GENE FLOW, REFUGIA, AND EVOLUTION OF GEOGRAPHIC VARIATION IN THE SONG SPARROW (MELOSPIZA MELODIA)

We surveyed mtDNA restriction-site variation in song sparrows taken from across their continental range. Despite marked geographic variation in size and plumage color, mtDNA variation was not geographically structured. Subspecies were not identifiable by mtDNA analysis. We suggest that postglaciation dispersal scattered mtDNA haplotypes across the continent, explaining the lack of mtDNA geographic patterns. Evolution of size and plumage coloration has probably proceeded faster than mtDNA evolution, leading to the well-structured continental pattern of morphological variation. We suggest that the nonordered geographic distribution of haplotypes reflects the recency of population establishment following completion of range expansion. Dispersal distance was estimated from the mtDNA data at 6.1 km per generation, an order of magnitude greater than that (0.3 km) estimated from demographic data. Island samples were not especially different from continental ones. Rooting the haplotype cladogram with a putative primitive haplotype identified Newfoundland and the Queen Charlotte Islands as potential sites of recent refugia. We question whether study of geographic variation in song sparrows leads to insights concerning speciation.