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Session 4

EVOLUTION

Darwin’s theory of natural selection

Darwin’s theory of natural selection states that all the organisms have great
capacity to reproduce
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All the organisms compete with each other for food and space
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The nature selects only those organisms which are fit
to live and the remaining disappears
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They give rise to the new species with their own specific characters
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Moreover,
he noted that each finch species was wellsuited for its environment and role
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Finally, he
observed that the finches (and other animals)
found on the Galápagos Islands were similar
to species on the nearby mainland of Ecuador,
but different from those found elsewhere in
the world
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However, some objections were raised against the theory
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Natural selection is not the only factor responsible for evolution
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Darwin did not mention any explanation about useful and useless
modifications
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There is no explanation about slow changes and abrupt changes
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Lamarckism Jean
Baptiste Lamarck proposed that morphological changes occurring in living
organisms are responsible for evolution and the reason behind those
morphological changes is activities or laziness of that organism
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The living organism can transfer the characters which it has acquired, to the next
generation
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Further, he said that the neck of giraffe has become too long due to browsing on
leaves of tall plants by extending their neck for several generations;
Similarly, shoulders of the ironsmith have become very strong due to frequent
hammering movements
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Legs of the birds like swan and duck have become useful for swimming due to
living in water
Snakes have lost their legs by modifications in their body for burrowing habit
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This is called as theory of inheritance of acquired
characters or Lamarckism
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The process by which new species develop from existing species is known as
speciation
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 Genetic drift due to drastic changes in the frequencies of particular genes
by chance alone
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Home work
I
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Lamarckism
2
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Read the following statements and justify same in your own words with
the help of suitable examples
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Geographical and reproductive isolation of organisms gradually leads to
speciation

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