An engine for global plant diversity: highest evolutionary turnover and emigration in the American tropics

Understanding the processes that have generated the latitudinal biodiversity gradient and the continental differences in tropicalbiodiversity remains a major goal of evolutionary biology. Here we estimate the timing and direction of range shifts of extant flowering plants(angiosperms) between tropicaland non- tropicalzones, and into and out of the major tropicalregions of the world. We then calculate rates of speciationand extinction taking into account incomplete taxonomic sampling. We use a recently published fossil calibrated phylogeny and apply novel bioinformatic tools to code species into user-defined polygons. We reconstruct biogeographic history using stochastic character mapping to compute relative numbers of range shifts in proportion to the number of available lineages through time. Our results, based on the analysis of c. 22,600 species and c. 20 million geo-referenced occurrence records, show no significant differences between the speciationand extinction of tropicaland non- tropicalangiosperms. This suggests that at least in plants, the tropicalbiodiversity gradient primarily derives from other factors than differential rates of diversification. In contrast, the outstanding species richness found today in the American tropics(the Neotropics), as compared to tropicalAfrica and tropical Asia, is associated with significantly higher speciationand extinction rates. This suggests an exceedingly rapid evolutionary turnover, i.e. Neotropical species being formed and replaced by one another at unparalleled rates. In addition, tropicalAmerica stands out from other continents by having ‘pumped out’ more species than it received through most of the last 66 million years. These results imply that the Neotropics have acted as an engine for global plant diversity.