Sexual Selection

Darwin's answer to the mysteries of the peacock' s tail and the stag's antlers was that they evolved because they contributed to an individual' s mating success, providing an advantage in the competition for desirable mates. The evolution of characteristics because of their mating benefits, rather than because of their survival benefits, known as sexual selection.

Sexual selection, according to Darwin, takes two forms. In one form, members of the same sex compete with each other , and the outcome of their contest gives the winner greater sexual access to members of the opposite sex. Two stags locking horns in combat is the prototypical image of this intrasexual competition. The characteristics that lead to success in contests of this kind, such as greater strength, intelligence, or attractiveness to allies, evolve because the victors are able to mate more often and, hence, pass on more genes.

In the other type of sexual selection— intersexual selection —members of one sex choose a mate based on their preferences for particular qualities in a mate. These characteristics evolve because animals that possess them are chosen more often as mates, and their genes thrive. Animals that lack the desired characteristics are excluded from mating, and their genes perish. Since peahens prefer peacocks with plumage that flashes and glitters, dull-feathered males get left in the evolutionary dust The leading theory is that peacocks today possess brilliant plumage because, over evolutionary history, peahens have preferred to mate with dazzling and colorful males (Trivers, 1985). The most likely explanation for why peahens prefer luminous plumage is because it's a signal of healthiness; peacocks that have a high prevalence of parasites look dull by comparison.

Success at same-sex competition leads to success at mating; traits that help to win these battles are passed on in greater numbers, and hence evolve in the population
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