ORIGINAL ARTICLE In and out of the Qinghai-Tibet Plateau: divergence time estimation and historical biogeography of the large arctic-alpine genus Saxifraga L. J. Ebersbach 1 , A. N. Muellner-Riehl 1,2 *, I. Michalak 1 , N. Tkach 3 , M. H. Hoffmann 3 , M. R€ oser 3 , H. Sun 4 and A. Favre 1 1 Department of Molecular Evolution and Plant Systematics & Herbarium (LZ), Institute of Biology, Leipzig University, D-04103 Leipzig, Germany, 2 German Centre for Integrative Biodiversity Research (iDiv) Halle-Jena-Leipzig, D-04103 Leipzig, Germany, 3 Institute of Biology, Geobotany and Botanical Garden, Martin Luther University Halle-Wittenberg, D-06108 Halle (Saale), Germany, 4 Kunming Institute of Botany, Chinese Academy of Sciences, Kunming 650204, Yunnan, China *Correspondence: Alexandra N. Muellner- Riehl, Department of Molecular Evolution and Plant Systematics & Herbarium (LZ), Institute of Biology, Leipzig University, Johannisallee 21-23, D-04103 Leipzig, Germany. E-mail: muellner-riehl@uni-leipzig.de ABSTRACT Aim Geologically dynamic areas often harbour remarkable levels of biodiver- sity. Among other factors, mountain building is assumed to be a precondition for species radiation, and yet, the potential role of immigration as a source of biodiversity prior to radiation is often neglected. Here, we studied the biogeo- graphical history of the large genus Saxifraga to unravel the role played by the Qinghai-Tibet Plateau (QTP) for the diversification of this genus and to under- stand factors that have led to the establishment of high biodiversity in and around this region. Location QTP and surrounding mountain ranges and worldwide distribution range of Saxifraga. Methods Using a total of 420 taxa (321 ingroup taxa) comprising more than 60% of extant Saxifraga species, we studied the evolutionary history of Saxi- fraga by performing phylogenetic analyses (maximum likelihood and Bayesian inference on nuclear ITS and plastid trnL–trnF, matK sequences), divergence time estimation (using uncorrelated log-normal clock models and four fossil constraints in beast) and ancestral range estimation (using BioGeoBEARS). Results Saxifraga originated in North America around 74 (64–83) Ma, dis- persed to South America and northern Asia during its early diversification and colonized Europe and the QTP region by the Late Eocene. The QTP region was colonized several times independently, followed in some lineages by rapid radiations, temporally coinciding with recent uplifts of the Hengduan Moun- tains at the southeastern fringe of the QTP. Subsequently, several lineages dis- persed out of Tibet. Main conclusions Immigration, recent rapid radiation and lineage persis- tence were all important processes for the establishment of a rich species stock of Saxifraga in the QTP region. Because floristic exchanges between the neigh- bouring areas and the QTP region were bi-directional, the spatio-temporal evolution of Saxifraga contrasts with the ‘out of QTP’ pattern, which has often been assumed for northern temperate plants. Keywords historical biogeography, in situ alpine diversification, molecular dating, phylogeny, Qinghai-Tibet Plateau, Saxifraga INTRODUCTION Disentangling abiotic and biotic processes which have shaped the Earth’s uneven distribution of biodiversity is one of the fundamental challenges of biogeography (Wen et al., 2013). A particular focus of biodiversity studies is on so-called bio- diversity hotspots, species-rich areas of conservation priority, characterized by high endemism and drastic habitat loss (Myers et al., 2000). High levels of biodiversity are com- monly associated with geologically dynamic environments ª 2016 John Wiley & Sons Ltd http://wileyonlinelibrary.com/journal/jbi 1 doi:10.1111/jbi.12899 Journal of Biogeography (J. Biogeogr.) (2016)