Significance Why do so many species occur in mountains? A popular but little-tested hypothesis is that tectonic uplift creates environmental conditions (new habitats, dispersal barriers, etc.) that increase the rate at which resident species divide and evolve to form new ones. In China’s Hengduan Mountains region, a biodiversity hotspot uplifted over the last 8 million years, this rate does in fact show a significant increase during that time, relative to the rate for adjacent older mountains, and to the rate of species immigration. The Hengduan Mountains flora is thus made up disproportionately of species that evolved within the region during its uplift, supporting the original hypothesis and helping to explain the prevalence of mountains as global biodiversity hotspots.