natural selection

Natural selection is an important factor of evolution, but not the only one. 

Charles Darwin was the propounder of this theory. Earlier natural selection was considered to be the only basic cause of evolution. But now it is understood that other factors also play important roles in evolution.

 

How natural selection acts : As a result of natural selection, the proportion of organisms in a species with characteristics that are adaptive to a given environment increases with each generation. Therefore, natural selection modifies the originally random variation of genetic traits in a species so that alleles that are beneficial for survival predominate, while alleles that are not beneficial decrease.