Genetic differentiation between insular and continental populations of migratory and resident warblers, the Great Reed Warbler Acrocephalus arundinaceus and Cetti’s Warbler Cettia cetti

Abstract

Island populations are frequently smaller than continental populations, have lower genetic diversity, are more inbred and show genetic differentiation from the mainland ones. However, sufficient numbers of immigrants may reduce the differentiation of insular populations and moderate the effects of genetic drift. In this study, we compared insular and continental populations of Cetti’s Warbler Cettia cetti and Great Reed Warbler Acrocephalus arundinaceus. We assessed the degree of genetic differentiation between insular and continental birds, their demographic history and genetic diversity. We compared the results, taking into account the differences in migration strategy and morphology of the two warblers. We found slightly lower genetic diversity in the insular populations than in the continental birds, possibly because of the lower population size or reduced immigration. The genetic differentiation between island and mainland birds was low, but higher in Cetti’s Warbler than in the Great Reed Warbler, suggesting differences in the species’ capability of crossing the sea. We found evidence for a past bottleneck in both the insular and continental populations of Cetti’s Warbler, while for the Great Reed Warbler we found no signs of past population reductions. A high dispersal capability of the Great Reed Warbler may have allowed high gene flow, which may explain the observed interspecific differences in the demographic history of the Great Reed Warbler and Cetti’s Warbler.