Published on Thu Jul 01 2021

How dominant alleles influence progress towardspeciation by natural selection

Munzur, A. D., Thompson, K. A.

Extrinsic post-zygotic isolating barriers occur when hybrids have poor fitness in the parental habitat(s) and ultimately arise as a consequence of interactions between the alleles underlying adaptation. Most models of speciation via natural selection consider only alleles that have additive phenotypic effects, even though many quantitative trait loci exhibit some dominance. We used computer simulations to evaluate how hybrid fitness differs when parent populations adapt from either (i) mutations that are strictly additive or (ii) mutations with a range of dominance coefficients. Compared to the additive case, we find that introducing dominance invariably reduces hybrid fitness when parents adapt to identical optima (i.e., parallel selection) because hybrids express transgressive phenotypes that are maladaptive in the common parental niche. The effect of dominance becomes less negative as selection tends toward completely divergent. When selection is completely divergent, dominance can increase hybrid fitness over the additive case because chance events result in unequal mean dominance among populations and cause hybrids to resemble one of the parents rather than a maladapted intermediate phenotype. However, when the adapting population has many traits (i.e., high `dimensionality') dominance always reduces hybrid fitness over the additive case because hybrids express maladaptive transgressive phenotypes in traits under stabilizing selection. Our results indicate that dominant alleles facilitate progress toward speciation via parallel natural selection, and might either increase or decrease the efficacy of divergent selection for speciation.