Phylogeography of a widespread North American migratory songbird (Setophaga ruticilla).

Genetic analyses for many widespread North American species have revealed significant east-west differentiation, indicating that many survived through the Pleistocene in 2 glacial refugia-1 in the eastern and 1 in the western part of the continent. It remains unclear, however, whether other areas may have served as important glacial refugia. Moreover, many such species exhibit widespread genetic similarity within eastern and western regions because of recent expansion from small refugial populations, making it difficult to evaluate current-day levels of gene flow. In this study, we used mitochondrial DNA (mtDNA) control region sequence and amplified fragment length polymorphism markers to survey genetic variation in a widespread migratory bird, the American redstart (Setophaga ruticilla). mtDNA analyses revealed a pattern that contrasts with that found for most other widespread species studied to date: most redstart populations across North America appear to have spread out from a single glacial refugium, possibly located in the southeastern United States, whereas populations in far-eastern Canada may have survived in a second glacial refugium located on the now-submerged Atlantic coastal shelf off the coast of Newfoundland. A pattern of isolation by distance in mtDNA suggested some constraints on current-day gene flow among extant redstart populations. This study thus reveals a recent evolutionary history for this species that differs from that of most other widespread North American passerines and provides evidence for limited gene flow in a species with potentially large dispersal distances.