Abstract

not-yet-known not-yet-known not-yet-known unknown Pleistocene glaciations have shaped much of the population divergence events in the coniferous forests of North America. However, while the evidence for forest fragmentation and population divergence associated with glacial cycles is well-established in western North America, whether glaciation has served a similar role in the boreal forests of eastern North America is unclear. Here, we present the first analyses for an avian hybrid zone in boreal eastern North America between two subspecies of Palm Warblers (Setophaga palmarum). Using both genomic and plumage datasets, we characterize the divergence history of the two subspecies and relate our findings to the larger biogeographic patterns in the region. The Palm Warbler subspecies diverged in the Pleistocene (around 775 KYA) and appears to have experienced near-continuous gene flow since initial splitting with little evidence for reproductive isolation. Demographic modelling indicates that following divergence, the western subspecies expanded eastwards in breeding range and displaced the genetic ancestry of its eastern counterpart, resulting in the current hybrid zone. The timing and patterns of divergence for the Palm Warbler subspecies is largely congruent with the sole other known case of avian divergence in the region, which likely reflects a shared biogeographic history involving multiple eastern boreal refugia. However, the apparent ongoing collapse of the Palm Warbler subspecies post-divergence suggests that the differentiation generated through these eastern refugia were likely not sufficient in establishing strong reproductive isolation, which perhaps explain why speciation events have been relatively rare in the eastern boreal forests of North America.