Balanced polymorphisms and the evolution of dominance

We explore the evolution of dominance at polymorphisms maintained either by overdominant selection or by migration-selection balance. At such balanced polymorphisms, heterozygotes remain at appreciable frequencies over long p eriods of time, allowing extensive modification of dominance to occur. The strength of selection favoring a modifier of dominance is roughly proportio nal to the probability that a modifier allele is found in a heterozygote at the locus subject to balancing selection times the heterozygote fitness in crease caused by the modifier. Using a two-locus model, we elucidate the in teresting ways in which recombination and migration cause departures from t his rough expectation. For example, with overdominance, a genetic associati on with the rarest allele favors a modifier that increases heterozygote fit ness because the modifier occurs more often in heterozygotes. With migratio n-selection balance, dominance evolves more readily in patches experiencing the strongest selection. We also find that, while there are more heterozyg otes in sink populations (which have higher rates of immigration than emigr ation), selection for dominance in sink and source populations is nearly eq ual because sink populations make a lower genetic contribution to future ge nerations. We conclude that the evolution of dominance is likely to occur w henever polymorphism is maintained by either overdominance or migration.