jats:titleAbstract</jats:title>jats:secjats:titleAim</jats:title>jats:pHow mountains accumulate species diversity remains poorly understood, particularly the relative role of in situ cladogenesis compared with colonization from lower elevations. Here, we estimated the contributions of in situ cladogenesis and colonization in generating biodiversity of a large mountain plant radiation and determined the importance of niche adaptation and divergence in these processes. We expected cladogenesis would accompany novel habitats formed by mountain uplift, but colonization would become more important with time as dispersal opportunities accrue.</jats:p></jats:sec>jats:secjats:titleLocation</jats:title>jats:pNew Zealand, Southern Alps.</jats:p></jats:sec>jats:secjats:titleTaxon</jats:title>jats:pjats:italicVeronica</jats:italic> sect. jats:italicHebe</jats:italic> (Plantaginaceae).</jats:p></jats:sec>jats:secjats:titleMethods</jats:title>jats:pWe estimated the most complete time‐calibrated phylogeny to date for jats:italicVeronica</jats:italic> sect. jats:italicHebe</jats:italic> to quantify rates of in situ cladogenesis and colonization of mountain habitat based on historical biogeographical models. We used environmental niche modelling to quantify species' climate niches and estimate niche disparity and divergence over time.</jats:p></jats:sec>jats:secjats:titleResults</jats:title>jats:pIn situ cladogenesis generated more species in the mountains than colonization from lowlands. Whereas cladogenesis slowed over time, colonization increased, especially in the alpine zone. Both adaptive ecological speciation along climate niche axes and non‐adaptive, vicariant speciation contributed to cladogenesis. However, climate niche disparity through time became saturated, suggesting competition for niche space was important. Colonization brought more divergent species into mountain niches.</jats:p></jats:sec>jats:secjats:titleMain Conclusions</jats:title>jats:pWe suggest mountain diversity accumulates through three main stages: high cladogenesis after initial colonization, decreasing cladogenesis with increasing competition and increasing colonization after niches saturate, likely promoted by niche divergence. Combining lineage and mountain uplift trajectories, these stages provide a conceptual model to understand how diversity accumulates elsewhere. Assuming these deep‐time findings apply to anthropogenic conditions, alpine specialists could struggle to outcompete colonizers facilitated by climate change, especially from generalist clades. Considering novel competitive interactions alongside niche traits and biogeographical processes will be crucial for predicting the fate of alpine biodiversity in a changing world.</jats:p></jats:sec>