Phenotypic and Genetic Introgression Across a Moving Woodpecker Hybrid Zone

We studied Red-breasted (Sphyrapicus ruber) and Red-naped (S. nuchalis) sapsuckers, which meet and hybridize along a narrow contact zone that stretches from northern California to southern British Columbia. We found strong evidence for changes in the structure of this hybrid zone across time, with significant temporal shifts in allele frequencies and in the proportions of parental phenotypes across the landscape. While we found these significant changes over time, we also found a significant bimodal distribution of hybrids across the contact zone, suggesting that premating barriers may be driving reproductive isolation, perhaps as a result of assortative mating based on plumage differences. Using museum specimens and genomic data, this study of sapsuckers provides insight into the ways in which phenotypic and genetic structure have changed over a 40-year period, as well as insight into the mechanisms that may contribute to the maintenance of the hybrid zone over time.