Sexual selection is a form of natural selection arising from differences in mating success or reproductive success associated specifically with obtaining mates or fertilisations. It was identified and developed as a major evolutionary concept by Charles Darwin, who used it to explain traits that seemed difficult to account for solely through survival advantages. Sexual selection can favour characteristics that increase an individual's probability of attracting mates, competing with members of the same sex or successfully securing fertilisations. Such traits can sometimes reduce survival while increasing reproductive success, creating an important distinction between natural selection through ecological survival and selection associated with reproduction. Examples include elaborate plumage, courtship displays, weapon-like structures, mating calls and intense competition among males in many species.
Sexual selection is commonly divided into intrasexual selection and intersexual selection, although the distinction is not absolute and modern evolutionary biology recognises considerable complexity. Intrasexual selection occurs when members of one sex compete with one another for access to mates, territories or reproductive opportunities. This can favour traits such as large body size, horns, antlers, enlarged jaws or aggressive behaviour when those traits increase success in contests.
Intersexual selection involves differences in mate choice, where individuals of one sex preferentially mate with particular members of the other sex. This can favour ornaments, colours, songs, courtship behaviours or chemical signals that influence mate attraction. Both mechanisms can operate within the same species and can interact with ecological selection.
Sexual selection can produce striking sexually dimorphic traits, meaning characteristics that differ systematically between males and females. In many species, one sex invests more in competition or display while the other exercises stronger mate choice, often reflecting differences in reproductive investment. However, sex roles are not universally fixed. Females can compete intensely with one another, males can be selective about mates, and both sexes can display ornaments or provide substantial parental care. The direction and intensity of sexual selection depend on factors such as operational sex ratio, parental investment, mating system, reproductive variance and ecological conditions. Consequently, sexual selection should not be reduced to a simple rule that males compete and females choose.
One important feature of sexual selection is that traits can increase reproductive success despite imposing survival costs. Large antlers may require substantial energy and may hinder movement, while elaborate plumage can make an individual more conspicuous to predators. If the reproductive advantage is sufficiently large, however, the trait can persist or increase in frequency. This produces an evolutionary trade off between survival and mating success. Sexual selection can therefore help explain otherwise apparently costly traits. The overall fitness consequence is determined by the balance between all components of reproductive success, including survival, mate acquisition, mating success, fertilisation and offspring production. A sexually selected trait does not need to improve survival to be favoured by evolution.
Mate choice can involve multiple sensory and behavioural characteristics. Potential mates may assess colour, size, symmetry, song, movement, scent, courtship behaviour, territory quality or parental ability. Preferences can be influenced by direct benefits, such as access to resources or protection, and indirect benefits, such as indicators of genetic compatibility or quality. Some theories propose that ornaments can become exaggerated through feedback between preference and trait evolution.
The Fisherian runaway process describes one theoretical mechanism in which a preference and a preferred trait can reinforce one another across generations. Other models emphasise costly signalling, in which maintaining a signal is difficult for low quality individuals, making the signal potentially informative about condition.
Sexual selection also includes sperm competition and cryptic female choice in many organisms. Sperm competition occurs when sperm from different males compete to fertilise eggs, creating selection on sperm number, morphology, physiology and reproductive anatomy. In some species, females can influence which sperm successfully fertilises eggs through post mating processes, a phenomenon discussed under cryptic female choice. Sexual selection can therefore continue after mating has occurred and is not limited to visible courtship or physical contests. Reproductive anatomy, gamete characteristics and mating behaviour can all evolve under selection generated by competition among gametes or differential fertilisation success. These mechanisms demonstrate that sexual selection can operate at several stages of reproduction.
Sexual selection is closely related to but distinct from natural selection in the narrower ecological sense. Both involve differential reproductive success and are evolutionary processes, but sexual selection specifically concerns variation in mating or fertilisation success. A trait can simultaneously be affected by both forms of selection. For example, body size may increase mating success but also increase energetic requirements and predation risk. The resulting trait reflects the balance of these pressures. Sexual selection can also influence speciation because divergent mating preferences, signals and reproductive structures can create or strengthen reproductive isolation between populations. Differences in courtship signals or mate preferences can therefore contribute to divergence even when populations remain geographically close.
Modern research treats sexual selection as a broad and diverse set of evolutionary processes rather than a simple explanation for extravagant male traits. Sexual selection occurs in mammals, birds, fish, insects, plants and many other organisms, although its mechanisms differ substantially among taxa. In plants, for example, pollen competition and differences in pollen success can generate selection associated with reproduction. In hermaphroditic organisms, sexual selection can operate through competition for reproductive roles or sperm competition.
Human beings also have traits influenced by complex evolutionary, cultural and developmental processes, so conclusions from sexual selection theory in other species should not automatically be applied to humans. Overall, sexual selection explains how competition and choice associated with reproduction can shape morphology, behaviour, physiology and reproductive systems across evolutionary time.
