Repeated Transitions in Migratory Behavior Across a Songbird Radiation Are Not Driven by Selection on Shared Variation in Candidate Genes

Understanding the mechanisms facilitating the repeated evolution of traits is a core question in evolutionary biology. While repeated evolution has been observed across many systems, behavioral phenotypes have been less well studied than morphological traits. Migration is a behavioral adaptation to seasonality that has evolved independently in many animals, and repeated gains and losses of migratory behavior have been especially prevalent throughout the diversification of birds. While migratory traits are heritable in many taxa and candidate genes underlying intraspecific variation in migratory behavior have been identified, whether similar genetic changes are responsible for the repeated evolution of migration remains poorly understood. We used a phylogenomic approach to investigate the relationship between variation in candidate genes and migratory propensity across nearly all species in the parulid warbler radiation (n = 101), a group of songbirds in which migratory behavior has been lost and gained multiple times. Comparisons of candidate gene tree topologies to the parulid warbler species tree revealed cases of discordance, though these were explained by differences in the number of phylogenetically informative sites among genes rather than by elevated genetic similarity among species sharing migratory strategies. Testing models of coding sequence evolution identified sites with elevated dN/dS in five candidate genes, suggestive of positive selection; however, variation at these sites showed no clear association with migratory strategy, nor did we detect recurrent selection across lineages where transitions in migratory behavior most likely occurred. Together, these results contradict the hypothesis that selection on shared variation (ancestral polymorphisms or introgressed alleles) facilitated the repeated evolution of migration in this clade.