Different timing of the adaptive radiations of North American and Asian warblers

The timing of speciation events among warblers (small insectivorous woodland birds) in the Himalayas of India and in the White Mountains of New Hampshire, USA, is compared. Sequence divergence in the mitochondrial cytochrome b gene for 13 New World species in six genera averages 2.6%, which according to standard calibrations places most of the diversification in the early Pleistocene. In contrast, eight Himalayan species in the single genus Phylloscopus differ by an average of 10.7% in the same gene sequence. The New Hampshire warblers appear to have undergone a relatively recent burst of speciation and morphological evolution, but there is a detectable slowing of speciation rates (or increase in extinction rates) within the community towards the present. Other North American passerine groups that have been studied show no signs of explosive diversification in the Pleistocene. In these groups, pairs of sister species are often older than the entire warbler radiation. Boreal forest migrated south from very high latitudes towards the end of the Pliocene and this apparently created conditions ripe for explosive diversification among the warblers, but not for many other groups.