English naturalist Charles Darwin revolutionized scientific thinking when he proposed that species evolve over time to become adapted to their environments by means of natural selection in his On the Origin of Species (1859). He was initially puzzled, though, by the seemingly useless exaggerated characters often found in animals, particularly males. The long and colorful tail of the peacock, for example, seemed to hinder rather than help its bearer survive. In his later work, The Descent of Man, and Selection in Relation to Sex (1871), Darwin proposed that some characters do not increase survival, but instead increase reproductive success. He called this sexual selection, which refers to the process that produces traits that affect an individual's reproductive success as a result of competition over mates.
While both sexual selection and natural selection are evolutionary processes that increase an organism's fitness, they differ in several important ways. Environmental, physical, or biological factors often drive natural selection, whereas sexual rivals and mates are the exclusive agents of sexual selection. Furthermore, the evolutionary effects of sexual selection differ markedly from those of natural selection. Sexual selection frequently produces sexual dimorphism and exaggerated male traits, often in opposition to the forces of natural selection.
For example, male widowbirds have extraordinarily long tails (more than twice their body length) that make flight more difficult. When researchers manipulated the tail length of several males, they found that female widowbirds preferred males with longer tails to males with short or normal length tails. Thus, while the long tails of widowbirds may be selected against by natural selection, they are favored by sexual selection.
There are two broad categories of sexual selection: intrasexual selection (members of one sex compete among themselves for reproductive opportunities with individuals of the other sex) and intersexual selection (members of one sex choose among members of the other sex).
Many examples of intrasexual selection are readily observable. Males of many species fight, display, vocalize, and otherwise compete for the opportunity to mate with available females. Male deer fight with their antlers and enormous male elephant seals fight with their bulk to establish dominance and
Post-mating competition also occurs. Male dragonflies often guard their mates after copulation to ensure that the female lays her clutch of eggs before remating. Male fruit flies sometimes transfer a substance to their mate that inhibits courtship by subsequent males. Male dunnocks (a small European bird) often peck the cloaca of their mate until she everts it, sometimes ejecting sperm.
Once the male has successfully rid the female of the sperm from a previous mate he will proceed to reinseminate her. Some male parasitic worms cement the genitalia of their mates after copulation to form a copulatory plug. These male worms take intraspecific competition one step further by occasionally "mating" with rival males and cementing the genitalia of their rivals to prevent subsequent sperm transfer.
For several decades after Darwin presented his theory of sexual selection, most naturalists discounted the importance of intersexual selection, or mate choice. However, in the 1950s a few scientists began to revisit this subject, and by the 1980s mate choice had gained wide popularity as a topic of study.
Many exaggerated male traits are now thought to have evolved as a result of female mate choice, although several competing hypotheses exist to explain the origin and maintenance of these female preferences. Ronald A. Fisher proposed an explanation called "runaway sexual selection" in The Genetical Theory of Natural Selection (1930). Fisher suggested that as females began to evolve a preference for a particular male trait, such as tail feather length, these females would be more likely to mate with males who displayed the preferred trait. The offspring of these matings would inherit the genes for both the male trait and the female preference, resulting in a genetic correlation between the preference and the trait. Consequently, as the male trait spreads because females prefer it, the female preference itself also spreads because it is linked with the male trait. This is called a self-reinforcing choice, and is one way that exaggerated male traits can evolve without conferring any direct benefits on the females who prefer them.
Another explanation for the evolution of female choice is called the handicap hypothesis. In his study "Mate Selection—A Selection for a Handicap" (1975), Amotz Zahavi suggested that exaggerated male traits indicate to females that the male is healthy enough to survive despite his substantial handicap. The exaggerated trait is a signal through which females can assess a male's genetic quality, and therefore is often called a good genes hypothesis. This is another way that exaggerated male traits can evolve without directly benefiting the females who prefer them.
Other explanations of the evolution of female mate choice include sensory bias (for example, female frogs prefer males who call loudly or in a low pitch because they can hear them better) and direct benefits (for example, females might prefer males who provide superior resources, defense, or parental care).
The refinement of several genetic analyses in the late 1980s and the 1990s have contributed greatly to the study of sexual selection. Using deoxyribonucleic acid (DNA) fingerprinting, microsatellite DNA typing, and related techniques, researchers can confidently assign paternity to offspring using genetic markers, whereas in the past they had to rely on behavioral cues. These techniques are particularly well used in avian studies, where scientists are learning that many birds thought to be monogamous actually have a high frequency (30 to 95 percent) of promiscuity. Using molecular techniques to definitively assign paternity has and will continue to further the study of sexual selection, particularly mate choice and sperm competition.
Michelle J. Solensky
Andersson, Malte. Sexual Selection. Princeton, NJ: Princeton University Press, 1994.
——. "Female Choice Selects for Extreme Tail Length in a Widowbird." Nature 299 (1982): 818–820.
Darwin, Charles. On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life. London: John Murray, 1859.
——. The Descent of Man, and Selection in Relation to Sex. London: John Murray, 1871.
Fisher, Ronald A. The Genetical Theory of Natural Selection. Oxford: Clarendon Press, 1930.
Zahavi, Amotz, and Avishag Zahavi. The Handicap Principle: A Missing Piece of Darwin's Puzzle. New York: Oxford University Press, 1997.