Morphological and molecular variation across a migratory divide in willow warblers, Phylloscopus trochilus

A migratory divide is a narrow region in which two populations showing diff erent migratory directions meet and presumably also mate and hybridize. Ban ding of willow warblers, Phylloscopus trochilus, in Europe has demonstrated a migratory divide latitudinally across central Scandinavia. In autumn, so uthern birds migrate southwest to tropical West Africa, whereas northern bi rds migrate southeast to East and South Africa. The migratory divide is ass ociated with concordant differences in size and plumage coloration. Based o n morphology, we estimate the width of the transition zone between northern and southern willow warblers to be less than 350 km. We found indication o f linkage disequilibria around the migratory divide, in that measures of bo dy size were correlated with plumage coloration within the contact zone, bu t uncorrelated within the populations south or north of the contact zone. T he presence of linkage disequilibria and the fact that several morphologica l dines occur together suggest that the hybrid zone is a result of secondar y contact between populations that have differentiated in allopatry. This i nterpretation is in accord with the knowledge of the recolonization pattern of the Scandinavian peninsula after the last glaciation; animals and plant s appeared to have colonized either from the south or from the north around the northern bay of the Baltic Sea. If northern and southern willow warble rs resided in allopatric populations during late Pleistocene glaciations an d the hybrid zone is a result of postglacial range expansions, we would exp ect some degree of genetic differentiation accumulated during the period in isolation. In contrast, northern and southern willow warblers are near pan mictic in the frequencies of alleles of mitochondrial DNA and at two micros atellite loci. The observed pattern, clear morphological and behavioral dif ferentiation without genetic differentiation at neutral loci, suggests eith er that the differences are maintained by strong selection on the expressed genes in combination with high levels of current gene flow or, in the case of weak gene flow, that the divergence in morphology and behavior is very recent.