Driving forces behind evolutionary radiations: Saxifraga section Ciliatae (Saxifragaceae) in the region of the Qinghai-Tibet Plateau

The driving forces behind evolutionary radiations in mountain ecosystems are only poorly understood. Using the species-rich alpine taxon Saxifraga section Ciliatae, which occurs predominantly in the region of the Qinghai-Tibet Plateau, we explore the driving forces behind elevated diversification rates previously observed in one of three subclades of section Ciliatae (clade 3). We use species distribution models to test for an influence of topographical heterogeneity on the diversity of section Ciliatae and investigate the characteristics of climatic niches in this section. In addition, we expand the dataset of known polyploids of section Ciliatae in order to investigate the effect of polyploidy on diversification in this group and estimate rates of niche evolution. Both polyploidization and topographical heterogeneity were associated with overall species richness in section Ciliatae, but neither was a likely driver of the observed radiation in clade 3 of section Ciliatae. Instead, larger niche breadth and high rates of niche evolution of taxa in clade 3 probably drove rapid diversification via pronounced adaptability for the biologically limiting climatic conditions of the alpine life-zone, presenting an important advantage during past periods of rapid climate change. Our study thus elucidates several aspects of the diversification process in this large alpine taxon and underlines the importance of climatic niche evolution for radiations in mountain regions.