Global biogeography and diversification of palms sheds light on the evolution of tropical lineages. II. Diversification history and origin of regional assemblages

Aim: Palms (Arecaceae/Palmae) are a model group for evolutionary studies in the tropics. In two companion papers, we aim to establish a general framework of palm evolution, exploring the geographical distribution of palm lineages and species diversity patterns at global and regional levels. In this, the second paper, we analyse the biogeographical events underlying regional palm assemblages and diversification processes across the family. We investigate the timing and location of diversification rate shifts and test the 'odd man out' pattern of low species diversity observed in the African palm flora. Location: Global. Methods: A complete, dated phylogeny of palm genera and ancestral area reconstructions were used to infer the biogeographical history of regional palm assemblages. Diversification rates for all genera were estimated and significant shifts in diversification rates across the family were identified under a maximum likelihood model. Results: Following their early diversification in Laurasia around 100 Ma, palms dispersed southwards before the end of the Cretaceous. Few dispersal events into Africa, North America and South America were inferred, whereas numerous lineages migrated between Eurasia, the Pacific and the Indian Ocean. Thirteen diversification rate increases were identified. High diversification rates were found only in lineages from Asia, the Americas, the Pacific and the Indian Ocean, but not in any African groups. Diversification decreases were not identified in any lineage. Main conclusions: Dispersal of palms into their present-day distribution started during the Late Cretaceous, consistent with the first fossil records for rain forests in Africa and South America. The few dispersal events into South America and Africa emphasize the isolated position of these continents for most of the Cenozoic. In contrast, high dispersal between Eurasia, the Pacific and the Indian Ocean, notably during the Miocene, can be attributed to geological activity in the region, especially in Malesia. Low species diversity in Africa relative to other regions is explained here by increased in situ diversification in Asia, the Americas, the Pacific and the Indian Ocean, rather than by diversification rate decreases in Africa. This may prove to be a general pattern in other organisms showing a similar disparity in richness in Africa.