Describe theory of natural selection ? Who gave this theory ?
 

Charles Darwin made a sea voyage around the world between $1831-1836$ in a sailship $HMS$ Beagle.

During that period Darwin explored the fauna and flora of a number of continents and islands.

Based on his observation, he concluded the following : $(i)$ There has been gradual evolution of life form with new forms arising at different periods of history of earth. $(ii)$ Varying degrees of similarities can be observed between existing life forms and those which existed million of years ago.$ (iii)$ Population of all organisms have variations in characteristics, which make them adapt to better environment. The characteristics which enable some population or individuals to survive better in natural conditions (foods, climate, physical factors would outbreed other survival of fittest).

Thus fitness of the individual, according to Darwin, refers ultimately to the reproductive fitness.

Such fit individuals leave more progeny than others.

Thus those organisms, which are better fit or adapt well, will survive more in nature and get selected by nature.

This is called natural selection.

Branching descent (eg. Darwin finches) and natural selection are the two key concepts of Darwinian theory of evolution.

Alfred Wallace, a naturalist worked in Malay Archipelago, came to a similar conclusions like Darwin.

Thus they both jointly propounded the theory of 'Natural selection' in $1858$ .

It is based on following observation which are factual : $(a)$ There are limited natural resources. $(b)$ Population are stable in size except for seasonal fluctuations. $(c)$Varying characteristics in members of a population i.e. no two members of a population are identical even though they look superficially similar. $(d)$ Variations are generally inherited. $(e)$ Population size has two facts i.e. theoretically it grows exponentially if everybody reproduced maximally and in reality, population size is limited due to the competition among the individual for resources.

Those which are better adapted could survive and will reproduce at the cost of others i.e. who are less adapted in the environment