Geographic variation in morphology that develops among closely related populations can help drive genetic divergence, and eventually speciation, when those morphological traits are the basis for social interactions that influence reproduction. The North American Dark-eyed Junco (Junco hyemalis) complex is an interesting case in speciation. The numerous subspecies have distinct breeding ranges and unique plumage coloration, but based on the presence of hybrid populations and recent genetic data, can be considered to belong to a single species. Research within various populations of juncos has shown first, that wing length and the amount of white on the tail feathers (“tail white”) influence an individual's dominance status and mating success, and second, that these traits can undergo rapid evolution when social and environmental conditions change. Here, I used museum specimens to examine tail white and body size, as measured by wing and tail length, of males and females within and among 13 geographically distinct Dark-eyed Junco subspecies. I documented geographic variation of mean values for each of these morphological traits, as well as patterns of trait co-variation and the degree of sexual dimorphism. I discuss these results in relation to what they may indicate about the generation and maintenance of divergence among the subspecies.
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