Epistatic interactionsare interactions amongst genes occurring at different loci on the genome. The effect of a certain gene or some alleles of a certain gene is frequently quantitatively and qualitatively dependent on the presence of quite specific alleles in a completely different locus. Genes and thus also polymorphism in these genes can be functionally interconnected and the fitness of the bearers of certain alleles can be dependent, not on their frequency in the population, but rather on the frequency of certain alleles in completely different loci in the genome.
For example, if allele a1 of gene A is selectionally more advantageous (than allele a2) in combination with allele b1 of gene B and selectionally less advantageous (than allele a2) in combination with allele b2 and if polymorphism is maintained in gene B by any of the above-mentioned mechanisms (for example, selection for heterozygotes), then polymorphism will exist permanently in gene A (Fig. VIII.10).If, for example, a certain form of the enzyme were to function better in black two-spotted lady beetles and a different form in red lady beetles (for example, because, as a consequence of differences in the degree of reflection and absorption of solar radiation, the body temperature of dark-colored lady beetles in the sun were higher than that of red lady beetles), then both alleles of the relevant gene would remain in the gene pool of the species A. bipunctata for prolonged periods of time.
As most genes are apparently functionally interconnected in various ways, it can be assumed that epitstatic interactions will be very important in maintaining the polymorphism of a great many traits and, because of the existence of gene linkage, also the maintenance of polymorphism in selectively neutral traits (mutations) (Kelly & Wade 2000). See also Frozen plasticity theory.