<p>Mountain regions harbor extraordinary biodiversity, yet the mechanisms underlying this pattern remain unclear. Here we use the phylogenetically and geographically replicated radiations of <i>Berberis</i> (barberries) in the Andes and Hengduan–Himalaya Mountains (HHM) to test how functional trait innovations, orogeny, and climate change influenced the buildup of species richness in each region. In barberries (<i>Berberis</i>), the successive evolution of deciduousness and small, densely-veined leaves enabled the lineage to take advantage of expanding alpine and subalpine habitats in the HHM during the Miocene. The resulting pulse in the origin of new species contrasts with the more modest rate of species accumulation in the Andes, where barberry species remained evergreen and the availability of high-elevation habitats was more limited. In this study, the phylogeny of barberries exemplifies context-dependent macroevolutionary dynamics, in which intrinsic innovations and extrinsic ecological opportunities interacted to generate exceptional plant diversity in a mountain hotspot.</p>