Genetic Differentiation, Geographic Variation and Hybridization in Gulls of the Larus glaucescens-occidentalis Complex

Survey of morphometric, colorimetric, and allozymic variation in the Glaucous-winged Gull (Larus glaucescens), the Western Gull (L. occidentalis) and their hybrids is based on 706 specimens taken from 33 colony areas located throughout the breeding range of both species. Whereas most morphometric characters overlap between taxa, colorimetric characters exhibited significant intraspecific and interspecific clinal variation. Canonical discriminant function analysis of colorimetric characters clearly separated pure and hybrid morphotypes and confirmed the intermediacy of character expression in hybrids. Plumage melanism of the mantle and primary tips are among the best discriminators of hybrids. Twenty-five of 32 presumptive genetic loci were polymorphic. Although there were no fixed differences between taxa, gene frequencies exhibited significant heterogeneity across geography. Despite moderately high gene flow (Nm = 2.8 individuals per generation), populations of the L. glaucescens-occidentalis complex are not panmictic, and the hybrid zone forms a partial barrier to gene flow. Hybrids are genetically more similar to L. glaucescens than to L. occidentalis. The hybrid zone extends from Juan de Fuca Strait, Washington, south to Coos Bay, Oregon. A broad zone of introgression extends from the Queen Charlotte Islands in British Columbia south to Coos Bay, Oregon. The hybrid zone appears to have expanded in recent years, yet its midpoint near Grays Harbor, Washington, has remained stable. Hybridization appears to be responsible for skewed introgression and elevated genic diversity in L. glaucescens. The fact that L. glaucescens shows high levels of introgression while maintaining its genetic identity lends support to the hypothesis that hybridization in birds may serve as a mechanism for increasing or restoring genetic diversity in forms that are adapted to local environments.