Comparing clines on molecular and phenotypic traits in hybrid zones: a window on tension zone models.

The study of zones of secondary contact provides insight into the maintenance of reproductive isolation. Tension zone theory supplies powerful tools for assessing how dispersal and selection shape hybrid zones. We present a multimodal analysis of phenotypic clines in conjunction with clines at molecular markers in a hybrid zone between Larus glaucescens and Larus occidentalis. We developed a new method to analyze simultaneously clines of quantitative traits and molecular data. Low linkage disequilibrium and the lack of coincidence between clines at six microsatellites, a mitochondrial DNA region, and two phenotypic traits indicated introgression. However, the hypothesis of neutral diffusion was rejected based on evidence that all of the clines were concordant and narrower than expected for neutral clines, indicating some indirect selection. The analysis of phenotypic variance gave evidence of restricted phenotypic introgression and together with the bimodal distribution of phenotypes suggested that disruptive selection is acting across the hybrid zone, especially on the coloration of bare parts. Multimodal analysis of phenotypic clines also highlighted a shift between the peak of intermediates and the cline center, left behind by hybrid zone motion. High-resolution analysis of phenotypes distribution thus proved useful for detecting hybrid zone movement even without temporal data.