In nature there are many different ways to select the two sets of genes that are combined to form new offspring.
The one we are most familiar with, because it occurs in all higher animal species, is known as dioecy. Individuals are either males or females, and one parent must be drawn from each class. Within this category there may be several variations, mostly behavioral in nature, that determine how animals choose mates. Monogamous species mate for life; in other species there is a ``harem'' structure with a dominant male and several females, and so on.
Many plants and some lower animals are monoecious. Here any one individual can be either a male or a female, but it takes two different individuals to create offspring. Related to this scheme is obligate selfing, in which the same individual is both the mother and the father -- it is not the same as asexual reproduction, in which the child is a clone of the parent, since in obligate selfing individuals carry two sets of genes and offspring are a combination of those genes.
Throughout the rest of this chapter, and in the computer models described in later sections, we will assume the species being modeled is monoecious.