A NEARLY COMPLETE PHYLOGENY REVEALS THE COMPLEX MACROEVOLUTIONARY UNDERPINNINGS OF CONIFER DIVERSITY HOTSPOTS

Regions of exceptional species richness can provide key insights into factors that drive diversification. High species richness may arise from high origination rates, low extinction rates, high migration, or a combination of these factors, but disentangling them is often difficult and requires detailed phylogenetic and geographic information. In this study, we focus on understanding the processes that generate species richness hotspots in one group of woody plants, the conifers, for which we have nearly complete phylogenetic and geographic information for extant species. We use a new time-calibrated molecular phylogeny that samples 90% of extant species, combined with geographic range data for every living species, to identify hotspots of species richness and characterize their phylogenetic structure using two standard metrics: evolutionary distinctiveness and net relatedness index. We use these metrics to ask if conifer hotspots are more generally “cradles” that have fostered diversification or “museums” that have preserved diversity. The fourteen hotspot regions that we identify, which span northern temperate to tropical environments, are diverse in their phylogenetic structure and many do not fall cleanly into either “cradle” or “museum” categories. Northern Hemisphere hotspots occupy a spectrum from regions composed of deeply branched, unrelated species to those composed of recently diverged and closely related species and these differences are driven primarily by precipitation; wet hotspots preserve more deeply divergent lineages while dry hotspots are dominated by a few lineages have recently radiated in harsher environments. Hotspots in tropical and Southern Hemisphere environments, in contrast, consist of species that are both deeply branched and closely related. This phylogenetic structure, which is absent in the Northern Hemisphere, reflects the slow but steady diversification of subclades within a few conifer genera that are well adapted to tropical forests. These results highlight the differences in conifer diversification dynamics between the hemispheres, the important role of high precipitation habitats in preserving deeply branched lineages in general, and the need for a nuanced terminology to describe the different ways in which regions of high species richness may arise.