Plumage and mitochondrial DNA haplotype variation across a moving hybrid zone

We analyze variation in phenotypes and mitochondrial DNA (mtDNA) haplotypes over the breeding ranges of hermit and Townsend's warblers and across two of their three hybrid zones. Within these two hybrid zones, we demonstrate that the placement, shape, and width of transitions in seven plumage charac ters are remarkably similar, suggesting that a balance between dispersal an d sexual selection keeps these hybrid zones narrow. A consistent asymmetry in these character transition curves suggests that Townsend's warblers have a selective advantage over hermit warblers, which is presumably due to the aggressive superiority of Townsend's over hermit males (Pearson and Rohwer 2000). An association between plumage and mtDNA haplotypes shows that pure Townsend's warblers, but not pure hermit warblers, immigrate into these hy brid zones, further supporting the competitive superiority of Townsend's wa rblers over hermit warblers. The mitochondrial haplotype transitions across these hybrid zones are much wider than the phenotypic transitions and prov ide no indication that the mtDNA haplotypes representing these two warblers are selectively maintained. More importantly, the phenotypically pure popu lations of Townsend's warblers throughout a 2000-km coastal strip north of the Washington hybrid zones contain a preponderance of hermit warbler mtDNA haplotypes. This result suggests massive movement of the hybrid zone betwe en these warblers during the 5000 years since their most recent interglacia l contact. We develop a model to explain the phenotypic and genetic diverge nce between these warblers and the evolution of their dramatic differences in aggressiveness; we also show how differences in male aggression, in comb ination with biased pairing patterns, can explain the haplotype footprint r ecording the historical movement of this hybrid zone.