Comparative phylogeography in North American birds

Modern molecular methods yield descriptions of the phylogenetic deployment of genetic variation within species, or phylogeography. A developing field is the comparison of geographic patterns of genetic variation in codistributed species, or comparative phylogeography. One determines whether species that currently share the same broad area exhibit congruent phylogeographic patterns, which would indicate that they were historically codistributed and differentiated in response to same geological or environmental events. I reviewed studies of mitochondrial DNA (mtDNA) variation in five codistributed, widespread species of North American birds. In addition to phylogeographic patterns, data were available on levels of genetic variation, gene flow, and evolutionary distance from common ancestors (an index of a species' relative age) for each species, all important factors involved in geographic differentiation. Two species, Canada goose (Branta canadensis) and fox sparrow (Passerella iliaca), exhibited incongruent phylogeographic patterns of mtDNA variation, whereas three other codistributed species, the song sparrow (Melospiza melodia), chipping sparrow (Spizella passerina), and red-winged blackbird (Agelaius phoeniceus), lacked geographically structured mtDNA patterns. Thus, the first two species have had demonstrably different histories, whereas the histories of the latter three are speculative. However, the three species without geographic variation in mtDNA exhibit shallow mtDNA haplotype trees, which are consistent with recent population expansion. Thus, lack of geographic variation in mtDNA is likely due to these three species arriving relatively recently at their current distributions, and after historical isolating events produced genetic divisions in the fox sparrow and Canada goose. Within the geographically uniform species, or parts thereof, dispersal seems a likely homogenizing factor; no consistent explanations emerged from consideration of levels of genetic variation. Lack of congruent phylogeographic patterns suggests that currently codistributed species have not had a long history of co-association. These results and comparison of other North American species for which less extensive mtDNA surveys are available, reveal that the North American avifauna is probably a composite of species with different histories.