The unequal reproductive success of offspring is the difference between those that have success in reproduction and those that do not. Inherited traits determine reproductive success, according to Sidwell Friends School. This is the basis of natural selection.
According to the Stanford Encyclopedia of Philosophy, natural selection posits "differential" traits against a "selective environment." There is typically an abundance of a species in comparison to the resources available to sustain the species. Those of the species born with the most robust genetic fitness receive more possibilities in terms of availability of mates. Entities of a species that do not have the best genes do not live as long, which also gives those with better genetic fitness more chances to reproduce. Within a species, those of both sexes have specific traits that work as signals to the opposite sex regarding the level of genetic fitness. The process by which creatures in a species choose a partner to reproduce is mate selection. The sex with the greater opportunity to choose a mate depends upon the parental investment. In humans, this is female choice.
Charles Darwin first explained the theory of natural selection and reproductive success. He explained his findings in his 1859 book, "On the Origin of Species."