Reduced abundance suggests endogenous selection against hybrids between two avian subspecies with different migratory programs

<p><strong>Poster Presentation: Speciation 2010 – First European Conference on Speciation Research</strong><br><strong>13-15 December 2010, IIASA Conference Center, Laxenburg, Austria</strong></p> <p><strong>Abstract</strong>:</p> <p>To what extent hybrid zones are maintained by endogenous or exogenous selection can be<br>difficult to disentangle. Endogenous selection is typically seen as the primary source of<br>selection hybrid zones. These hybrids zones are thought to represent tension zones where<br>reduce hybrid fitness is balanced by dispersal by parental taxa. Theory suggests that tension<br>zones are not static and tend to move and often become trapped in regions of low abundance. These areas of low abundance are often associated with environmental gradients. Two subspecies of willow warblers (<em>Phylloscopus trochilus trochilus</em> and <em>P. t. acredula</em>) in central Sweden are a good example of a narrow hybrid zone that likely resulted from secondary contact and is hypothesized to represent a tension zone. Whereas these two subspecies meet along a north-south gradient, the landscape surrounding the willow warbler hybrid zone is characterized by a perpendicular gradient in climate and habitats. In this study, we set out to use this hybrid zone combined with a long-term nationwide monitoring dataset, the Swedish Breeding Bird Survey, to test hypotheses and predictions derived from the tension zone model. Specifically, we tested (i) whether the willow warbler hybrid zone was associated with a region of low abundance, (ii) whether the hybrid zone was coincident with an environmental gradient indicative of exogenous selection, and (iii) if the single locus tension zone model could be used to predict hybrid zone abundance, supporting endogenous selection. Our results demonstrate that low abundance occurs in the willow warbler hybrid zone that is oriented on a north-south latitudinal gradient, and coincides with four previously described willow warbler trait clines. The elevation, climatic patterns, and the associated habitats for this region exist in an east-west longitudinal gradient, perpendicular to the cline in abundance, suggesting a limited role of exogenous factors in the location and maintenance of our tension zone. Further, the overall abundance of willow warblers in the hybrid zone closely matches the predicted abundance expected from the reduced hybrid fitness calculated from the single locus tension zone model.