ABSTRACT
Biodiversity is distributed unevenly from the poles to the equator, and among branches of the tree of life, yet how those enigmatic patterns are related is unclear. We investigated global speciation-rate variation across crown Mammalia using a novel time-scaled phylogeny (N=5,911 species, ~70% with DNA), finding that trait- and latitude-associated speciation has caused uneven species richness among groups. We identify 24 branch-specific shifts in net diversification rates linked to ecological traits. Using time-slices to define clades, we show that speciation rates are a stronger predictor of clade richness than age. Speciation is slower in tropical than extra-tropical lineages, but only at the level of clades not species tips, consistent with fossil evidence that the latitudinal diversity gradient may be a relatively young phenomenon in mammals. In contrast, species tip rates are fastest in mammals that are low dispersal or diurnal, consistent with models of ephemeral speciation and ecological opportunity, respectively. These findings juxtapose nested levels of diversification, suggesting a central role of species turnover gradients in generating uneven patterns of modern biodiversity.
Footnotes
Statement of authorship: NSU and WJ conceived the study; NSU and JAE collected and curated the data; NSU performed all analyses and wrote the first draft of the manuscript, with contributions to revisions from all co-authors.
Data and material availability: All data and code is available in the manuscript and after publication on Dryad (also to be available at github.com/n8upham/).
Manuscript revised for another round of review; added more detailed explanations of several concepts in the Discussion.
