Population Divergence in the Eastern North American Boreal Forests: Extensive Gene Flow Characterises a Hybrid Zone Between Palm Warbler Subspecies (Parulidae: Setophaga palmarum).

Pleistocene glaciations have shaped much of the population divergence events in the coniferous forests of North America. However, unlike in western North America, the evidence for forest fragmentation and population divergence associated with glacial cycles is less clear in the boreal forests of eastern North America. Here, we present the first genomic analysis of an avian hybrid zone in the eastern North American boreal forest between two subspecies of Palm Warblers (Setophaga palmarum). Using both genomic and plumage datasets, we characterise the divergence history of the two subspecies and relate our findings to the larger biogeographic history of the region. We find the Palm Warbler subspecies to have diverged in the mid-Pleistocene (~831 KYA) and have experienced near-continuous gene flow since initial splitting, resulting in a wide hybrid zone and little evidence of reproductive isolation. Coalescent modelling further indicates that the divergence of these two subspecies is congruent with the effects of glacial cycles and suggests a shared biogeographic history with the sole other known example of avian divergence in the region. However, in addition to the general paucity of such events, both of the known divergence events in the eastern boreal forests are characterised by low differentiation at present. This suggests that the conditions in the proposed eastern glacial refugia may not have been sufficient in establishing strong reproductive isolation, which perhaps explains why speciation events have been relatively rare in the eastern boreal forests of North America.