2. Evolution

Evolution

 The sequence of gradual changes that take place in an organism over a million years and leads to the formation of new species is known as evolution.
J.B. Lamarck was the first scientist who gave the theory of evolution. He gave the theory of inheritance of acquired characters. Later on, Charles Darwin came and gave the theory of natural selection or Darwinism. According to his theory, evolution occurs through natural selection.

Theory of Lamarckism

This theory is also known as the “Theory of inheritance of acquired characters”. Lamarckism is based on the following postulates-

• Living organisms tend to increase in size.
• New needs lead to the formation of new organs.
• Continued use of a particular organ makes it more developed and disuse of an organ leads to its degeneration.
• New characters are acquired by individuals during their lifetime.

Theory of Darwin
Darwin's theory was also known as the “Theory of Natural selection”.
Postulates of Darwin Theory

• Speciation (formation of species) - Useful variations from generation to generation gives rise to the formation of new species.
• The struggle of existence due to the multiplication of organisms and limited food and space, there exists competition among the organisms.
• Survival of the fittest or Natural selection Nature selects those characteristics or organisms that are useful and are best adapted to the prevailing conditions. “For example, industrial melanism observed in peppered moths in Britain”

Speciation
The origin of new species from already existing species is known as speciation. Speciation can take place through-

• Gene flow can lead to speciation. It is a transfer of genetic variation from one population to another
• Random change in allele frequency known as genetic drift can also lead to speciation.
• Natural selection is another reason through which speciation can take place
• Geographical barriers such as mountains, rivers can also lead to speciation. This is known as geographical isolation.