More than 100 species of birds have Holarctic distributions extending across Eurasia and North America, and many of them likely achieved these distributions by recently colonizing one continent from the other. Mitochondrial DNA (mtDNA) and five nuclear introns were sequenced to test the direction and timing of colonization for a Holarctic duck, the gadwall (Anas strepera). Three lines of evidence suggest gadwalls colonized North America from Eurasia. First, New World (NW) gadwalls had fewer alleles at every locus and 61% of the allelic richness found in Old World (OW) gadwalls. Second, NW gadwalls had lower mtDNA allelic richness than other NW ducks. Third, coalescent analysis suggested that less than 5% of the ancestral population contributed to NW gadwalls at the time of divergence. Gadwalls likely colonized North America during the Late Pleistocene (∼81,000 years ago), but the confidence interval on that estimate was large (8500–450,000 years ago). Intercontinental gene flow and selection also likely contributed to genetic diversity in gadwalls. This study illustrates the use of multiple loci and coalescent analyses for critically testing a priori hypotheses regarding dispersal and colonization and provides an independent datapoint supporting an OW to NW bias in the direction of colonization.
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Vol. 62 • No. 6