Latitudinal phylogenetic and diversity gradients are explained by a tropical-temperate transitional region

The distribution of ecological and evolutionary forces throughout space bring about the patterning of biodiversity. In large geographical areas, this causes the regionalization of biodiversity into structured units known as bioregions. In order to understand how such patterns emerge, a clear delineation of bioregions is required. We use tree species as model taxa in order to analyze the global distribution of biodiversity and understand how latitudinal gradients of biodiversity, specifically the latitudinal phylogenetic and diveristy gradients are formed. By compiling an extensive dataset of tree species distributions and their phylogenetic relationships, we use a data-driven approach to delineate global bioregions of similar evolutionary histories, termed phyloregions. Our analysis reveals the presence of a region between the tropical and temperate regions, coined bridge phylore-gion, with a unique evolutionary composition and characteristically weaker association to climatic and environmental parameters. Through simulations, we show that the pres-ence of latitudinal phylogenetic and diversity gradients are much more likely to emerge in the presence of an independent ecological region between tropical and temperate regions, suggesting that its role as a stepping-stone in colonization of species between distinct climatic zones has shaped latitudinal gradients. This study highlights that accurate de-lineation of evolutionary structures of biodiversity can reveal previously cryptic regions with fundamental evolutionary roles in the formation of biodiveristy patterns. 1 Significance StatementBiodiversity patterns are shaped by geography, forming distinctive units known as bioregions. At global scales there are important gradients in terms of species diversity and phylogenetic relations that are well documented, but the underlying processes that generate them are unclear. We comprised an extensive dataset of tree species distributions and phylogeny, and identified the bioregions from the dataset. We analyzed the diversity and the phylogenetic gradients and identified a previously cryptic bioregion between tropical and temperate zones. Though less climatically distinct, we show through simulations that the presence of this transition zone better explains latitudinal biodiversity patterns. Our findings refine the global dispersal dynamics of species between tropical and temperate regions and highlight the role of this intermediate region in shaping global biodiversity patterns.