Non-random mating can be an important cause of evolutionary change within populations (see, e.g. Partridge, 1983). While several factors can lead to non-randomness in mating – dispersal patterns, intrasexual competition, mating preferences, etc. – it is the possible role of mate choice in differential mating success that has excited the greatest interest among evolutionary biologists. In part this is because, while mate choice is at the heart of part of Darwin's (1859, 1871) original theory of sexual selection, attempts to model the evolution of mating preferences have proved contentious and tests of models often equivocal (Read, 1990; Bateson, 1983; Bradbury & Andersson, 1987). Nevertheless, there is no shortage of suggestions as to the criteria on which mating preferences might be based (e.g. Halliday, 1978; Hamilton & Zuk, 1982; Bateson, 1983) and, in some cases, convincing empirical support has been forthcoming (e.g. Semler, 1971; Andersson, 1982; Majerus, 1986).

A number of lines of argument point to the degree of relatedness between potential mates as a criterion in mate choice (Bateson, 1983, 1988; Smith, 1979; Shields, 1982, 1983; Partridge, 1983). The degree to which individuals of sexually reproducing species outbreed and therefore mate with others of differing genotype is likely to have important consequences for their reproductive success, mainly through the effects of dispersal costs, changes in the level of homozygosity and indirect fitness (e.g. Smith, 1979; Bateson, 1983; Partridge, 1983). It seems likely that extremes of inbreeding or outbreeding will incur both advantages and disadvantages for individual reproductive success and that an optimal balance between them might be expected under selection.