Rhododendron exhibits an exceptional diversity in south-west China and the Pacific islands indicating that the group might have originated in these regions. However, fossil record suggest that the group were abundant in the higher latitudes 40-50 million years ago. In order to find plausible explanation for this phenomenon, we traced the origin and dispersal history of the genus. We compiled the global distributions of all Rhododendron species (> 1,000 species), and constructed a dated molecular phylogeny (~500 species) using 16 genes. By integrating these two large datasets, we estimated the global patterns of Rhododendron diversity, mean net diversification rates (spatial and temporal: see Figure 1 below), and mean species age. We also reconstructed the geographic ancestral area of the genus and assessed the relationships among species diversity, age, diversification rates and contemporary environment. The results suggested an interesting into-the-tropics colonization of the genus from high northern latitudes and a rapid diversification in the tropics/subtropics following orogeny events therein. The extant pattern of Rhododendron distribution and its diversification rates both were strongly associated with habitat heterogeneity variables indicating the roles of heterogeneous topography in in-situ speciation.
Figure 1. Spatial patterns of mean net diversification rates of global Rhododendron (a) and the temporal trends of net diversification rates in different biogeographical regions (b). [Shrestha et al. 2018, Global Ecology and Biogeography 27: 913-924]
High species diversity in regions far from its origin center suggested that longer time does not necessarily yield more species. This is in contrast to the long accepted concept on species accumulation as suggested by the time-for-speciation hypothesis. The findings on Rhododendron evolutionary history, therefore, opened up new insights on how species tend to accumulate in the tropics/ sub-tropics particularly in the tropical and sub-tropical mountains. The findings suggest that tropical mountains are important evolutionary and diversity centers of temperate clades. The integrative framework adopted in this research also highlights the importance of considering both ecological and evolutionary processes in gaining better understanding of the factors shaping the global patterns of species diversity.
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