobe that processes memory formation.
• Occipital lobe:
  • It is situated at the back of the brain and is primarily engaged in vision.
  • It is the main area for visual processing.
  • It helps in recognition and identification of the visual world. 

Functions of cerebrum:

• The cerebrum directs the body’s conscious or voluntary motor functions.
• Inside the primary motor cortex and other frontal lobe motor areas where actions are scheduled, these functions originate.
• Upper motor neurons in the primary motor cortex transmit their axons to the lower motor neurons, which innervate the muscles, to synapse into the brain stem and spinal cord.
• In some forms of motor neuron disease, damage to motor areas of the cortex can result.
• Instead of complete paralysis, this sort of harm results in loss of muscle strength and accuracy.
• The olfactory sensory system is unusual in that its axons are sent directly to the olfactory cortex by neurons in the olfactory bulb, rather than to the thalamus first.
• A deterioration of the sense of smell results in damage to the olfactory bulb.
• From such brain areas as the amygdala, neocortex, hippocampus, locus coeruleus, and substantia nigra, the olfactory bulb also receives ‘top-down’ knowledge.
• Its’ possible functions can be classified into four non-exclusive categories:
  • distinguishing between odors
  •  improving odor detection sensitivity
  • filtering out background odors
  • allowing higher areas of the brain involved in arousal and attention to alter odor detection or discrimination.
• Sections of the cerebral cortex are largely attributed to speech and language.
• In the frontal lobe, motor portions of language are attributed to Broca’s field.
• Wernicke’s area, at the temporal-parietal lobe junction, is due to speech comprehension.
• Damage to the area of Broca results in verbal aphasia (non-fluent aphasia), whereas receptive aphasia results in damage to the Wernicke’s area.

2. Amygdala:

• This is another aspect of the limbic system that is a functional portion of the cerebrum.
• In the temporal lobes, the amygdala lies and is engaged in several functions of the autonomic nervous system, including physiological responses to fear (the “Fight or Flight response) and hormone secretion.
• In each hemisphere, one amygdala is found (plural: amygdalae).
• In particular, this structure is concerned with controlling the emotions of fear, anger, and pleasure.
• The amygdala is the structure which decides what memories are stored and where they are stored in the brain.
• The amygdala is the mechanism which decides what memories are stored and where they are stored in the brain.

3. The thalamus:

• It is a gray matter which is located between the cerebral cortex and the midbrain.
• The thalamus, placed under the cerebral cortex, is involved in various sensory and motor functions.
• It also regulates the circadian rhythm partly by signaling to the brain during sleep to decrease those physiological functions.

4. Hypothalamus:

• It is a small but necessary region of the brain.
• It is situated at the base of the brain, in the proximity to the pituitary gland.
• It is located in the third ventricle’s floor and is the autonomic system’s master control.
• In regulating behaviors such as starvation, hunger, sleep, and sexual response, it plays a major role.
• It also controls the temperature of the body, blood pressure, emotions, and hormone secretion.

5. Basal ganglia:

• The basal ganglia are a collective term for a group of subcortal nuclei that are central to motor control, learning and executive functions as those regulated by the frontal lobe.
• The putamen, caudate, globus pallidus, subthalamic nucleus, and substantia nigra are the parts which make up the basal ganglia collectively.
• The caudate and putamen are referred to as the striatum together.
• These systems are best known for their role in movement, considering all they’re associated with.

6. Olfactory bulb:

• The olfactory bulb is a structure found in the anterior part of the brain, in the lower (bottom) part of the cerebral hemisphere.
• There is one olfactory bulb per hemisphere, and an elongated structure known as the olfactory stalk or the olfactory peduncle binds them to the cerebrum.
• The olfactory bulb is as the name implies, central to the sense of smell and is also partially involved in taste.

7. Brain stem:

• The distal portion of the brain that consists of the midbrain, pons, and medulla oblongata is the brain stem (brain stem).
• Each of the three parts has a particular structure and feature of its own.
• Together, they help to monitor breathing, heart rate, blood pressure, and a variety of other main functions.
• A stalk-like projection that stretches caudally from the base of the cerebrum is the brainstem.
• It favors the communication between the cerebrum, cerebellum, and the spinal cord.
• At its proximal end, the brainstem is broader and becomes smaller towards the distal end.
• Three parts of the brainstem exist:

i) Medulla oblongata:

• The narrowest and most distal section of the medulla is the medulla oblongata.
• The narrowest and most caudal portion of the brainstem is the medulla oblongata or medulla.
• It is a funnel-like structure that stretches from the decussation of the broad pyramids to the inferior pontine sulcus (pontomedullary groove) through the foramen magnum (which is the largest of all the foramina and fissures of the skull).
• The narrowest and most caudal portion of the brainstem is the medulla oblongata or medulla.
• It is a funnel-like structure that stretches from the decussation of the broad pyramids to the inferior pontine sulcus (pontomedullary groove) through the foramen magnum (which is the largest of all the foramina and fissures of the skull).
• As the medulla in the posterior cranial fossa proceeds upward it stops at the fourth ventricle inferior pontine sulcus (anteriorly) and medullary striae (posteriorly).

ii) Pons:

• To the anterior, pons lies in the middle segment of the brainstem.
• Another comparatively short part of the brainstem located in the posterior cranial fossa is the pons.
• The arrangement of approximately 2.5 cm rests against the skull clivus, below the tentorium cerebelli.
• The metencephalon, which is a secondary brain vesicle developed from the hindbrain (rhombencephalon), produces the pons.
• Caudal to the mesencephalon, and cranial to the myelencephalon (medulla), the metencephalon is located.

iii) Midbrain:

• The midbrain is the section that is broader and superior.
• The midbrain is the brainstem’s shortest section.
• It stretches caudally from the thalamus base to the fourth ventricle’s superior roof.
• In the tentorium cerebelli (an extension of the dura mater), it passes through an opening.
• The structure emerges from the mesencephalon, which is one of three primary brain vesicles that emerge (above the fourth pair of somites) from the cranial portion of the closed neural tube.
• Between the forebrain (prosencephalon) and the hindbrain (rhombencephalon) the mesencephalon is located.
• This part of the brainstem is split into tectum (the Latin word for roof), tegmentum (the Latin word for covering) and crus cerebri.
• The tectum is the dorsal component of the midbrain to the cerebral aqueduct of Sylvius (a conduit linking the third and fourth ventricles).
• In comparison, the tegmentum is ventral to the aqueduct of Sylvius.
• Wide arrays of ascending and descending tracts moving to and from the brain are the crus cerebri.
• The substantia nigra separates each crus from the tegmentum.
• This is a pigmented lamina that consists of neurons that are dopaminergic and GABAergic.
• These are cells that create unique neurotransmitters they are named for.
• It is important to note that the tegmentum is continuous throughout the midline of the midbrain, even though each crus cerebri is separate from each other.
• Some texts collectively refer to the crus cerebri and tegmentum as the cerebral peduncles.

Functions of brain stem:

• The brainstem has three essential functions:
• To act as a conduit for pathways to and from the brain to ascend and descend
• To house the nuclei of cranial nerves
• To merge the roles of many critical systems with each other

8. Cerebellum:

• It is a comparatively small region of the brain, about ten percent of the total weight, but it includes approximately half of the neurons of the brain, specialized cells that convey electrical signal information.
• The cerebellum is split into two lateral hemispheres, which are connected by a medial portion called the vermis.
• Each of the hemispheres is composed of a white matter central core and a gray matter surface cortex and is divided into three lobes.
• The flocculonodular lobe is the first portion of cerebellum to develop.
• The flocculonodular lobe receives sensory input from the vestibules of the ear.
• From the spinal cord, the anterior lobe receives sensory input.
• The posterior lobe is last to arise and receives nerve impulses from the cerebrum.
• Within the cerebellar cortex, all these nerve impulses are incorporated.
• The superior, middle, and inferior peduncles that link the cerebellum with the midbrain, pons, and medulla, respectively, transmit information to and from the cerebellum via three paired bundles of nerve fibres.
• The cerebellum has traditionally been regarded a motor structure as cerebellar damage contributes to impairments in motor function and posture and as the majority of the outputs of the cerebellum are part of the motor system.
• In the cerebellum, motor commands are not initiated; rather to make movements more adaptive and precise, the cerebellum modifies the motor commands of the descending pathways.

Functions of cerebellum:

• Balance and posture management:
  • In order to maintain equilibrium, the cerebellum is essential for making postural changes.
  • It modulates commands to motor neurons through its feedback from vestibular receptors and proprioceptors to compensate for changes in body position or changes in load on muscles.
  • Balance problems are suffered by patients with cerebellar injury, and they frequently develop stereotyped postural strategies to compensate for this issue (e.g. a broad-based stance).
• Voluntary movement synchronization:
  • Most movements consist of a variety of distinct muscle groups behaving in a temporarily organized manner together.
  • One of the cerebellum’s key functions is to coordinate the timing and force to produce fluid limb or body movements of these various muscle groups.
• Motor learning:
  • In adjusting and fine-tuning motor programs to make precise movements in a trial-and-error process e.g. learning to hit a baseball), the cerebellum plays a major role.
• Cognitive roles:
  • Though the cerebellum is most commonly understood in terms of its contribution to motor regulation, some cognitive functions, such as language, are also involved.
  • The cerebellum is therefore regarded historically as part of the motor system, like the basal ganglia, but its functions extend beyond motor control in ways that are not yet well understood.

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• The glands that secrete digestive juices for the digestion of food are termed digestive glands.
• Besides the number of gastric glands present in the lining of the stomach, there are many other related digestive glands that pour their secretions into the alimentary canal.
• The digestive glands include salivary glands, gastric glands, liver, pancreas, and intestinal glands.

1. Salivary glands:

• Three pairs of salivary glands are present.
• They are the parotid, submandibular and sublingual glands.
• Parotid glands:
  • The largest of the salivary glands are parotid glands.
  • They are located one on each side of the face, just below and in front of the ears.
  • Parotid glands secrete saliva via a parotid duct into the mouth and help in mastication and swallowing,
  • The secretion of each parotid gland passes through Stensen’s duct which opens into the mouth opposite the site of second upper molar tooth.
  • Mumps is the disease caused by the viral infection of parotid gland.
  •  It is characterized by swelling, irritation, and pain.
• Submandibular glands:
  • These are located embedded in the mucous membrane on the floor of the buccal cavity, under the tongue.
  • Duct of these glands open into the sublingual part of the mouth under the tongue.
  • It secretes mixture of the serous fluid and mucus.
  • This secretion passes into the oral cavity via the submandibular duct or Wharton’s duct.
  • Despite being much smaller than the parotid glands, it accounts for the production of 65-70% of saliva in the oral cavity.
• Sublingual glands:
  • It is the smallest of all the salivary glands.
  • It also includes the parotid and submandibular glands.
  • It is located between the muscles of the oral cavity.
  • It is different from other salivary glands as it lacks striated ducts, hence the saliva is secreted directly via the ducts of Rivinus.
  • It is the only unencapsulated major salivary gland.
  • Primarily, it secretes a thick mucinous fluid that lubricates the oral cavity and aids for swallowing, buffering the pH, initiate digestion, and dental hygiene.

Saliva and its fuctions:

• The salivary glands secrete saliva which is a viscous fluid.
• The saliva of man is a viscous, colorless, cloudy, and opalescent liquid.
• The optimum pH value is 6.8 with a range of 5.6 to 7.6 and specific gravity ranges from 1.002 to 1.008.
• It is uniformly secreted in small quantities to keep the buccal cavity moist.
• When food is present the rate of secretion is increased as the saliva helps both moistening the food and lubricating its subsequent passage through the alimentary canal.
• It also initiates digestion.
• The saliva contains 98.5 to 99% water and 1 – 1.5 % of a dense residue.

Enzymes of saliva:

• Saliva possesses a large number of enzymes such as amylase, lysosome acid phosphatase, aldolase, cholesterase, lysozyme, maltase, catalase, lipase, urease, and protease.
• Among them, amylase and lysozyme show physiological importance.
• Salivary amylase carries out the hydrolysis of starch and glycogen to maltose, isomaltose, dextrin and some glucose.
• Some particular complex polysaccharides present in the cell wall of different species of bacteria are hydrolysed by lysozyme, thereby killing and dissolving them.

Functions of saliva:

• The saliva is responsible for a number of functions:
• The dry food is moistened and facilitates swallowing by a lubricating action. It prevents desiccation of the oral mucosa, since water evaporates slowly from saliva.
• One of the enzymes present in saliva, salivary amylase, or ptyalin play a role in the digestion of starch.
• It keeps the mouth and teeth clean.
• The soluble substance such as sugar and salts get dissolved by it.
• It makes the food delicious to taste.
• The excretion of certain substances such as lead, mercury, and iodides take place in the saliva.
• It makes rapid articulation possible by facilitating movements of the tongue and lips.
• It enhances the sense of taste by acting as a solvent. The taste buds can be stimulated only when the substances having pleasant taste are actually present in the solution.
• It consists of three buffering systems, bicarbonate, phosphate, and mucin of which the bicarbonate is most important.
• When the salivary flow increases particularly at the time of eating, the concentration of bicarbonate and the buffering system rises.

2. Gastric glands:

• The wall of the stomach has various gastric glands.
• They are simple or branched tubular glands.
• About 2-3 litres of gastric juice is secreted daily by these gastric glands in adults.
• At least three different types of gastric glands are present in the gastric mucosa. These are: parietal cells (oxyntic cells), chief cells, and mucous cells.
• The parietal cells supply the hydrochloric acid of the gastric juice.
• The chief cells provide pepsin and other enzymes such as rennin and gastric lipase and the mucous cells secrete mucin.
• The Castle’s intrinsic gastric factor is secreted by the parietal cells that help in the absorption of vitamin B₁₂.
• All these secretions altogether form an acidic gastric juice with pH=2 and low specific gravity ranging from 1.002 to 1.006.
• The gastric juice comprises about 0.5% of solid matter including sodium chloride with traces of potassium chloride and phosphates, mucin and enzymes pepsin, rennin and gastric lipase, hydrochloric acid and mucin.
• The secretion of gastric juice is under both nervous and hormonal control.
• Gastric secretion is triggered by the thought of food, the smell of food, chewing of food and contact of food with the stomach wall.
• The gastric glands also have two uncommon types of gland cells:
• Argentaffin cells that are normally situated at the base of the glands and secrete serotonin, a potent vasoconstrictor.
• Endocrine cells that are present in the pyloric antrum and produce gastrin. The gastrin triggers the secretion of enzymes and HCl.

3. Liver:

• It is the largest gland of the body.
• It generally weighs about 1.6 kg.
• It is chocolate colored and is located just behind the diaphragm on the right side of the upper abdominal cavity.
• The liver has two main lobes, the right lobe which is larger, and the left lobe along which is smaller with two small lobes, the quadrate lobe and the caudate lobe present behind the main lobes.
• Hepatic artery supplies to the liver.
• The blood is drained into the inferior venacava by the hepatic vein.
• On the undersurface of the right lobe of liver, a pear-shaped structure is located called as a gall bladder.
• The gall bladder serves as a reservoir that stores the bile juice secreted by the liver.
• The right and left hepatic ducts from the liver join to form a common bile duct or ductus choledochus.
• The common bile duct and pancreatic duct join to form the hepatopancreatic duct, near the duodenum.
• This opens in the duodenum via an opening guarded by the sphincter of Oddi.
• The dilation of short common hepatopancreatic duct forms hepatopancreatic ampulla or Vater’s ampulla.
• The ampulla opens into the duodenum.
• The bile duct is surrounded by a strong sphincter muscle of Boyden before it is joined by the pancreatic duct.
• When there is no food in the duodenum, this muscle closes.
• This forces the bile into the gall bladder through the cystic duct for storage.

Functions of the liver:

• The liver being a multipurpose organ, performs several important functions:
• Production of bile:
  • Liver secretes bile which is an alkaline dark green-colored fluid having several organic and inorganic salts as well as some waste substances.
  • The bile performs the following functions:
  • It makes the chyme alkaline, better suited for the action of pancreatic juice.
  • It is responsible for the emulsification of fats.
  • It helps in removing the excretory products like bile pigments, inorganic salts, toxins, etc. from the body.
  • It stimulates peristalsis.
  • Some bile salts are necessary for the absorption of vitamin K and other vitamins soluble in fats.
  • Bile acts as an antiseptic, therefore, it does not allow the growth and multiplication of bacteria.
• Regulation of blood sugar level:
  • Liver regulates the blood sugar level (normal 90-120mg per ml of blood) either by the process of glycogenesis or by the glycogenolysis.
  • Glycogenesis is the conversion of glycogen into glucose by the liver cells with the help of glucagon secreted by the pancreas.
  • It is also a center for gluconeogenesis and glyconeogenesis.
• Lipogenesis:
  • Liver also controls lipogenesis. The conversion of the excess of glucose and amino acids into the fats is termed as lipogenesis. It also takes place into the liver.
• Deamination:
  • Liver decomposes the excess and harmful amino acids of blood into toxic NH₃ and metabolically useful keto acids by the process of deamination in the presence of oxidase enzyme.
  • In the liver cells, toxic NH₃ is combined with CO₂ to form less toxic urea by the process of detoxification.
  • Liver also detoxifies the alcohol and converts it into acetaldehyde and then harmless acetyl CoA.
• Absorption and metabolization of bilirubin:
  • Liver transforms hemoglobin of dead RBCs into bile pigments such as biliverdin and bilirubin which are egested out along with feces. Thus, liver helps in excretion.
• Aids in blood clotting:
  • Liver produces an anticoagulant called heparin which prevents the coagulation in the blood vessels.
  • Liver produces two main proteins fibrinogen and prothrombin which help in clotting of blood at injury to check excess of bleeding.
• Erythropoesis and hemolytic function:
  • Liver acts as an erythropoietic organ. It forms RBCs in the foetus.
  • Liver also acts as hemolytic organ. It breaks old RBCs.
• Immunological functions:
  • Kupffer’s cells of liver act as phagocytes which feed on dead cells and bacteria by the phagocytosis process.
• Vitamin and mineral storage:
  • Liver synthesizes vitamin A from beta carotene in the presence of an enzyme carotenase. Beta carotene is an orange yellow substance of carrot.
  • Liver stores minerals like copper and iron, vitamins like A, D, E, K and B12, glycogen, fats and water.
• Maintains body temperature:
  • Liver is the major heat producing centre of the body. Because of high metabolic activities of the liver, sufficient heat is generated, which is very important for maintaining the optimum body temperature.
• Blood reservoir:
  • It serves as the second reservoir of blood.
• Angiotensinogen synthesis:
  • Liver secretes a protein called angiotensinogen which helps kidney to maintain body fluid, osmoregulation.
• Blood filtration:
  • It helps in eliminating several unwanted substances like carbolic acid, cresol, etc. from the blood coming from alimentary canal.
  • It is the important seat of lymph formation.
  • It also produces certain substances which check anemia.

4. Pancreas:

• The pancreas is an elongated and yellowish gland.
• It is located behind the stomach lying horizontally in the curvature of duodenum.
• It is about 12-15cm long and 2.5 cm wide and weighs about 60gms.
• It comprises of head, body and tail.
• The head is located in the curve of the duodenum, the body lies behind the stomach and the tail links the spleen.
• The smaller ducts within the pancreas form the main pancreatic duct.
• Into the hepatopancreatic ampulla (ampulla of Vater), the main pancreatic duct opens.
• In pancreas, an accessory pancreatic duct (duct of Santorini) is also present and opens directly into the duodenum.
• The pancreas is exocrine as well as endocrine in nature.
• A large number of branching tubules or lobules are present in the exocrine tissue called acini.
• Acini are embedded in connective tissue containing blood and lymph vessels, nerves and pancreatic ductules.
• The acinus comprises of cuboidal epithelial cells and secretes an alkaline pancreatic juice with pH 8.8.
• About 500-800 ml of pancreatic juice is secreted every day.
• The main pancreatic duct carries the pancreatic juice into the duodenum through the hepatopancreatic ampulla.
• The pancreatic juice is directly poured into the duodenum by the accessory pancreatic duct.
• The pancreatic juice contains sodium bicarbonate, three proenzymes: trypsinogen, chymotrypsinogen and procarboxypeptidase and several enzymes: pancreatic amylase, pancreatic lipase and nucleases such as DNAse and RNAse.
• The pancreatic juice aids in the digestion of starch, proteins, fats and nucleic acids.
• The activity of chyme caused by HCl is neutralized by the sodium bicarbonate.
• The endocrine tissue of pancreas consists of several group of cells, called Islets of Langerhans, found situated in between the acini.
• Each Islet of Langerhans comprises of the following types of cells.  
• These secrete hormones to be poured in the circulating blood.
• i) Alpha cells (a-cells) are more numerous towards the periphery of islet and constitute about 25% of the Islet of Langerhans. These cells produce glucagon hormone which converts glycogen into glucose in the liver.
• ii) Beta cells (b-cells) are more numerous towards the middle of the islet and constitute about 60% of the islet and constitute about 60% of the islet of Langerhans. These cells supply insulin hormone that converts glucose into glycogen in the liver and muscles. Deficiency of insulin causes diabetes mellitus.
• iii) Delta cells (d-cells) are found towards the periphery of islet and constitute about 10% of islet of Langerhans. These cells produce somatostatin hormone which halts the secretion of glucagon by alpha cells and to some extent secretion of insulin by beta cells.
• iv) Pancreatic polypeptide cells (PP cells) are also found in the pancreas. Pancreatic polypeptide (PP) are secreted by these cells which inhibits the release of pancreatic juice.
• Thus, the pancreas serves two main functions:
  • i) secretion of pancreatic juice which contains digestive enzymes and
  • ii) production of hormones

5. Intestinal glands:

• They are numerous, microscopic glands present in the mucosa of small intestine.
• They are of two types i.e. crypts of Lieberkuhn and Brunner’s glands.
• Crypts of Lieberkuhn:
  • These are simple tubular glands.
  • They occur throughout the small intestine between villi.
  • Digestive enzymes and mucus are secreted by it.
  • These secretions are slightly alkaline with pH in the range of 7.5 to 8.0.
  • They contain several types of cells that secrete mucus and a large number of enzymes.
  • The secretion of these glands is termed as succus entericus or intestinal juice.
  • Each day about 2-3 litres of intestinal juice is secreted.
  • It is alkaline (pH 8.3) in nature and is poured into the intestine.
  • The intestinal juice contains several enzymes: aminopeptidases, intestinal amylase, maltase, isomaltase, dipeptidases, limit dextrinase, sucrase, intestinal lipase, nucleotidases or nucleophosphatases, lactase, nucleosidases and enteropeptidase (enterokinase).
  • These enzymes digest all types of food.
• Brunner’s glands:
  • These are branched tubular glands.
  • These are restricted to the submucosa of the duodenum.
  • These glands secrete mucus.
  • The mucus lubricates and sticks the food together.
  • The digestive tract is therefore lubricated so that food quickly slips into it. This protects the duodenal mucosa from the stomach-derived acidic chyme.
  • The mucus also protects mucosal cells from digestive enzymes.
  • Along with the glands mentioned, the entire alimentary canal has mucous glands that produce mucus.
  • The mucus lubricates the food and digestive tract and protects the mucosa of the stomach from damage.

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• Earthworm has a well-developed nervous system.
• The nervous system is metamerically segmented.
•  The nervous system is divisible into central, peripheral and autonomic nervous system (sympathetic nervous system).

Central nervous system of earthworm:

• It includes a nerve ring and a nerve cord.

i) Nerve ring of earthworm :

• Nerve ring is an oblique ring around the pharynx in 3^rd and 4^th segments.
• Its mid-dorsal part comprises of a pair of small and fused supra-pharyngeal ganglia, also called cerebral ganglia or brain.
• Likewise, its mid ventral part is formed of a pair of small and fused sub-pharyngeal ganglia.
• The dorsal and ventral ganglionic parts are connected by a pair of loop-like circum-pharyngeal or peri-pharyngeal connective to complete the ring.

ii) Nerve cord of earthworm:

• Nerve cord arises from the sub-pharyngeal ganglia.
• It extends behind up to the posterior end of body in mid ventral axis beneath the ventral vessel.
• Both cords appear as a single nerve cord as they are fused and enclosed in a common sheath.
• Starting from the 5^th segment behind, the nerve cord has a ganglionic swelling in the posterior part of each segment. It is known as segmental ganglion.
• Histologically, the double nerve cord is solid and formed of nerve cells and fibres.
• Nerve cord is surrounded by epineurium (a common sheath of connective tissue).
• Outside epineurium, there is a layer of longitudinal muscle fibers.
•  This is finally surrounded by a layer of visceral peritoneum.
• Nerve cells are mainly present in ventral and lateral sides of the nerve cord.
• Nerve fibers are mainly present in dorsal and middle portion of nerve cord.
• Nerve fibers are of two types ordinary fibers and giant fibers/neurocords.
• Four peculiar giant fibers (one median, one sub-median, and 2 laterals) are found mid-dorsal to the ventral nerve cord.
• Giant fibers functions for the rapid conduction of impulses throughout the nerve cord.
• In lateral giant fibers, impulse is conducted antero-posteriorly.
• However, in median and sub-median fibers, the impulse is conducted postero-anteriorly.

Peripheral nervous system (PNS) of earthworm:

• All nerves arise from CNS and supply to various parts of body.
• All the nerves present in earthworm are of mixed type as they contain both afferent and efferent nerve fibres and adjustors (association neurons).
• From the cerebral ganglia/brain, 8-10 pairs of nerves arise.
• From the peri-pharyngeal or circum-pharyngeal connectives, 2 or 3 pairs of nerves arise.
• From the sub-pharyngeal ganglia, 3 pairs of nerves arise.
• Each segmental ganglion of nerve cord gives out 3 pairs of nerves in each segment.
• First pair arises just in front of setal ring while the other 2 pairs arise closely but behind setal ring.

Autonomic nervous system (ANS) of earthworm:

• ANS of Pheretima posthuma possess sympathetic nervous system only with extensive nerve plexus that are connected to the nerves from circum-pharyngeal connectives.

Nervous activities in Earthworm:

• All earthworm operations are regulated by the nervous system, but not necessarily by the brain.
• They have both sensory and motor neurons, like larger animals.
• Sensory fibers start in the epidermis from sensory cells or receptor organs and end in fine branches in the ventral nerve cord.
• Similar branches of motor fibers arise near the branches of the cord and form a synapse with them, running outward and ending in muscles.
• Stimuli or sensory impulses are conducted to the ventral nerve cord from the receptor through sensory fibers, from where motor impulses are reflected along efferent fibers to contracting muscles.
• A simple reflex arc forms the circuit of stimuli or impulses.
• The motion of the circular and longitudinal muscles is coordinated, so the contraction of one causes the other to relax.
• Giant fibres of nerve cord conduct impulses more rapidly than other fibres, resulting in the sudden contraction of the entire earthworm body when strongly stimulated at one point.

The post Nervous system of Earthworm appeared first on Online Biology Notes.

• Earthworm are monoecious or hermaphrodite or bisexual.
• However, self-fertilization doesn’t occur because of relative location of male and female reproductive organs as well as it is protandrous.
• Protandrous refers the condition where male reproductive organs mature earlier than the female ones.
• Thus, cross fertilization occurs in earthworm.
• It comprises of male reproductive organs and female reproductive organs.

Male Reproductive Organs of Earthworm:

• The male reproductive organs comprises of testes, testis sacs, seminal vesicles, spermiducal funnel, vasa deferentia, prostate glands and accessory glands.
• The male reproductive organs include:

i. Testes:

• Testes are very minute structures that are whitish in colour.
• There are two pairs of testes and they are lobed.
• One pair each is present in segment 10^th and 11^th that are found attached with the posterior surface of 9/10 and 10/11 inter-segmental septa.
• Each testis possess 4-8 finger like lobules that contains rounded cells in masses termed as spermatogonia.
• Testes are enclosed within testis sac.
• During the young stage of the earthworm, the testes are fully developed whereas they get degenerated in the adult stage.

ii. Testis sacs:

• Testis sacs are whitish, wide bilobed sacs that encloses testis.
• They are thin-walled and fluid-filled sacs.
• On the ventrolateral sides of the ventral nerve cord, the two testis sacs are located in the 10^th and 11^th segments beneath the stomach.
• In the testis sac, a large spermatic funnel having folded and ciliated margins is present behind each of the four testes.
• The testis sacs links with a pair of seminal vesicles.
• The spermatogonia are casted into the testis sacs then, they are passed into the seminal vesicles.

iii. Seminal vesicles:

• These are large, whitish spherical structures.
• These are found in two pairs, each pair is located in 11^th and 12^th segment.
• The testis sacs open into seminal vesicles by a narrow duct.
• The posterior seminal vesicles are larger than the anterior ones.
• The seminal vesicles of the 11^th segment are present enclosed in the posterior larger testis sac.
• The seminal vesicles of the 12^th segment are exposed in the coelomic cavity.
• These are located ventro-laterally below the stomach.
• They are also termed as septal pouches, as they develop as septal outgrowths.
•  The spermatogonia from testis sac are received by seminal vesicles.
• Seminal vesicles aids for nourishment to the sperm.
• Here, the spermatogonia matures and form spermatozoa.

iv. Spermiducal funnel/ spermatic funnel:

• They are cup like curvature in structure and are present in two pairs.
• Each spermiduct funnel is internally ciliated.
• It is located below each testis in the segment 10^th and 11^th segment.
• It is found enclosed within the same testis sac.
• It helps in conduction of spermatozoa.
• After the maturation, sperms from seminal vesicles revert to testes sac and pass into vasa differentia via spermiducal funnel.

v. Vasa deferentia:

• It is thin, long, narrow, thread like tubular structure.
• Posteriorly, each spermiducal funnel leads to vasa deferens.
• These are found in two pairs and each pair is located on the either side of the alimentary canal.
• Both the pair of vasa deferentia runs in close proximity and laterally to the nerve cord below the alimentary canal on the ventral body wall.
• It extends from 12^th to 18^th segment and meets prostatic duct in the 18^th segment forming common prostatic and spermatic duct.
• The sperms from spermatic funnel are collected by the vasa deferentia and are supplied to prostate glands.

vi. Prostate glands:

• Prostate glands are large, whitish, flat solid, irregularly shaped glands.
• These are found in pair and are located one on either side of the alimentary canal in the segments from 16^th to 20^th or 17^th to 21^st .
• Maximum portion of prostate glands are glandular region while a small part is non-glandular region.
• A thick curved prostatic duct arises from each prostate gland in 18^th segment.
• The prostatic duct is connected to the two vasa deferntia of its own side and forms a common prostatic and spermatic duct.
• On the ventral side of 18^th segment, prostatic duct opens via a male genital aperture.
• Hence, each genital aperture has three distinct apertures, two of the vasa deferentia and one of the prostatic gland.
• In earthworm the prostatic secretion is useful for the activation of sperms.
• It also aids in motility of sperms.

vii. Accessory Glands:

• Accessory glands are rounded structures and are found in two pairs.
• Each pair is located in the segments 17^th and 18^th on the ventral body wall at the lateral sides of the nerve cord.
• These glands are exposed to outside by a collection of small ductless glands.
•  On the two pairs of genital papillae positioned externally on either side of the mid-ventral line, these glands open in the 17^th and 19^th  segments.
• The secretion of these glands is thought to hold the two worms together during the copulation process.
• After the completion of spermatogenesis, tailed spermatozoa are formed.
• These spermatozoa again enter the testis sacs and reach the vasa deferentia via the spermatic funnels.
• Then they are discharged through the male genital apertures along with the prostate glands’ secretion.
• Male genital pores are found in pairs and is located in 18^th segment.

Female Reproductive Organs of Earthworm:

• Female reproductive organs consist of the ovaries, oviducal funnel, oviducts and spermathecae.
• Female reproductive organ includes:

i. Ovaries:

• Ovaries are white, small, lobulated structures that are found in a pair.
• In the 13^th segment, ovaries are located one on either side of the ventral nerve cord.
• It is attached with the posterior region of the inter-segmental septum of 12/13 segments.
• Each ovary consists of various finger like projections where developing ova are found in a row, giving the beaded appearance.
• In each ovarian lobe, the ova exhibits several stages of its development where mature ones lie in the distal part and the immature ones are found in the proximal part.

ii. Oviducts:

• A small, ciliated ovarian funnel with folded margins are present below each ovary in 13^th segment.
• Each ovarian funnel opens into a short and conical oviduct.
• Oviducts are ciliated.
• The oviducts of both the sides unite below the nerve cord.
• Here, it open by a single median female genital pore ventrally in the 14^th segment.
• The ova after maturation are released from the ovaries and are received by the ovarian funnel.
• Then, it passes through the oviduct and that is discharged out via the female genital pore.

iii. Spermathecae:

• Spermathecae are four pairs and each pair is located ventro-laterally in the 6^th, 7^th, 8^th, and 9^th segments.
• Each spermatheca is flask shaped structure.
• Each spermathecum is made of ampulla and neck (upper short tubular part).
• the main body is the ampulla.
• At the junction of the ampulla and neck, a small sac is found.
• This sac is termed as diverticulum.
• Spermathecae are also termed as seminal receptacles as they are designed for receiving sperms from another worm during copulation and temporary storage of sperms.
• The diverticulum of spermathecae in Pheretima posthuman stores sperms, which reach here after copulation.
• But in other species of earthworm, sperms are store in ampulla.
• In Pheretima posthuma ampulla aids nourishment to the sperm but in other species of earthworm, diverticulum provides nourishment.
• Spermathecal duct opens outside by small pores situated in the grooves of 5/6, 6/7, 7/8, and 8/9 segments.

iv. Female genital pore:

• It is single pore and is located in the 14^th segment.

Copulation in Earthworm:

• Copulation has been seen in several species of earthworms.
• It commonly take place in the rainy season during the months of July to October, in the morning hours before sunrise.
• During copulation, two earthworms lie opposite to each other in such a way that their ventral surfaces remain in touch and male genital openings of one comes just opposite to the spermathecal openings of the other and vice versa.
• Both the earthworms receive sperms and prostatic secretion in all of their spermathecae by a protuberance that arises from the male genital opening.
• Copulation lasts for about an hour.
• Sperms are stored in the spermathecae.

Cocoon formation in Earthworm:

• Fertilization only occurs in the cocoon or ootheca and is always external.
• After maturation of ovaries, cocoon or ootheca formation starts after copulation.
• Three varieties of glands are present in the epidermis of clitellar segments, i.e., 14^th , 15^th and 16^th segments.
•  They are mucous glands that secrete mucus for copulation, cocoon-secreting glands that produces the wall of the cocoon and albumen glands that secrete albumen in which eggs are deposited in the cocoon.
• A cocoon of earthworm contains many fertilized eggs.
• However, only one develops into embryo, while other fertilized eggs serve nutritive/ nurse cells.
• Cocoon-secreting glands of the clitellum secretes a membranous girdle.
• This girdle after hardening, the deposition of albumin between the girdle and the body wall takes place.
• The worm begins to withdraw itself backwards from the girdle.
• As the girdle moves over the female genital pore, it receives eggs, and when it passes over the spermathecae, sperms are emitted into it through spermathecal pores.
• Finally, the girdle is removed off from the anterior end of the worm.
• In a short time, the elasticity of its wall closes up its two ends forming a cocoon or ootheca.
• Several cocoons formation occurs after each copulation as the spermatozoa present in the spermatheca do not move out all at one time.
• The cocoons are oval in structure, light yellow in colour and are about 2 to 2.4 mm in length and 1.5 to 2 mm in breadth.

Fertilization in earthworm:

• Fertilization occurs inside the cocoon and normally there is only one embryo in a cocoon.
• Cocoon is found in moist and cool places and young one hatch out after 2-2.5 months.
• Albumen cells provide nutrition to the embryo.
• Cleavage is holoblastic and unequal.
• Development is direct without any larval stage.

Summary points on Reproductive system of earthworm:

• Earthworm are monoecious or hermaphrodite or bisexual and protandrous.
• Each testis consists of 4-8 fingers like projections and are situated inside testis sac.
• Each spermiduct funnel is internally ciliated and helps in conduction of spermatozoa.
• Seminal vesicles in earthworm are also called septal pouches.
• Seminal vesicles aids for nourishment to the sperm.
• In earthworm the prostatic secretion is useful for the activation of sperms.
• Each ovary is whitish in colour and consists of ovarian tubules.
• Ovaries are larger than the testes.
• Spermathecae are flask shaped structure and are found one pair in each 6^th, 7^th, 8^th and 9^th segments.
• Each spermathecum is made of ampulla and neck (upper short tubular part).
• At the junction of the ampulla and neck, a small sac is found termed as diverticulum.
• This diverticulum of spermathecae in Pheretima posthuman stores sperms, which reach here after copulation.
• But in other species of earthworm, sperms are store in ampulla.
• In Pheretima posthuma ampulla aids nourishment to the sperm but in other species of earthworm, diverticulum provides nourishment.
• Spermathecal duct opens outside by small pores situated in the grooves of 5/6, 6/7, 7/8, and 8/9 segments.
• Spermathecae/ seminal receptacles are designed for receiving sperms from another worm during copulation and temporary storage of sperms.
• It is assumed that, the secretion of accessory glands helps to keep the two earthworms together during copulation.
• Copulation takes place between two earthworms generally at night during rainy season.
• During copulation, two earthworms lie opposite to each other in such a way that their ventral surfaces remain in touch and male genital openings of one lies just opposite to the spermathecal openings of the other worm and vice versa.
• Both the earthworms receive sperms and prostatic secretion in all of their spermathecae by a protuberance that arises from the male genital opening.
• Sperms are stored in the spermathecae.
• Fertilization only occurs in the cocoon or ootheca and is always external.
• The glandular clitellum forms cocoons.
• A cocoon of earthworm contains many fertilized eggs.
• However, only one develops into embryo, while other fertilized eggs serve nutritive/ nurse cells.
• Cocoon is found in moist and cool places and young one hatch out after 2-2.5 months.
• Albumen cells provides nutrition to the embryo.
• Cleavage is holoblastic and unequal.
• Development is direct without any larval stage.

Reproductive system of Earthworm

The post Reproductive system of Earthworm appeared first on Online Biology Notes.

• In earthworm, the blood vascular system is of closed type.
• Blood vascular system is different in first 13 segments as regards to number, arrangements and nature of blood vessels.
• It comprises of the blood vessels, hearts, loops, blood capillaries and the blood glands.

Blood of earthworm:

• The respiratory pigment haemoglobin is dissolved in plasma, and hence the blood of the earthworm is red in colour.
• The plasma also includes other corpuscles that are colourless and bear nucleus.

Blood Vessels of earthworm:

• The blood vessels are of two distinct types.
• They can either be collecting blood vessels or distributing blood vessels.
• The vessels are closed tubes and possess definite wall.
• They further break down into capillaries and are divaricated in the various parts of the body.
• The arrangement of blood vessels in the anterior thirteen segments slightly differs from that behind the thirteen segment, i.e., in the region of intestine.
• Thus, the simplified headings for the study of blood vessels are:
  • Blood vessels and its arrangement in the segments behind 13th, i.e., intestinal region.
  • Blood vessels and its arrangement in the anterior thirteen segments.

A] Blood Vessels behind 13th segments in Intestinal Region:

• The blood vessels of intestinal region comprises of:
  1. Median longitudinal blood vessels
  2. The intestinal blood plexus
  3. The commissural vessel
  4. The integumentary vessel
  5. The nephridial vessels

1. Median Longitudinal Blood Vessels:

• (i) Dorsal blood Vessel:
  • It is located above the intestine in mid dorsal line.
  • The direction of flow of blood in dorsal vessel is from posterior to anterior.
  • Dorsal blood vessel is the largest blood vessel of the body in earthworm.
  • It is the thickest blood vessel having contractile muscular walls.
  • It is visible as a dark line from the thin and semi-transparent body wall.
  • It is contractile and rhythmically operates to force the blood from the posterior to the anterior side.
  • It has a pair of valves internally in each segment which check the backward flow of blood.
  • Dorsal blood vessel is regarded as true heart in earthworm.
  • Dorsal blood vessel is distributive in segments 1-13 and main collecting vessel in the segments 14 onwards.
  • From the posterior segment up to the 14th segment, It collects blood from 2 pairs of dorso-intestinal vessels from the intestine in each segment and a pair of commissural vessels from the sub-neural vessel.
  • Behind each septum, the commissural vessels create a loop and receive blood from the body wall, nephridia and prostate glands.
  • Through a septo-intestinal branch, the commissural vessels distribute the blood in each segment into the intestine.
• (ii) Ventral blood Vessel:
  • Ventral blood vessel is found below the alimentary canal and above the ventral nerve cord.
  • It is also a long blood vessel that runs from second segment to the last segment of the body.
  • It is the main distributive blood vessel of earthworm.
  • It has thin wall and lacks muscles and valves.
  • The direction of the flow of blood is anterior to posterior side.
  • In the anterior segments (i.e. first 13 segments), ventral blood vessel supplies blood to the body wall, septa, nephridia, and reproductive organs of the same segment through the ventro-tegumentaries (a pair in each segment).
  • Ventral blood vessel supplies blood to the intestine in the intestinal region through the single median ventro-intestinal in each segment.
  • Similarly, it gives off a pair of ventro-tegumentaries, one on each side, in front of septum in each segment.
  • This ventro-tegumentaries supply blood to the body wall, septa and integumentary nephridia, septal nephridia, gonads, spermathecae and seminal vesicles.
  • Behind the 13th segment, the ventral vessel also branch out a ventro-intestinal vessel in each segment.
  • These carry blood to the lower part of the intestine.
  • The blood plexuses are formed by the branches in intestine consisting of two networks in the intestinal wall.
• (iii) Sub-neural Vessel:
  • It is long and thin vessel and is situated mid-ventrally below the ventral nerve cord.
  • Sub-neural vessel runs from 14^th segment up to last segment, below the ventral nerve cord.
  • It lacks muscular walls and internal valves.
  • It is mainly a collecting blood vessel.
  • The direction of blood flow is from anterior to posterior end.
  • It receives a pair of slender branches in each segment which bring blood from the ventral body wall and the nerve cord.
  • It branch off a pair of commissural vessels in each segment which join the dorsal vessel.
  • Hence, it receives blood from the ventral body wall and supplies some blood to the intestine.

2. Intestinal Blood Plexus:

• The blood capillaries supply the intestine of earthworm that forms a close network.
• The intestinal blood plexus is comprised of a close network of capillaries in the wall of intestine.
• Two capillary networks are present in the intestine i.e. (i) External and (ii)Internal
• The capillary network found at the outer surface of intestine is termed as external plexus.
• External plexus receives blood from the ventral vessel through ventro-intestinal and passes it on to the internal plexus.
• The capillary network found between the circular muscle layer of intestine and its internal epithelial lining is referred as internal plexus.
• Internal plexus is linked with dorsal blood vessel through the dorso-intestinals.
• Internal plexus functions to absorb the nutrients from the gut.

3. Commissural Vessels:

• Commissural vessels link the dorsal and sub-neural vessels.
• These vessels receive blood from nephridia, body wall and reproductive organs through capillaries.
• Then, they supply it to dorsal blood vessel.

4. Integumentary Vessels:

• These vessels originate from ventral vessels.
•  It supplies the blood to integument for aeration and the aerated blood is collected by various capillaries of commissural vessel in each segment.
• Hence, there exists an intimate parallelism among venous and arterial capillaries throughout the body wall.

5. Nephridial Vessels:

• Nephridial vessels arises from the ventro-tegumentary vessels of ventral vessel.
• It supplies the blood to the nephridia.

B] Blood Vessels anterior to 13 Segments:

• The blood vascular system in the first thirteen segments is modified considerably and differs markedly from that of the intestinal region.
• It comprises of :
  1. Median longitudinal vessels;
  2. Hearts and anterior loops;
  3. Blood vessels of the gut.
• The function of collecting blood from the anterior region of the gut is taken over by a new vessel supra-oesophageal, while the blood from the peripheral structures is collected by the right and left lateral oesophageal.

1. Median Longitudinal Blood Vessels:

• (i) Dorsal blood vessel:
  • In anterior 13 segments, dorsal blood vessel becomes distributive in function.
  • It is similar in structure as in posterior segments, however it lacks dorso-intestinalis and commissural vessels opening into it.
  • It exports all the collected blood from the posterior region of the body into hearts and the anterior region of the gut.
  • Here, it gives off three branches that is distributed over the pharyngeal bulb and the roof of the buccal chamber.
  • However, it supplies blood to stomach, gizzard, oesophagus, pharynx and other related parts.
• (ii) Ventral blood vessel:
  • Ventral blood vessel is again distributve in these segments as well.
  •  However, it extends only up to the second segment.
  • Due to the absence of ventrointestinals, it does not supply to the alimentary canal in this region.
  • In the anterior segments (i.e. 1^st 13 segments), ventral blood vessel supplies blood to the body wall, septa, nephridia, and reproductive organs of the same segment through the ventro-tegumentaries (a pair in each segment).
• (iii) Supra-oesophageal vessel:
  • It is smallest vessel of body lying from 9^th to 13^th segment.
  • It is located above the stomach.
  • It is the main transverse vessels of first 13 segments.
  • It gets blood from the lateral oesophageals through two pairs of anterior loops that surround the stomach in the 10^th  and 11^th  segments.
  • It carries collected blood by the latero-oesophageal hearts in 12^th and 13^th segments to the ventral vessel.
• (iv) Lateral oesophageals:
  • In the 14^th segment, the sub-neural vessel actually bifurcates to form two lateral oesophageals.
  • In the anterior thirteen segments, these vessels are comparatively thick and located along the ventro-lateral margins of the alimentary canal.
  • These vessels are tightly connected to the wall of the stomach from the 10^th  to 13^th  segments and interact with the ring vessels.
  • However, in the gizzard region and further forwards, they exist free from the wall of the alimentary canal.
  • They continue receiving branches from it in each segment.
  • In each segment, these vessels receive a pair of branches that hold blood from the body wall and the septum.
  • They also collect blood from the reproductive organs and nephridia, thus acting like the posterior region’s sub-neural and commissural vessels, i.e. these are vessels that collect.
  • It collects blood from seminal vesicles present in 11^th and 12^th segments.

 2. Hearts and Anterior Loops:

• The dorsal and ventral blood vessels have no direct connections in the posterior segments behind 13^th segment.
• However, in the anterior region both these vessels are connected together by 4 pairs of pulsatile hearts.
• The hearts are neurogenic i.e. the heart beat arises in the nerve cells of the heart.
• The hearts are contractile and surround the alimentary canal, they are present in the segments 7^th , 9^th , 12^th  and 13^th.They can further be simplified as:
• Lateral hearts:
  • One pair of lateral hearts in 7^th and one pair in 9^th segment.
  • Each lateral heart possesses 4 pairs of valves that permits blood to flow downwards only.
  • It sends blood from dorsal vessel to ventral vessel.
• Lateral oesophageal hearts:
  • One pair lateral oesophageal hearts in 12^th and one pair in 13^th segment.
  • Each lateral oesophageal heart has thick muscular walls.
  • It possesses 3 pairs of valves and sends blood from supraoesophageal and dorsal vessel to ventral vessel.
  • A pair of valves is present at each junction with the dorsal vessels and supra-oesophageal vessel, and remaining pair of valves at the ventral end.
  • These valves allow flow of blood to downwards only.
• Anterior loops:
  • Along with four pairs of hearts, there are two pairs of loop-like vessels that connects the supra-oesophageal with the lateral oesophageals.
  • These vessels are neither muscular nor pulsatile and are termed as anterior loops.
  • These lack valves.
  • One pair is present in 10^th and one pair in 11^th segment.
  • It sends blood from lateral oesophageal vessel to supraoesophageal vessel into ventral vessel through the hearts of 12th and 13th segments.

3.     Blood Vessels of the Gut:

• These are ring-like vessels that connect the supra-oesophageal and lateral-oesophageal vessels.
• These are located on either side of stomach.
•  Through these vessels blood flows upwards from the lateral- oesophageals into the supra-oesophageal.
• Dorsal blood vessel supplies blood to buccal cavity, pharynx and gizzard directly.
  

How is blood circulated in earthworm?

• The blood collected by the dorsal vessel through the dorsointestinals, intestinal blood plexuses, and commissurals is distributed partly to the anterior alimentary canal, but primarily to the ventral vessel through the heart.
• The blood flows forward to the anterior region in front of the hearts in the ventral vessel.
• However, the largest proportion of blood flows backwards, which is distributed to the body wall and the organs in the coelom via ventrotegumentaries and to the alimentary canal via the ventrointestinal vessels.
• In other terms, ventral vessel supplies blood to all parts.
• The sub-neural collects blood from the ventral body wall, which also receives some blood from the anterior region via the lateral-oesophageal region.
• This blood passes from the sub-neural to the dorsal vessel via the commissurals.
• The lateral oesophageals also send blood to the supra-oesophageal vessel through the anterior loops, which then passes it to the ventral vessel through the latero-oesophageal heart.

What are the functions of blood in earthworm?

• The blood distributes digested food to different regions of the body and absorbs waste materials such as nitrogen waste and CO2 that are administered to nephridia, skin and coelomic fluid.
• Respiration takes place in almost all aquatic and terrestrial oligochaetes by the diffusion of gases through the integument, which comprises a capillary network in the outer epidermal layer in larger forms.
• The film of moisture needed for the diffusion of gases is provided by mucous glands, coelomic fluid, and nephridial excretions.
• Plasma haemoglobin absorbs O2 from the capillaries of the skin, but moist skin must be available where O2 can be mixed with haemoglobin in order to be carried by blood.
• Haemoglobin is an appropriate pigment and can absorb O2 either from the ambient air or from a relatively oxygen deficient environment.
• Earthworms can also survive in well-aerated water and do not drown.
• They can also survive without O2 for several hours, potentially carrying on anaerobic respiration in this state.

Blood Glands in Earthworm:

• Blood glands are various groups of small rounded follicles of red colour.
• The follicles have a syncytial wall surrounding a capsule containing a mass of loose cells.
• The blood glands are present in the 4^th, 5^th, and 6^th segments of Pheretima above the pharyngeal mass.
• Blood glands synthesizes blood corpuscles and hemoglobin are synthesized.
• Blood glands are related with pharyngeal nephridia and with salivary glands.
• Hence, these glands may be excretory as well.

Lymph glands in Earthworm:

• On both sides of dorsal blood vessel from 26^th segment and those behind it (one pair per segment, small and whitish), lymph glands are found.
• Lymph glands are responsible for producing certain phagocytic cells.

Summary points on Earthworm circulatory system:

• In earthworm, the blood vascular system is of closed type.
• Blood vascular system is different in first 13 segments as regards to number, arrangements and nature of blood vessels.
• Dorsal blood vessel is distributive in segments 1-13 and main collecting vessel in the segments 14 onwards.
• Ventral blood vessel is the main distributive blood vessel of earthworm.
• Supra oesophageal is smallest vessel of body lying from 9^th to 13^th segment.
• Lateral oesophageals collects blood from seminal vesicles present in 11^th and 12^th segments.
• One pair of anterior loops is present in 10^th and one pair in 11^th segment.
• Dorsal blood vessel supplies blood to buccal cavity, pharynx and gizzard directly.
• The blood glands are present in the 4^th, 5^th, and 6^th segments of Pheretima above the pharyngeal mass.

Circulatory system of Earthworm

The post Circulatory system of Earthworm appeared first on Online Biology Notes.

• Details on digestive and excretory system of earthworm

Structure of Alimentary canal of Earthworm

• Alimentary canal is a long straight tube extending from the first segment to the last segment of an earthworm’s body.
• It represents a tube within a tube body plan.
• It begins with an anterior mouth or prostomium and ends in the posterior anus.
• Along with it, it includes buccal cavity, pharynx, oesophagus, gizzard, stomach and intestine.
• Mouth:
  • It is a semicircular structure situated in the first segment called the peristomium just below the hood like prostomium.
  • It is highly elastic in nature and can protrude out and in.
  • Buccal/oral cavity:
  • It lies within the second and third segments.
• Pharynx:
  • Following the buccal cavity, the pharynx lies in the 4^th, 5^th, and 6^th segments.
  • It is thick and highly muscular.
  • The pharyngeal cavity is divided into two types:
  • Dorsal chamber (ciliated): They possess salivary glands in the outer region, that are formed by the chromophil cells (secrete saliva, i.e., mucus) and proteolytic enzymes for the digestion of the proteins.
  • Ventral chamber (non-ciliated): It is non-glandular and is termed as conducting chamber.
  • The effective organ for food digestion in earthworm is the pharynx.
• Oesophagus:
  • It extends from 5^th to 7^th segments.
  • They are quite thin, non-glandular and do not possess any muscular folds.
• Gizzard:
  • It lies in the 8^th and 9^th segments.
  • It acts as grinder hence termed as grinding machine, i.e. it aids in grinding of soil particles mixed with organic matter and other decayed materials.
  • The wall of the gizzard is made up of an outer layer of the circular muscles, a single layer of epithelial cells and an innermost thick layer of cuticle that is secreted by epithelial layer.
  • The contraction and relaxation of circular muscles cause the easy mastication of food and soil.
• Stomach:
  • The gizzard leads to the stomach, that is present from second half of 9^th segments upto 14^th segments.
  • It is a longer tube with short narrow cavity termed as glandular chamber.
  • The wall of stomach consists of calciferous glands whose secretion causes the neutralization of acidity of soil.
• Intestine:
  • It extends from 15^th to the last segment.
  • An internal long fold of dorsal wall is present after the 26^th segment which is termed as typhlosole.
  • Typhlosole is responsible for increasing the surface area of absorption.
  • Intestine can be divided into three types on the basis of typhlosole:
  • i) Pre-typhlosolar region:
  • It extends from 15^th to the 25^th segment.
  • It is highly glandular.
  • A pair of short and conical intestinal caecae is present on the 26^th segment.
  • ii) Typhlosolar region:
  • It lies from 26^th to 75-95last segment.
  • It is glandular and is highly absorptive.
  • iii) Post-typhlosolar region:
  •  It is the region after typhlosole.
  • It lies in the last 23-25 segments (76-96 to 100-120).
  • It is also termed as rectum and is absorptive in nature.
• Anus:
  • It is present in last/anal/pygidial segment.
  • It is a vertical, slit-like, small terminal aperture.
  • The defecation of worm-castings occurs through this aperture.

Physiology of digestion in Earthworm :

• In pharynx the food is mixed with saliva secreted by glandular cells of pharyngeal bulbs.
• Saliva contains mucin and proteolytic enzymes.
• Within gizzard, food is grinded into fine powder. The main reason for ingesting soil is to produce friction during breakdown of food.
• From gizzard the food reaches into stomach.
• The gland cells of stomach secrete proteolytic enzymes, which convert proteins and small peptides into amino acids.
• In stomach, the neutralization of food by calcites (CaCO₃) and digestion of rest of the proteins takes place.
• In intestine complete digestion of proteins, polysaccharides, fat, chitin, and cellulose takes place.
• Proteins when acted by proteases break down to peptones and proteoses.
• Further, peptones and proteoses are acted upon by proteases to form amino acids.
• The amylase enzyme is responsible for breakdown of polysaccharides into disaccharides.
• The lipase enzyme converts fats or lipids into fatty acids.
• Cellulose is also converted to disaccharides by lipase.
• Chitin is converted to disaccharides by lipase.
• After completion of digestion, both the digested and undigested foods pass to typhlosolar region.
• Here, the digested substances are absorbed by typhlosole and are circulated throughout the blood vascular system.
• The portion of food and soil that remained, passes to post typhlosolar region for storage.
• Finally, such substances are removed from the anus in the form of castings.

Excretory system of Earthworm:

• The nephridia are the excretory organs of earthworm.
• They are ectodermal in origin.
• Nephridia are analogous to kidneys of vertebrates.
• Nephridia are present in all segments of the body except in first 3 segments and last segments.
• In earthworm, the nephridia functions for the removal of the excretory wastes both from blood and the coelomic fluid.
• There are 3 types of nephridia based on the structure and location:
  • Septal nephridia or typical nephridia: Enteronephric nephridia
  • Integumentary nephridia: Exo-nephric nephridia
  • Pharyngeal nephridia: Enteronephric nephridia

1. Septal nephridia (Enteronephric nephridia):

• Septal nephridia are the largest nephridia.
• They are located in both sides of septum in each segment, behind the 15^th to 2^nd last segment.
• Each inter-segmental septum possess two rows of septal nephridia numbering 80-100 on each septum.
• Each septal nephridium has 4 parts: Nephrostome (nephridiostome or ciliated funnel), neck, body of nephridium and terminal duct.
• Septal nephridium is distinguished from pharyngeal nephridia in having nephrostome.
• Septal nephridia are the only nephridia with nephrostome or funnel.
• The terminal duct opens into septal excretory canal.
• These canals in turn open into two supra-intestinal excretory canals, so called enteronephric nephridia.

2. Integumentary nephridia:

• These are smallest nephridia.
• These are V-shaped in structure and are the most numerous types of nephridia.
• Integumentary nephridia are scattered in the body wall in all segments except in the first 7 segments and last segment.
• In each segment, there are about 200-250 integumentary nephridia.
• However, in the clitellar segments, the number is 10 times more than in ordinary segments.
• Hence, clitellar region is also termed as the forest of nephridia.
• As the terminal duct of integumentary nephridium is internally closed, each nephridium opens upon the body surface through nephridiophores.
• Hence, these nephridia are referred as exonephric nephridia.
• No nephridiopores are found in integumentary nephridia.

3. Pharyngeal nephridia:

• Pharyngeal nephridia (3 pairs) occurs as paired tufts on either side of pharynx and oesophagus.
• One pair each is present in 4^th, 5^th, and 6^th segments.
• Each of these tufts comprises of hundreds of pharyngeal nephridia as coiled tubes only.
• The terminal duct of nephridia of each tuft open into a common pharyngeal nephridial duct or the common excretory duct.
• Thus, there are three pairs of common pharyngeal nephridial duct.
• They run upward parallel with ventral nerve cord and open into alimentary canal.
• Ducts from 4^th and 5^th segments open into the pharynx in 4^th while those from 6^th segment open into buccal cavity in 2^nd.
• As these nephridia directly open into gut (buccal cavity and pharynx), they are of enteronephric type.
• Pharyngeal nephridia are also termed as pepto-nephridia.
• Earthworms are mainly ureotelic as their chief excretory product is urea (urea-50% and ammonia-45% and other 5%).
• The chloragogen cells excrete silicates consumed along with the food by Pheretima.

Summary points on Earthworm digestive and excretory system:

• The alimentary canal comprises of mouth, buccal cavity, pharynx, oesophagus, gizzard, stomach, intestine and anus.
• Pharynx lies in the 4^th, 5^th, and 6^th segments.
• The role of typhlosole is to increase the surface area of absorption.
• Gizzard acts as a grinder for soil particles along with food.
• Various enzymes like amylase, proteases and lipase are engaged during digestion.
• Pharynx is the effective organ for digestion.
• Nephridia are the excretory organs for earthworm and are of three types.
• Septal nephridia are the largest nephridia and integumentary nephridia are the smallest ones.
• One pair each of pharyngeal nephridia is present in 4^th, 5^th, and 6^th segments.
• Earthworms are mainly ureotelic.

Digestive and excretory system of Earthworm

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