In sexual reproduction, random mating is the idea that mates are chosen at random from the pool of all possible mates, with no preference given to any genetic characteristic, ancestry or trait. When pine trees mate, for example, and the males release pollen into the wind, the chance that any given male is the one to produce the pollen that fertilizes a certain female is essentially random. There is no choosing on the part of the female, but rather, the pollen lands wherever it lands.

In contrast, nonrandom mating means that certain mates from a given pool of potential mates have a greater chance than others of mating with a specific individual of the opposite sex. The ways in which nonrandom mating actually occurs are varied. There are three primary situations which result in nonrandom mating: stratification, assortive mating, and consanguinity.

Stratification means that the population...