Secondary contact between Lycaeides idas and L. melissa in the Rocky Mountains: extensive admixture and a patchy hybrid zone

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Abstract
Studies of hybridization have increased our understanding of the nature of species boundaries, the process of speciation, and the effects of hybridization on the evolution of populations and species. In the present study we use genetic and morphological data to determine the outcome and consequences of secondary contact and hybridization between the butterfly species Lycaeides idas and L. melissa in the Rocky Mountains. Admixture proportions estimated from structure and geographical cline analysis indicate L. idas and L. melissa have hybridized extensively in the Rocky Mountains and that reproductive isolation was insufficient to prevent introgression for much of the genome. Geographical patterns of admixture suggest that hybridization between L. idas and L. melissa has led to the formation of a hybrid zone. The hybrid zone is relatively wide, given estimates of dispersal for Lycaeides butterflies, and does not show strong evidence of cline concordance among characters. We believe the structure of the Lycaeides hybrid zone might be best explained by the patchy distribution of Lycaeides, local extinction and colonization of habitat patches, environmental variation and weak overall selection against hybrids. We found no evidence that hybridization in the Rocky Mountains has resulted in the formation of independent hybrid species, in contrast to the outcome of hybridization between L. idas and L. melissa in the Sierra Nevada. Finally, our results suggest that differences in male morphology between L. idas and L. melissa might contribute to isolation, or perhaps even that selection has favoured the spread of L. melissa male genitalia alleles.