Signals of Recent Tropical Radiations in Cunoniaceae, an Iconic Family for Understanding Southern Hemisphere Biogeography

Extratropical angiosperm diversity is thought to have arisen from lineages that originated in the more diverse tropics, but studies of dispersal between tropical and extratropical environments and their consequences for diversification are rare. In order to understand the evolutionary effects of shifts between the tropics and extratropics, defined here as areas that do versus do not regularly experience freezing temperatures, we studied the biogeographic history and associated diversification patterns of Cunoniaceae. We mapped the distribution of all species in the family and combined this with a newly constructed phylogeny for the family. The family shows a long evolutionary association with both tropical and extratropical environments, the tropics house considerably greater species richness of Cunoniaceae. Indeed, both tropical and extratropical environments appear to have had a similar number of lineages until 12 Ma, after which time the number of lineages in tropical areas increased at a faster rate. In addition, community phylogenetic approaches show that tropical regions have markedly less phylogenetic diversity than expected given their species richness, which is further suggestive of recent species radiations in tropical areas. The Cunoniaceae show an atypical pattern for angiosperms of frequent shifts between tropical and extratropical environments, but despite this, shows a more conventional pattern of higher, although recent, diversification rates in the tropics. Our results support the idea that high angiosperm species richness in the tropics may result from the tropics acting as a cradle of recent angiosperm diversification.