Sexual selection is the theory proposed by Charles Darwin that states that certain evolutionary traits can be explained by intraspecific competition. Darwin defined sexual selection as the effects of the "struggle between the individuals of one sex, generally the males, for the possession of the other sex". Biologists today distinguish between "male to male combat" (it is usually males who fight each other), "mate choice" (usually female choice of male mates) and sexual conflict. Traits selected by male combat are called secondary sexual characteristics (including horns, antlers, etc.) and sometimes referred to as "weapons"; and traits selected by mate choice are called "ornaments". Much attention has been given to cryptic female choice, a phenomenon in internally fertilising animals such as mammals and birds, where a female will get rid of a male's sperm without his knowledge. The equivalent in male-to-male combat is sperm competition.
Direct competition between members of one sex (usually males) for mates is also classified as intrasexual selection, while mate choice is known as intersexual selection.
Females often prefer to mate with males with external ornaments—exaggerated features of morphology. These can plausibly arise because an arbitrary female preference for some aspect of male morphology initially increased by genetic drift, creating, in due course, selection for males with the appropriate ornament. This is known as the sexy son hypothesis. Alternatively, genes that enable males to develop impressive ornaments or fighting ability may simply show off greater disease resistance or a more efficient metabolism—features that also benefit females. This idea is known as the good genes hypothesis.
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