Darwin's problem and two mechanisms
Darwin puzzled over why traits such as the peacock's enormous tail, which draws predators' attention and hampers movement, had evolved; he wrote that the sight of a feather in a peacock's tail made him sick. His answer, set out in The Descent of Man (1871), was sexual selection: a trait can spread if it helps in obtaining mates, even at some cost to survival. Darwin distinguished two mechanisms. Intrasexual selection is competition within one sex, usually males, for access to mates, producing weapons such as antlers and large body size. Intersexual selection, or mate choice, is one sex, usually females, choosing partners with particular traits, producing ornaments such as colorful plumage and song. Because mate choice seemed to credit animals with aesthetic taste, it long met with skepticism.
Why one sex competes and the other chooses
In experiments with fruit flies, Bateman (1948) found that male reproductive success rises steeply with the number of mates while female success does not, the Bateman principle. Robert Trivers (1972) generalized this into parental investment theory: the sex that invests more in offspring, through eggs, gestation and care, chooses mates carefully, while the sex that invests less competes for mates. In species such as seahorses and jacanas, where males do more of the care, sex roles reverse and females are more ornamented and competitive, a good test of the theory. Competition continues after mating: Parker (1970) described sperm competition, in which sperm from several males compete to fertilize eggs.
Models of mate preference and modern research
Why do females prefer particular ornaments? Ronald Fisher (1930) proposed a runaway process in which preference and ornament become genetically linked and reinforce each other, later formalized by Lande (1981). In good-genes models the ornament indicates male genetic quality. Amotz Zahavi's (1975) handicap principle held that costly ornaments are honest signals because low-quality individuals cannot fake them; initially met with skepticism, it gained support from Alan Grafen's (1990) mathematical model, and it parallels signaling theory in economics. Hamilton and Zuk (1982) argued that bright plumage reveals resistance to parasites. Michael Ryan (2018) and others developed the sensory bias hypothesis, in which ornaments exploit preexisting biases in sensory systems. Experimentally, Malte Andersson (1982) showed directly that artificially lengthening male widowbirds' tails won them more mates. Researchers now see these models not as mutually exclusive but as points on a continuum within an integrated framework (Andersson, 1994; Kokko et al., 2006).
FAQ
- What is sexual selection in simple terms?
- It is the evolutionary principle that a trait spreads across generations if it helps win mates, even when it does nothing for survival or is a handicap. The peacock's showy tail is the classic example.
- How does sexual selection differ from natural selection?
- Natural selection spreads traits that aid survival and reproduction in an environment; sexual selection spreads traits that give an edge specifically in competing for and being chosen by mates.
Sources reviewed: 2026-10-26
References
- Darwin, C. (1871). The Descent of Man, and Selection in Relation to Sex. London: John Murray.
- Fisher, R. A. (1930). The Genetical Theory of Natural Selection. Oxford: Clarendon Press.
- Bateman, A. J. (1948). Intra-sexual selection in Drosophila. Heredity, 2(3), 349–368.
- Trivers, R. L. (1972). Parental investment and sexual selection. In B. Campbell (Ed.), Sexual Selection and the Descent of Man, 1871–1971 (pp. 136–179). Chicago: Aldine.
- Zahavi, A. (1975). Mate selection: A selection for a handicap. Journal of Theoretical Biology, 53(1), 205–214.
- Grafen, A. (1990). Biological signals as handicaps. Journal of Theoretical Biology, 144(4), 517–546.
- Hamilton, W. D., & Zuk, M. (1982). Heritable true fitness and bright birds: A role for parasites? Science, 218(4570), 384–387.
- Andersson, M. (1982). Female choice selects for extreme tail length in a widowbird. Nature, 299(5886), 818–820.
- Andersson, M. (1994). Sexual Selection. Princeton: Princeton University Press.
- Lande, R. (1981). Models of speciation by sexual selection on polygenic traits. Proceedings of the National Academy of Sciences, 78(6), 3721–3725.
- Parker, G. A. (1970). Sperm competition and its evolutionary consequences in the insects. Biological Reviews, 45(4), 525–567.
- Kokko, H., Jennions, M. D., & Brooks, R. (2006). Unifying and testing models of sexual selection. Annual Review of Ecology, Evolution, and Systematics, 37, 43–66.
- Ryan, M. J. (2018). A Taste for the Beautiful: The Evolution of Attraction. Princeton: Princeton University Press.