Morph-specific selection pressures drive phenotypic divergence in a color polymorphic bird

Abstract

There is a long tradition of using heritable color polymorphisms in natural populations to study selection, gene flow, and other evolutionary processes. However, we still have only limited knowledge on how continuous color variation within genetically discrete morphs affects selective dynamics, which narrows our understanding of how color polymorphisms persist. Our comprehensive analysis of 43 years of plumage color scores from a bi-morphic Finnish population of tawny owls (Strix aluco) reveals that intra-morph variation is substantial, but also unexpectedly dynamic. We show that both morphs recently diverged in their plumage coloration: while the brown morph is on a steady trajectory toward more intense plumage pigmentation, the gray morph has recently shifted toward lighter plumage, following a series of extreme winters. Evidence suggests that this divergence is driven by the brown morph's higher reproductive success and greater plasticity in response to seasonality, while the gray morph, with its more genetically determined plumage color and limited standing variation, has a reduced capacity to respond to selection and track new phenotypic optima. Our study confirms the presence of substantial and dynamic phenotypic variation within genetically discrete color morphs, and demonstrates its relevance for the evolutionary potential of populations to respond to environmental change.