Implications for evolutionary trends from the pairing frequencies among golden-winged and blue-winged warblers and their hybrids.

Extensive range loss for the Golden-winged Warbler () has occurred in areas of intrusion by the Blue-winged Warbler () potentially related to their close genetic relationship. We compiled data on social pairing from nine studies for 2,679 resident to assess evolutionary divergence. Hybridization between pure phenotypes occurred with 1.2% of resident males for sympatric populations. Pairing success rates for Golden-winged Warblers was 83% and for Blue-winged Warblers was 77%. Pairing success for the hybrid Brewster's Warbler was significantly lower from both species at 54%, showing sexual selection against hybrids. Backcross frequencies for Golden-winged Warblers at 4.9% were significantly higher than for Blue-winged Warblers at 1.7%. More frequent backcrossing by Golden-winged Warblers, which produces hybrid phenotypes, may contribute to the replacement of Golden-winged by Blue-winged Warblers. Reproductive isolation due to behavioral isolation plus sexual selection against hybrids was 0.960. Our analyses suggest that plumage differences are the main driving force for this strong isolation with reduced hybrid fitness contributing to a lesser degree. The major impact of plumage differences to reproductive isolation is compatible with genomic analyses (, 2016, 26, 2313), which showed the largest genetic difference between these phenotypes occurred with plumage genes. These phenotypes have maintained morphological, behavioral, and ecological differences during two centuries of hybridization. Our estimate of reproductive isolation supports recognition of these phenotypes as two species. The decline and extirpation of the Golden-winged Warbler in almost all areas of recent sympatry suggest that continued coexistence of both species will require eco-geographic isolation.