Random mating is a crucial condition which allows a population to remain in Hardy-Weinberg equilibrium. The term refers to a situation in which an individual choses its mate arbitrarily, that is, in which any given combination of two gametes of opposite sexes is as likely to occur as any other combination. Although some species are more selective in mate choice than others, the very act of mating requires that two motile organisms meet, or that the motile gametes of two immotile organisms meet. As a result, random mating can never entirely be achieved, because an organism which can traverse the greatest area in search of a mate, or produce the greatest quantity of gametes, will have a greater likelihood of mating than an organism that is not hardy enough to do anything but seek food or escape predators, or one that cannot muster the metabolic resources required to produce a large number of gametes.

Non-random mating is, however, a relative concept. In some species, males actively fight over harems of females, leading to the genetic propagation of the strongest individuals. In many avian species, females select mates based on the brightness of their plumage, a factors which reflects a male individual’s overall health. More random are simpler animals, for instance, hermaphroditic earthworms who mate with the first individual of their species that they encounter. And among the most random of all are plants whose male gametes are wind borne, and aquatic invertebrates which release their gametes en mass into the water.

While it is debatable whether the current means of selection employed are those which will lead to the greatest adaptive evolutionary gain, humans do not mate randomly either. We chose our mates based on a variety of factors- consciously recognized and not consciously recognized- including physical appearance, i.e. the health neccesary to produce viable offspring, income, i.e the economic security neccesary to provide for offspring, and intelligence, i.e. the adaptability neccesary to protect offspring.

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