Abstracts (first author)
Environmental tolerances, latitudinal gradients and the potential for speciation in mammals and birds
Janzen (1967) proposed that the broader temperature tolerances of temperate species should facilitate dispersal across the cold peaks of mountain barriers and therefore should promote latitudinal differences in the potential for speciation. This elegant hypothesis has since been extended to other environmental barriers to dispersal and suggested as a possible driver of latitudinal diversity gradients. Nevertheless, a key prediction –that gene flow across mountain passes, or other environmental gradients, is more likely in species with broader environmental tolerances– has never been tested in a large-scale comparative analysis that controls for the effects of phylogeny and other relevant factors. Here we test these ideas through phylogenetically-informed analyses of the potential for speciation in terrestrial mammals (3136 species) and birds (6694 species). We show that although greater tolerance to rainfall unpredictability reduces significantly the potential for speciation (as measured by the number of subspecies per species), greater tolerance to temperature variability has the opposite effect, even among species whose ranges are dissected by mountains. In addition, we show that the net potential for speciation is higher in temperate than tropical mammals, and that it does not vary significantly with latitude in birds. We discuss the implications of these findings for our understanding of the effects of climate change on global patterns of diversity.