HYBRID ZONES WITH DOBZHANSKY-TYPE EPISTATIC SELECTION

Dobzhansky's model of epistatic selection assumes that viable genotype s form ''clusters'' in genotype space so that populations can evolve f rom one state to a reproductively isolated state following a ''ridge'' of well-fit genotypes without crossing any deep adaptive valleys. Rec ently, the importance of Dobzhansky-type models in evolutionary studie s has been reemphasized by Gavrilets (1997a) and Gavrilets and Gravner (1997) who argue that the existence of ''ridges'' of well-tit genotyp es connecting reproductively isolated genotypes is actually a general property of multidimensional adaptive landscapes. Using rigorous techn iques and numerical simulations, I analyze dines in the frequencies of selected anti neutral alleles maintained by a balance of migration an d Dobzhansky-type epistatic selection acting on two diallelic loci. I show that Dobzhansky-type epistatic selection can build up a very stro ng barrier to neutral gene flow. I describe properties of clines that are indicative of Dobzhansky-type selection.