Sexual Selection: Mate Choice, Competition, and the Evolution of Ornaments and Armaments

Sexual selection is a special form of natural selection that arises from differences in mating success. While natural selection favors traits that enhance survival, sexual selection favors traits that increase an individual’s chances of obtaining mates and reproducing. Sexual selection explains some of the most striking and seemingly paradoxical traits in nature: the elaborate tail of the peacock, the massive antlers of elk, the complex songs of birds, and the brilliant colors of many fish and insects. These traits often appear costly and even harmful to survival, yet they evolve because they confer advantages in the competition for mates. This guide explores the mechanisms of sexual selection, the diversity of traits it produces, the theoretical frameworks that explain its operation, and the ongoing debates about its role in evolution.

The Two Forms of Sexual Selection

Darwin distinguished two forms of sexual selection. Intrasexual selection involves competition between members of the same sex for access to mates. This is most commonly competition between males for females, and it favors traits that are useful in direct combat or competition. Larger body size, weapons such as antlers and horns, and aggressive behavior are products of intrasexual selection. The winners of these contests gain access to mates and pass their traits to offspring.

Intersexual selection, or mate choice, involves the preferences of one sex for certain traits in the other. This is most commonly female choice of males, and it favors traits that attract females. Elaborate plumage, complex courtship displays, and nutritious offerings are products of intersexual selection. Females choose males based on these traits, and males with the preferred traits have higher mating success.

Why Females Usually Choose

The asymmetry in sexual selection, with females typically being the choosy sex and males the competitive sex, is explained by parental investment theory developed by Robert Trivers. Females typically invest more in offspring than males, through the production of large eggs, gestation, and often parental care. Because females have more to lose from a poor mating choice, they are more selective about mates. Males, who invest less in each offspring, maximize their fitness by mating with as many females as possible.

This asymmetry is reversed in species where males invest more in offspring. In seahorses, where males carry the developing young in a brood pouch, females are the more competitive and showy sex. In birds like phalaropes, where males provide all parental care, females are larger and more colorful and compete for access to males.

The Peacock’s Tail and the Problem of Costly Ornaments

The peacock’s tail is the iconic example of a sexually selected trait. The tail is large, colorful, and energetically expensive to produce and carry. It makes the peacock more visible to predators and impedes his ability to escape. How could such a costly, apparently harmful trait evolve? Darwin’s answer was sexual selection: females prefer males with more elaborate tails, and this preference outweighs the survival costs.

The evolution of costly ornaments poses a theoretical puzzle because it requires both a preference for the ornament and the ornament itself to evolve. Several hypotheses have been proposed to explain this. The good genes hypothesis suggests that costly ornaments are honest signals of male quality because only high-quality males can afford to produce them. The handicap principle, proposed by Amotz Zahavi, argues that the cost of the ornament is precisely what makes it an honest signal: a male that can survive despite carrying a costly ornament must be of high quality.

Runaway Selection

Ronald Fisher proposed that female preferences and male ornaments could coevolve in a runaway process. If females have a preference for a particular male trait, males with that trait have higher mating success, and their offspring inherit both the trait and the preference. This creates a genetic correlation between the trait and the preference, and both can evolve rapidly in a positive feedback loop. The process can continue until the costs of the trait balance the benefits of mating success.

Fisher’s runaway selection model has been supported by theoretical models and empirical studies. The process can explain why sexually selected traits are often exaggerated beyond what would be expected if they signaled male quality alone. The genetic correlation between male traits and female preferences has been demonstrated experimentally in several species.

Weapons and Armaments

Intrasexual selection has produced some of the most impressive weapons in the animal kingdom. Deer antlers are among the most familiar, used in contests between males for access to females. Antler size correlates with male fighting success and mating success. Elephant seal males fight fiercely for beach territories where females give birth, and the largest males achieve dominance and mate with many females.

The evolution of weapons involves trade-offs. Larger weapons are more effective in competition but more costly to produce and carry. The size of weapons is often correlated with overall body size, and males in good condition can afford larger weapons. The allometry of weapons, where weapon size increases faster than body size, is common and indicates the intensity of sexual selection.

Mating Systems

Sexual selection shapes the mating systems of species. Monogamy, where one male and one female form a pair bond, is relatively rare in mammals but common in birds, where male parental care is important. Polygyny, where one male mates with multiple females, is the most common mammalian mating system and is associated with strong sexual selection on males. Polyandry, where one female mates with multiple males, is rare but occurs in species where males provide parental care.

The operational sex ratio, which is the ratio of sexually active males to receptive females at any time, is a key determinant of the intensity of sexual selection. When the operational sex ratio is male-biased, as is common, competition among males is intense and sexual selection is strong. When the ratio is female-biased, sexual selection on females may be stronger.

Frequently Asked Questions

Does sexual selection always favor males? No. Sexual selection acts on whichever sex has greater variance in mating success. In most species, males have higher variance, but in species where females compete for mates, sexual selection acts on females.

Are sexually selected traits always harmful to survival? Not necessarily. Some sexually selected traits, such as the bright colors of male birds, can also serve as signals of quality that reduce conflict. Even costly traits can be maintained if the mating advantages outweigh the survival costs.

How is sexual selection different from natural selection? Sexual selection is a subset of natural selection that specifically involves competition for mates. Natural selection also includes survival selection, which favors traits that enhance survival. The two can conflict, with sexually selected traits sometimes reducing survival.

Can sexual selection lead to speciation? Yes. Divergent sexual selection can contribute to reproductive isolation and speciation. Differences in mate preferences between populations can reduce gene flow and lead to the evolution of new species.

Conclusion

Sexual selection is a powerful evolutionary force that has produced some of the most remarkable and beautiful traits in nature. Understanding sexual selection requires recognizing that the goal of evolution is not survival but reproduction, and that traits that enhance mating success can evolve even at a cost to survival. Sexual selection theory continues to be refined through empirical studies and theoretical advances, providing insights into the evolution of mating systems, communication signals, and the extraordinary diversity of courtship behaviors and ornaments observed across the animal kingdom.