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Aim Aridification of western North America from the Miocene to the Recent has spurred plant migrations, as species invaded and/or diversified into newly formed floristic regions. The 51 species of Mentzelia section Bartonia (Loasaceae) inhabit most of these regions, providing an opportunity to investigate lineage-specific diversification among the western floras. Lineage-specific formation of the modern western North American flora was investigated by applying statistical phylogenetic methods to molecular data. Location Western North America, including the intermountain region, south-western deserts, Rocky Mountains and North American Prairies Province. Methods Internal and external transcribed spacer region data for all taxa in section Bartonia and outgroups were analysed with maximum likelihood and Bayesian inference methods for phylogenetic relationships. Distribution and phylogenetic data were then analysed by Bayesian, maximum likelihood, parsimony, and stochastic mapping optimization methods for inference of biogeographical transitions among floristic regions. Model fitting was used to assess the symmetry of transitions from mesic to arid environments. Results Multiple transitions were inferred, especially those associated with the Colorado Plateau and the Rocky Mountains. The ancestral area for section Bartonia was estimated equivocally to be all regions except the Great Basin in parsimony reconstructions, but to be the Colorado Plateau with highest probability with the Bayesian approach. Throughout these regions, transitions between mesic to arid habitats were consistent with expectations under a null model, suggesting no directional bias in transitions. In section Bartonia, within-region diversification occurred in all areas except the Great Basin and Sonoran Desert, the diversity of which is explained as the result of movement into these regions. Main conclusions The floristic regions of western North America have exchanged numerous migrants throughout their history. Among them, the Colorado Plateau, Great Basin and Rocky Mountains were particularly important floristic sources and recipients of migrants. Although previous studies have revealed a strong association among the south-western deserts, these results suggest that the plant diversities in these deserts can be influenced more by adjacent provinces than by each other.