The shifting roles of dispersal and vicariance in biogeography

Dispersal and vicariance are often contrasted as competing processes primar ily responsible for spatial and temporal patterns of biotic diversity. Rece nt methods of biogeographical reconstruction recognize the potential of bot h processes, and the emerging question is about discovering their relative frequencies. Relatively few empirical studies, especially those employing m olecular phylogenies that allow a temporal perspective, have attempted to e stimate the relative roles of dispersal and vicariance. In this study, the frequencies of vicariance and dispersal were estimated in six lineages of b irds that occur mostly in the aridlands of North America. Phylogenetic tree s derived from mitochondrial DNA sequence data were compared for towhees (g enus Pipilo), gnatcatchers (genus Polioptila), quail (genus Callipepla), wa rblers (genus Vermivora) and two groups of thrashers (genus Toxostoma). Dif ferent area cladograms were obtained depending on how widespread and missin g taxa were coded. Nonetheless, no cladogram was obtained for which all lin eages were congruent. Although vicariance was the dominant mode of evolutio n in these birds, approximately 25% of speciation events could have been de rived from dispersal across a preexisting barrier. An expanded database is now needed to estimate the relative roles of each process. Applying a molec ular clock calibration, nearly all speciation events are of the order of a million or more years old, much older than typically presumed.