NONRANDOM MATING
The result of nonrandom mating is that some individuals have more opportunity to mate than others and thus produce more offspring (and more copies of their genes) than others. Random mating works against evolutionary change by guaranteeing that the genes of a population are thoroughly mixed among the members of the next generation.

Random mating is unlikely to occur for a variety of reasons. One is that it is simply easier to mate with a nearby individual, as opposed to one that is farther away. Also, especially in animals, individuals compete for mates and active selection of mating partners occurs. This goes directly against the concept of randomness.