What is Sexual selection?

Darwin’s second mechanism: selection on traits that win mates rather than survival. Its root is anisogamy — a female’s reproduction is limited by the eggs she can make and provision, a male’s by the females he can fertilise — so that in most species a male’s fitness rises steeply with his number of matings and a female’s hardly at all (the Bateman gradient); males then compete for females and females choose among males. Competition gives horns, antlers, tusks, large size and the contests of the previous section. Choice gives ornaments, and three accounts of why females prefer them. Fisher’s runaway: a preference for a slightly longer tail and the longer tail spread together, each making the other advantageous, until the tail’s cost in survival balances its gain in matings. Zahavi’s handicap: an ornament that only a healthy male can afford to grow is an honest signal of his condition, precisely because it is costly — a peacock with a full train has, demonstrably, fewer parasites. Good genes and direct benefits: the female gains, through her offspring’s inheritance or through the territory and food the male provides. The two sexes’ interests differ, and sexual conflict — over mating frequency, over who cares for the young, over the fate of a previous mate’s sperm — is an arms race run within a species: the seminal fluid of male fruit flies shortens the female’s life and raises her egg-laying, and females evolve resistance.