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| Summary |
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 Organic evolution refers to the slow and gradual process by which living organisms have changed from the simplest unicellular form to the most complex multi-cellular forms that are existing today. |
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 Organic evolution primarily involves modifications in the existing organisms and the inheritance of these modifications. |
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 There are several theories that try to explain the mechanism of organic evolution. |
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 Lamarckism is one of the earliest theories on evolution proposed by J.B.Lamarck. According to Lamarckism, organic evolution occurs due to the inheritance of acquired characters. |
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 Darwinism is the theory proposed by Charles Darwin. Darwinism explains evolution in terms of over-production, struggle for existence, variations, survival of the fittest and natural selection. |
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 Mutation theory proposed by Hugo DeVries attempts to find the sources of variations in mutation occurring in individual organisms. |
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 Neo-Darwinism is the modern theory on organic evolution that combines the ideas of Darwinism and mutation theory. |
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 Neo-Darwinism attempts to explain evolution in terms of genotypic variations, natural selection and isolation. |
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 The occurrence of organic evolution is established by the innumerable evidences that are available from the different branches of biology. |
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 The study of extinct organisms (palaeontology) provides direct evidences in favour of evolution. |
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 Fossils indicate the relationships between different groups, of organisms. They also help in constructing the story of life's journey on this planet. |
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 Indirect evidences are available from the branches like morphology, taxonomy, embryology, physiology and so on, by a comparative study of extinct organisms. |
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 Variations: the differences that occur in the characteristics between members of the same species, form the raw materials for evolution. |
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 Variations can be somatic or blastogenic. Somatic variations are non- heritable, while blastogenic variations are heritable. |
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 Mutations: recombinations, genetic drift, natural selection and migration are the main sources of variation. |
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 Origin of a new species from the existing one is called as speciation. |
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 Speciation can be multiplicative involving splitting of one species into two or more or phyletic involving replacement of one species by another. |
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 Instant speciation may occur due to hybridisation and polyploidy. |
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 Isolation is the separation of populations by certain barriers, which prevent interbreeding. |
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 Isolation may occur due to geographical barriers such as land masses, mountains and rivers. |
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 Isolation may also be reproductive isolation. It may involve preventing of mating (pre-reproductive) or mechanisms that operate after mating (post-reproductive). |
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