Barn swallows before barns: population histories and intercontinental colonization.

The barn swallow (Hirundo rustica) is one of most widely distributed swallows, owing in part to its recent switch from natural nest sites to human structures. We conducted phylogenetic analysis of mitochondrial (mt) and nuclear DNA to explore the recent evolutionary history of this species. Strongly supported mtDNA clades corresponded to Europe, Asia and North America plus the Baikal region of Asia. Analysis of sequence data from a sex-linked nuclear gene was unable to recover the phylogenetic splits in the mtDNA tree, confirming that the main clades evolved recently. The phylogenetic pattern suggests that the ancestral area of the barn swallow was the holarctic; most divergence events are consistent with vicariance. Most unexpectedly, analyses show that barn swallows from North America colonized the Baikal region in the recent past (one fixed substitution). This dispersal direction is opposite of that for most nearctic-palearctic taxon exchanges. Although this invasion was envisioned to coincide with the appearance of new types of human dwelling in the Baikal region, calibration of molecular divergence suggests an older dispersal event. A recent history of gene flow within the main palearctic clades is consistent with range and population expansion owing to new nesting opportunities provided by human settlements. Contrary to expectation, populations in North America appear historically larger and more stable than those in the palearctic. The Baikal population apparently has not increased greatly since colonization.