The four principles of natural selection are variability among members of a species, heritability of the variable features, differences in population members' ability to reproduce and survival of the fittest in terms of variability, heritability and reproduction. Natural selection is one of evolution's four mechanisms.
Natural selection acknowledges the fact that members of a population are capable of producing more offspring than are needed to maintain a healthy population size. Nature controls the population's size by empowering species' best specimens to survive and reproduce.
Variability among population members refers to differences in such traits as eye color or hair color, and it serves as natural selection's foundation. The variable traits must be heritable, which means they are passed down from one generation to the next. Differences among population members' ability to reproduce refers to the fact that some population members are able to reproduce and some are not. These three factors combine to help ensure the survival of the fittest, which means that the population members with the most desirable traits reproduce and create new members that share those traits, whereas members with undesirable traits fall into obscurity.
Other mechanisms that contribute to evolution include genetic mutations among members, migration of the fit populations and genetic drift, which is a random process that results when one species' genetic material finds its way into other populations.