A major goal of ecology is to explain differences in species richness between regions and among clades. The diversification rate of clades is a key parameter for understanding both patterns. Here, we combine phylogenetic and climatic data for 250 species of plethodontid salamanders and show for the first time that rapid species diversification is associated with accelerated climatic-niche evolution among species. Both rates are particularly rapid in tropical regions, and where few clades geographically overlap. These results offer a surprising ecological explanation for why diversification rates are often higher in the tropics: rapid shifts between climatic regimes, rather than specific environmental conditions (e.g. high productivity, energy) drive higher diversity. They also suggest that climatic-niche evolution may be particularly rapid in regions where climate is particularly stable. Finally, these results indicate that evolutionary conservatism in climatic niches may be influenced by interactions between species and clades, rather than physiological tolerances alone.