Stable Underdominance and the Evolutionary Invasion of Empty Niches

Abstract

Complex adaptations are frequently polygenic; they cannot evolve in a single step, but rather must pass through several relatively poorly adapted intermediate stages. Evolutionists frequently invoke density- and frequency-dependent selection to explain this process. The initial poorly adapted morphs are assumed to have high fitness because they use an unexploited resource or otherwise occupy an "empty niche." We explore this process with a single-locus selection model based on the differential utilization of two distinct resources. Not surprisingly, we show that unexploited niches can be invaded by poorly adapted morphs. Less obviously, we show that heterozygotes often have the lowest fitness at stable polymorphic equilibria. We also show by numerical example that stable two-locus polymorphisms exist, in which the most heterozygous individuals are the least fit. In general, maladaptive phenotypes are frequently maintained in these models. Thus, they may present a biologically plausible framework in which to investigate evolutionary modifications of dominance, linkage, and mating that reduce the frequency of maladaptive phenotypes.