Phylogenetic approaches to understanding grass diversity have recently provided new insights into the evolution of grasses. Although some recent studies substantially improved our understanding of the radiation of grasses since their origin in warm and closed habitats, the discrepancies observed in the responses of different grass lineages along climatic and latitudinal gradients remain poorly understood. Here we used the geographic variation in the phylogenetic diversity and structure at global scale to gain insight into grass evolution. We gathered geographic, functional trait and phylogenetic information for the Poaceae family at the species level, c. 11000 species. We mapped the phylogenetic diversity (PDI) and clustering (NRI) of the two major lineages: the PACCMAD (C4 and C3 species) and BOP (C3 species) clades. Through analysis of diversification and of the geographic variation in the phylogenetic diversity and structure of grass assemblages, we tested how evolution of cold tolerance and adaptation to arid environment in grasses explain the unusually shallow latitudinal gradient.
Results/Conclusions
Global-scale spatial patterns of grass diversity in the two clades showed contrasting patterns resulting from differences in long-term historical drivers. Decreasing temperature and precipitations were related to more clustered BOP assemblages. This relationship is similar to those observed in other groups of Angiosperms and suggests the result of a strong environmental filtering. However, in the PACCMAD clade, higher clustering was associated with, on average, higher annual and seasonal precipitation. Diversification analyses showed that diversification rates have been higher for freezing-exposed species, supporting the hypothesis that the evolution of cold tolerance is a major driver of the spatial distribution of grass diversity. Together, our results provide a better understanding of the current spatial distribution of grass assemblages and established the grass family as a model group for macroecological studies, on par with the major vertebrate groups.