DOI: 10.5061/dryad.n2j6c20
title: Data from: Cross-decades stability of an avian hybrid zone

description: Hybrid zones are particularly valuable for understanding the evolution of
 partial reproductive isolation between differentiated populations. An
 increasing number of hybrid zones have been inferred to move over time,
 but in most such cases zone movement has not been tested with long-term
 genomic data. The hybrid zone between Townsend’s Warblers (Setophaga
 townsendi) and Hermit Warblers (S. occidentalis) in the Washington
 Cascades was previously inferred to be moving from northern S. townsendi
 southward toward S. occidentalis, based on plumage and behavioral patterns
 as well as a 2000-km genetic wake of hermit mitochondrial DNA (mtDNA) in
 coastal Townsend’s Warblers. We directly tested whether hybrid zone
 position has changed over 2-3 decades by tracking plumage, mtDNA, and
 nuclear genomic variation across the hybrid zone over two sampling periods
 (1987-94 and 2015-16). Surprisingly, there was no significant movement in
 genomic or plumage cline centers between the two time periods. Plumage
 cline widths were narrower than expected by neutral diffusion, consistent
 with a “tension zone” model, in which selection against hybrids is
 balanced by movement of parental forms into the zone. Our results indicate
 that this hybrid zone is either stable in its location or moving at a rate
 that is not detectable over 2-3 decades. Despite considerable gene flow,
 the stable clines in multiple phenotypic and genotypic characters over
 decades suggest evolutionary stability of this young pair of sister
 species, allowing divergence to continue. We propose a novel biogeographic
 scenario to explain these patterns: rather than the hybrid zone having
 moved thousands of kilometers to its current position, inland Townsend’s
 met coastal Hermit Warbler populations along a broad front of the British
 Columbia and Alaska coast and hybridization led to replacement of the
 Hermit Warbler plumage with Townsend’s Warbler plumage patterns along this
 coastline. Hence hybrid zones along British Columbia and Alaska moved only
 a short distance from the inland to the coast, whereas the Hermit Warbler
 phenotype appears stable in Washington and further south. This case
 provides an example of the complex biogeographic processes that have led
 to the distribution of current phenotypes within and among closely related
 species.
description: HZ.stable.MS.July2019.genomicEV.80sGenomic hybrid index of each site along with the distance (Km) from isocline (distr) and corrected distr (distr.corrected=distr+1200) for 1987-94 sampling.HZ.stable.MS.July2019.genomicEV.156Genomic hybrid index of each site along with the distance (Km) from isocline (distr) and corrected distr (distr.corrected=distr+1200) for 2015-16 sampling.sites.80s.N.W.distr.mtdata.meade&sievertMitochondrial DNA mean hybrid index of each site along with distr (Km away from the isocline) and distr.corrected (distr+1200Km) for 1980s sampling.sites.156.N.W.distr.mtdataMitochondrial DNA mean hybrid index of each site along with distr (Km away from the isocline) and distr.corrected (distr+1200Km) for 2015-16 sampling.TOWAHEWA_pres_hist.ageParentalcorrectedHI.sitemean.distrPlumage mean hybrid index of each site along with distr (Km away from the isocline) and distr.corrected (distr+1200Km) for 1980s and 2015-16 sampling.