Sexual selection is the evolutionary process whereby traits evolve because they increase mating success, either through competition with same-sex individuals (intrasexual selection) or by being attractive to potential mates (intersexual selection). Unlike natural selection for survival, sexual selection can favor traits that reduce survival but increase reproductive opportunities. Sexual selection explains sexually dimorphic traits (e.g., body size differences, secondary sexual characteristics) and reproductive behaviors.
Sexual selection operates through two main pathways: (1) Intrasexual selection (same-sex competition): traits that confer advantage in competition for mates (e.g., male-male combat, sperm competition). This creates selection for larger size, weapons, aggression, and mate-guarding behaviors. (2) Intersexual selection (mate choice): traits that increase attractiveness to opposite sex (e.g., peacock tail, human intelligence). Mate choice often favors 'honest signals' of genetic quality (e.g., symmetry, health indicators, cognitive abilities). Sexual selection can be stronger than natural selection, leading to traits that reduce survival but increase reproduction (e.g., bright coloration attracting both mates and predators). The intensity of sexual selection relates to parental investment: the sex investing less in offspring (typically males) experiences stronger intrasexual competition.
Sexual selection shaped human traits relevant to health: high intelligence (selected as attractive trait), body hair loss (parasite defense and mate attraction), concealed ovulation (promoting pair bonding), and sexual dimorphism in body composition. Understanding sexual selection helps explain behaviors like risk-taking in young males (competition for status/mates), female mate preferences affecting male health behaviors, and the evolution of cooperation vs. competition. It also explains why some traits persist despite health costs (e.g., high testosterone increasing disease risk but attractiveness).
- Can favor traits reducing survival but increasing mating success
- Intrasexual selection creates same-sex competition; intersexual selection involves mate choice
- Stronger in sex with lower parental investment (typically males)
- Human intelligence likely under sexual selection as attractive trait
- Loss of body hair in humans may relate to sexual selection (parasite defense and attraction)
- Explains sexual dimorphism in body size, composition, and behaviors
- Red-back spider males exhibit suicidal mating behavior due to extreme sexual selection
- natural selection — sexual selection is subset of natural selection favoring reproductive success
- evolutionary fitness — sexual selection increases fitness through enhanced mating success
- reproduction — sexual selection directly targets reproductive success over survival
- intelligence — human intelligence may be sexually selected trait (attractive to mates)
- parental investment — sexual selection intensity relates to asymmetry in parental investment
- testosterone — testosterone-dependent traits are often sexually selected despite survival costs
- body size — sexual dimorphism in body size results from intrasexual selection
- Pagel and Bodmer hypothesis — loss of body hair may involve sexual selection for parasite resistance and attraction
- parasites — parasite resistance is honest signal of health under sexual selection
- aggression — male aggression is sexually selected for intrasexual competition
- brain size — large human brain may be costly but sexually selected for intelligence
- cognitive function — cognitive abilities attractive to mates under sexual selection
- concealed ovulation — human concealed ovulation may promote pair bonding (reducing male-male competition)
- monogamy — sexual selection pressure differs between monogamous and polygynous species
- risk-taking — male risk-taking behavior reflects sexual selection for status and access to mates
- immune function — testosterone suppresses immunity but is sexually selected signal of quality
- cooperation — human cooperation may be sexually selected (attractive in mate choice)
- phenotype — sexual selection acts on phenotypes that signal reproductive fitness
- mate selection — mate choice drives intersexual selection
- sperm competition — intrasexual selection at level of sperm competition in multi-mate species