Phenotypic integration and modularity evolve along with plumage coloration across populations in a widespread songbird

Patterns of interdependence among traits such as phenotypic integration and modularity can have profound evolutionary consequences. Despite their importance in evolutionary theory, large-scale empirical evidence for processes that lead to population divergence in phenotypic integration and modularity across a species range remains rare. Here, we analyze patterns of correlations between plumage color traits sampled from many populations across the global range of the barn swallow (Hirundo rustica). Population divergence in phenotypic integration is associated with both genomic divergence and the evolution of ventral coloration. We also demonstrate that throat coloration acts as a separate module from other ventral plumage patches, and populations with darker throat color exhibit increased modularity. These results provide rare empirical evidence to support the theory that selection on one set of traits can lead to the evolution of modularity. Our study takes a unique, comparative approach that links phenotypic integration and modularity to both phenotypic and genomic divergence among populations within a species known to experience divergent sexual selection pressures. Finally, our study offers insight into the evolution of phenotypic integration and modularity in the context of animal coloration, as well as the structure and function of complex signal traits in general.