Sexual selection

Sexual selection favors traits that increase an individual's success at getting mates and fathering or bearing offspring, rather than its chances of staying alive. Charles Darwin proposed it to explain features that seemed to make no sense for survival, such as the enormous tail of the peacock. It is usually treated as one form of natural selection, because in both, heritable traits that raise reproductive success become more common.

It works in two main ways. In intrasexual selection, members of one sex, usually males, compete directly with each other, which favors large size, weapons such as antlers and horns, and fighting skill, as in elephant seals battling for beaches. In intersexual selection, or mate choice, members of one sex, usually females, choose among partners, which favors bright colors, elaborate songs, dances and displays. Females are often the choosier sex because eggs and pregnancy cost more than sperm, so male mating success varies much more widely. In a few species, such as phalaropes and some pipefish, these roles are reversed.

Sexual selection explains sexual dimorphism, the differences in size, color and ornament between the males and females of one species. Because showy traits can also attract predators, the balance between winning mates and staying alive is a common exam topic.