Class 12 Biology Evolution Biological evolution

Biological evolution

Darwin’s theory of evolution

  • The essence of Darwinian Theory about evolution is natural selection.
  • Theory of natural selection states that individuals that are less adapted to the environment are eliminated and selecting those better adapted by the nature.
  • The rate of appearance of new forms is linked to the life cycle or the life span.
  • There must be a genetic basis for getting selected and to evolve.
  • Some organisms are better adapted to survive in an otherwise hostile environment.
  • Adaptive ability is inherited and it has genetic basis.
  • Nature selects for fitness.
  • Fitness is the end result of the ability to adapt and get selected by nature.
  • Fitness is based on characteristics which are inherited.
  • Branching descent and natural selection are the two key concept of Darwinian Theory of evolution.

Lamarck’s theory of evolution

  • Lamarck had said that evolution of life forms had occurred but driven by use and disuse of organs.
  • Lamarck gave the example of Giraffes who in an attempt to forage leaves on tall trees had to adapt by elongation of their necks and they passed on this acquired character of elongated neck to succeeding generations.
  • Giraffe, slowly over the years came to acquire long necks.

Fig. elongated necks of giraffe due to an attempt to forage leaves on tall trees  

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