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| Theories of Organic Evolution |
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| Some of the formal theories on evolution are discussed in brief here. |
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| The theory of evolution as put forth by French biologist Lamarck has come to be known as Lamarckism. This theory has two salient features: |
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| Use and Disuse of Parts |
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| According to Lamarck, continuous use of a part results in it being well-developed and disuse of a part over a long period of time will result in its degeneration. For example, giraffes were forced to extend their necks and stretch their legs to reach higher vegetation over a period of time. This resulted in every generation having a little longer neck and legs than the previous one. Webbed feet in aquatic birds are thought to have developed due to constant spreading of toes and the stretching of the skin between. Flatfish also are thought to have developed their shape due to lying on their sides in shallow water. |
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| Inheritance of Acquired Characters |
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| According to Lamarck, the characters that an organism acquired due to a change in their environment such as long neck, webbed feet, flat bodies, etc. were passed on to the next generation. In this way, evolution from simpler to complex forms took place. |
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| However, this theory was not widely accepted as it is known that acquired characters are only phenotypic changes and not genotypic. Thus, while the cases of giraffe, aquatic birds and flatfish do show that evolution has occurred, Lamarckism does not provide a satisfactory answer to the mystery of evolution. |
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| Theory of Continuity of Germplasm |
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| This was proposed by Weismann who did not agree with Lamarck's theory of inheritance of acquired characters. To prove his point, he cut off the tails of many successive generations of mice. This resulted in forced disuse of the tail. According to the theory of use and disuse, the tails should have become progressively shorter. However, this did not happen. |
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| According to Weismann, the changes affected only the somatic (vegetative) cells and did not affect the germ cells or the gametes. Only the changes that affect the germ cells and the germplasm (the collection of genes) will be inherited by successive generations. |
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| The theory of natural selection was put forth by Charles Darwin in his book 'On the Origin of Species by Means of Natural Selection', co-authored by Alfred Russel Wallace. According to Darwin, nature has its own ways of selecting the best from the available species for continuation of life. The mechanism of natural selection works as follows: |
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 Individuals of a species produce more offspring than necessary to replace themselves. |
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This results in competition and struggle for existence among the individuals. Within the species itself there is variation that results in minor differences between the individuals. |
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Thus in the struggle for existence only the ones with the variations best adapted to their environment survive. |
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In this manner nature ensures survival of the fittest. |
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| The theory put forth by Darwin and Wallace gained wide acceptance. However, in the light of modern evidences, it was slightly modified and called the neo-Darwinism. In neo-Darwinism, organic evolution is by natural selection of inherited characters. It utilises evidences from various fields such as genetics, palaeontology, molecular biology, ecology and ethology (study of behaviour). |
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