Phylogenetic relationships of toads in the Bufo bufo species group from the eastern escarpment of the Tibetan Plateau: a case of vicariance and dispersal.

Phylogenetic relationships among Tibetan populations of the Bufo bufo species group are investigated using 1063 bases of mitochondrial DNA sequence from the genes encoding ND1 (subunit one of NADH dehydrogenase), tRNA(Ile), tRNA(Gln), tRNA(Met), and ND2. The aligned sequences contain 181 phylogenetically informative characters across all taxa sampled. Two hypotheses for colonization of the Tibetan Plateau are tested. A vicariant hypothesis predicts monophyly of populations from high elevations. A dispersalist hypothesis predicts monophyly of populations in each of two river drainages (Yangtze and Yellow rivers), which requires nonmonophyly of populations from high elevations. Both hypotheses are rejected in favor of a third hypothesis that combines elements of vicariance and dispersal. The most parsimonious phylogenetic tree places the high-elevation species, B. andrewsi, as the sister taxon to the other Asian Bufo populations; these high-elevation populations are postulated to have had a vicariant origin approximately 5 million years before present. The high-elevation population recognized as B. minshanicus is nested within low-elevation populations of B. gargarizans and is suggested to have dispersed onto the Tibetan Plateau more recently.