Sexual Selection

Definition

Evolution is driven not just by the survival of the fittest (natural selection) but also by the reproduction of the sexiest (sexual selection). If an animal finds food and avoids predators but can't find a mate, the animal is an evolutionary dead end. Its genes will die out when it dies. This is why sexual selection is so important: It is the evolutionary gateway to genetic immortality. Every one of your ancestors managed not just to survive to adulthood but also to attract a willing sexual partner. Every one of your 30,000 genes has passed through thousands of generations of successful courtship, mating, and parenting. Sexual selection is another term for reproductive competition, competition to attract more high-quality mates than one's sexual rivals, to have more high-quality offspring.

History and Background

Charles Darwin discovered sexual selection and published a massive book about it in 1871, but sexual selection was usually ignored in biology until the 1970s and in psychology until the 1990s. Since then, biologists have realized that many traits in animals have been shaped by sexual selection, either as sexual ornaments to attract mates (e.g., the peacock's tail, the nightingale's song, the female baboon's bright red bottom) or as weapons for sexual competition against rivals (e.g., deer antlers, gorilla muscles, big male baboon teeth). Since about 1990, evolutionary psychologists have also realized that many human traits have been shaped by sexual selection. These sexually selected traits include (a) socially salient physical traits such as female breasts and buttocks, and male beards, upper-body muscles, and penises; (b) personperception abilities to judge the attractiveness of potential mates, including their beauty, kindness, intelligence, and status; (c) self-presentation abilities (ways of showing off in courtship) such as language, art, music, and humor; and (d) social emotions such as lust, love, jealousy, anger, and ambition.

Importance

Sex differences in bodies and brains are usually the result of sexual selection. Male mammals can produce offspring just by having sex for a few minutes if they find a willing female, whereas female mammals can only produce offspring if they get pregnant for a long time and produce milk for their offspring. Thus, males can potentially have a lot more offspring than females can. This makes fertile females a much more precious, limited resource than fertile males are. For these reasons, male mammals typically compete much more intensely to attract mates than females do, and females are typically much more choosy about their mates than males are. This leads to many human sex differences that appear across all known cultures, including stronger male motivations to seek status, kill rivals, seduce multiple partners, and take conspicuously heroic risks for the public good.

Yet, sexual selection is not restricted to explaining sex differences. Sexual selection can also explain mating-related traits that are shared by both sexes, including many uniquely human physical traits (e.g., long head hair, everted lips, smooth hairless skin) and mental traits (e.g., creativity, language, social intelligence, moral virtues). Humans can feel lust for other people's bodies, but humans typically fall in love with other people for their impressive minds, great personalities, and social virtues. Or, humans fall out of love with other people because they realize the other people are stupid, boring, selfish, or violent. Thus, human mate choice (choice of sexual partners) depends a lot on the social psychology shared by both sexes, the way people perceive what others are thinking and feeling.

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