Natural selection is a process by which biological populations are altered over time, as a result of the propagation of heritable traits that affect the capacity of individual organisms to survive and reproduce. It is one of several mechanisms that give rise to the evolution of biological species (other mechanisms include genetic drift and gene flow.) However, natural selection has a special significance because it is believed to be the one responsible for organisms being adapted to their environment. The theory of natural selection was proposed by Charles Darwin and Alfred Russel Wallace in 1858, though vaguer and more obscure formulations had been arrived at by earlier workers.
Natural selection can be subdivided into two types: (i) ecological selection, which arises from the portion of an organism's environment not related to direct sexual competition (such as the availability of food, the presence of predators, and so forth); and (ii) sexual selection, which arises from the competition for mates between individuals of the same sex. The reason for this division is that the effects of sexual selection can produce results that seem counterintuitive from the point of...
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