Nonrandom Mating

The fourth requirement of genetic equilibrium is random matings, without regard to genetic makeup. However, many species do not mate randomly. Mate selection is often influenced by geographic proximity, which can result in mates with some degree of kinship. Matings of related individuals can amplify certain traits and can result in offspring with disorders caused by recessive genes, which, although rare, may be present in the genomes of related individuals.

In another example of nonrandom mating, individuals may select a mate that has traits similar to their own traits. This mate would probably have some similar genes. The selection of a mate based on similarity of traits is called assortative mating. Nonrandom mating affects which alleles will be combined within individuals, but it does not affect overall allele frequencies within a population.

Sexual Selection

In many species of birds, the males are brightly colored and often heavily plumed, such as the peacock shown in Figure 16-7. These elaborately decorated males are easy for predators to see. Why would natural selection work in favor of an organism being conspicuous to a predator? Females tend to choose the males they mate with based on certain traits. This tendency is referred to as sexual selection. In order to leave offspring, a male must be selected by the female. The peacock's gaudy plumage increases his chances of being selected. Extreme traits, such as heavy, brightly colored plumage, may give the female an indication of the quality of the male's genes or his fitness in his environment. Remember that natural selection acts upon differences in survival and reproduction. Natural selection favors an increase in the genes of successful reproducers, rather than merely those of successful survivors.

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