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Organic Evolution Summary

  • 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.
  • Organic evolution primarily involves modifications in the existing organisms and the inheritance of these modifications.
  • There are several theories that try to explain the mechanism of organic evolution.
  • 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.
  • 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.
  • Mutation theory proposed by Hugo DeVries attempts to find the sources of variations in mutation occurring in individual organisms.
  • Neo-Darwinism is the modern theory on organic evolution that combines the ideas of Darwinism and mutation theory.
  • Neo-Darwinism attempts to explain evolution in terms of genotypic variations, natural selection and isolation.
  • The occurrence of organic evolution is established by the innumerable evidences that are available from the different branches of biology.
  • The study of extinct organisms (palaeontology) provides direct evidences in favour of evolution.
  • Fossils indicate the relationships between different groups, of organisms. They also help in constructing the story of life's journey on this planet.
  • Indirect evidences are available from the branches like morphology, taxonomy, embryology, physiology and so on, by a comparative study of extinct organisms.
  • Variations: the differences that occur in the characteristics between members of the same species, form the raw materials for evolution.
  • Variations can be somatic or blastogenic. Somatic variations are non- heritable, while blastogenic variations are heritable.
  • Mutations: recombinations, genetic drift, natural selection and migration are the main sources of variation.
  • Origin of a new species from the existing one is called as speciation.
  • Speciation can be multiplicative involving splitting of one species into two or more or phyletic involving replacement of one species by another.
  • Instant speciation may occur due to hybridisation and polyploidy.
  • Isolation is the separation of populations by certain barriers, which prevent interbreeding.
  • Isolation may occur due to geographical barriers such as land masses, mountains and rivers.
  • 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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