A gene's eye view of epistasis, selection and speciation

In this mini-review, I discuss the effects of gene interaction or epistasis from a 'gene's eye view.' By a 'gene's eye view' of epistasis, I mean that I will consider a single, bi-allelic locus, A, whose effects on fitness result only from its interactions with alleles of another, unknown locus, X. I will show how changes in the frequencies of alleles at the background locus affect the relationship of alleles at the A-locus to fitness. Changing the genetic background changes the fundamental characteristics of the A-locus, such as the magnitude and sign of allelic effects on fitness, and, consequently, it changes the strength and pattern of selection. I consider each of the four kinds of pair-wise interactions between two loci and show that some kinds of epistasis are more sensitive than others to population genetic subdivision. Lastly, I show that some kinds of epistasis are more likely than others to affect the process of speciation and contribute to or be responsible for general genetic features of interspecific hybrids, such as Haldane's rule.