Darwin considered traits selected according to their role in mating to be separate from those acted on by natural selection. He considered this a different type of selection that he called "sexual selection." Sexual selection occurs in two ways: through contests and through choice.
Contests are competitions between members of the same sex for access to the other sex. This competition may take many different forms. A common example is direct combat between males. These fights may determine hierarchies in which the dominant males get first access to sexually receptive females or to establish territories, whose resources help attract females. In cases like these, horns, antlers, or other combative devices are acted upon by sexual selection because they are directly involved in helping an individual obtain a mate.
While contests involve direct physical competition between members of the same sex, choice involves competition for attention from the opposite sex. For example, the tail feathers of the peacock are a sexually selected trait, but they are not used to attack or fend off other males. Instead, they are intended to attract peahens. The peahens can use such showy displays as an indicator of a male's quality or fitness, because the male has been able to survive even with the added cost of an otherwise useless and sometimes dangerously conspicuous display. Signaling displays such as the peacock's tail are discussed further in the Signaling and Communication SparkNote of animal behavior.