Genetic evidence for the effect of a postglacial population expansion on the phylogeography of a North American songbird

Phylogeographical studies of Nearctic songbirds conducted to date have yiel ded unexpectedly low levels of genetic differentiation and weak phylogeogra phical structure in mitochondrial DNA lineages as compared with species stu died in Neotropical areas. Factors leading to this pattern may include (i) gene flow, (ii) population expansions from bottlenecked populations, and (i ii) selective sweeps. Here we provide evidence for the role played by Pleis tocene postglacial population expansions on the phylogeography of MacGilliv ray's warbler (Oporornis tolmiei), a long-distance migratory bird. Samples from 12 breeding localities in the temperate USA were compared with those f rom two localities in north-eastern Mexico. The former showed evidence of a Late Pleistocene population expansion as indicated by low haplotype and nu cleotide diversity, a star-like phylogeny of alleles, and a mismatch distri bution indicating a sudden increase in effective population size. By contra st, the Mexican population showed high levels of genetic diversity and a mi smatch distribution as expected for a population unaffected by sudden demog raphic change. Haplotypes from the two regions formed two distinct phylogro ups which separated roughly one million years ago according to a convention al molecular clock for songbirds. This study provides support for the Pleis tocene expansion hypothesis in MacGillivray's warbler and suggests that pos tglacial expansion of bottlenecked populations is responsible for the lack of variation and structure reported for most North American songbird specie s.