Latitudinal variation in speciation mechanisms in frogs.

Speciation often has a strong geographical and environmental component, but the ecological factors that potentially underlie allopatric and parapatric speciation remain understudied. Two ecological mechanisms by which speciation may occur on geographic scales are allopatric speciation through niche conservatism and parapatric or allopatric speciation through niche divergence. A previous study on salamanders found a strong latitudinal pattern in the prevalence of these mechanisms, with niche conservatism dominating in temperate regions and niche divergence dominating in the tropics, and related this pattern to Janzen's hypothesis of greater climatic zonation between different elevations in the tropics. Here, we test for latitudinal patterns in speciation in a related but more diverse group of amphibians, the anurans. Using data from up to 79 sister-species pairs, we test for latitudinal variation in elevational and climatic overlap between sister species, and evaluate the frequency of speciation via niche conservatism versus niche divergence in relation to latitude. In contrast to salamanders, we find no tendency for greater niche divergence in the tropics or for greater niche conservatism in temperate regions. Although our results support the idea of greater climatic zonation in tropical regions, they show that this climatic pattern does not lead to straightforward relationships between speciation, latitude, and niche evolution.