Sexual Selection

Sexual selection is the evolutionary theory that traits aiding an organism's success in mating, rather than its general survival, can spread through a population even when those traits carry a survival cost. Charles Darwin introduced the concept in On the Origin of Species in 1859 to explain conspicuous features such as the peacock's tail that natural selection alone struggled to account for, and developed it fully in his 1871 book The Descent of Man, and Selection in Relation to Sex. Darwin distinguished two mechanisms: intrasexual selection, in which members of one sex compete directly with each other, typically males fighting for access to mates, and intersexual selection, in which one sex, typically females, chooses mates based on particular traits. Ronald Fisher's work in the 1930s formalized how a female preference and an exaggerated male trait could reinforce one another in a runaway process, and sexual selection remains a major branch of evolutionary biology distinct from, though complementary to, natural selection.