Cross-decades stability of an avian hybrid zone

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. Plumag...