Some differences between human populations may be attributed to the ways in which people choose their mates. Individuals usually prefer to marry someone who is like themselves, a process called assortative mating—in other words, tall men marry tall women, and light-skinned women marry light-skinned men. When two populations differ in obvious physical characteristics, the number of matings between them may be small if the traits of one population are considered unattractive to members of the other population. As-sortative mating tends to maintain and even emphasize physical differences between populations.
In addition to the tendency to mate assortatively, men and women within a population may have preferences for particular physical features in their mates. When a trait influences the likelihood of mating, that trait is under the influence of a form of natural selection called sexual selection. Sexual selection is responsible for many of the differences between males and females within a species. For instance, the enormous tail on a male peacock resulted from female peahens choosing mates with showier tails. In humans, there is some evidence that the difference in overall body size between men and women is a result of sexual selection—namely, a widespread female preference for larger males. In both cases, female choice for particular mates has led to the evolution of the population.
Some scientists have hypothesized that a trait common in the Khoikhoi people of South Africa evolved as a result of sexual selection. Women in this population store large amounts of fatty tissue in their buttocks and upper thighs, giving them a body shape that is considerably different from other African populations. Men in these populations prefer women with this body shape and appear to have caused selection for this trait in the population. The morbid fascination that many Europeans had for this body type and one consequence of that fascination are the subject of Essay 10.2. It is possible that many physical features that are unique to particular human populations evolved as a result of sexual selection.
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