New study systems and tools are needed to understand how divergence and speciation occur between lineages with gene flow. Migratory birds often exhibit divergence despite seasonal migration, which brings populations into contact with one another. We studied divergence between 2 subspecies of Northern Saw-whet Owl (Aegolius acadicus), in which a sedentary population on the islands of Haida Gwaii, British Columbia (A. a. brooksi), exists in the presence of the other form (A. a. acadicus) during migration but not during the breeding season. Prior research showed fixed mtDNA divergence but left open the question of nuclear gene flow. We used 2,517 ultraconserved element loci to examine the demographic history of this young taxon pair. Although we did not observe fixed single nucleotide polymorphism differences between populations among our genotyped individuals, 100% of the birds were diagnosable and δaδi analyses suggested the demographic model best fitting the data was one of split-bidirectional-migration (i.e. speciation with gene flow). We dated the split between brooksi and acadicus to ∼278 Kya, and our analyses suggested gene flow between groups was skewed, with ∼0.7 individuals per generation coming from acadicus into brooksi and ∼4.4 going the opposite direction. Coupled with an absence of evidence of phenotypic hybrids and the birds' natural history, these data suggest brooksi may be a young biological species arising despite historic gene flow.
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Vol. 136 • No. 2