Abstract
The uneven distribution of species in the tree of life is rooted in unequal speciation and extinction among groups. Yet the causes of differential diversification are little known despite their relevance for sustaining biodiversity into the future. Here we investigate rates of species diversification across extant Mammalia, a compelling system that includes our own closest relatives. We develop a new phylogeny of nearly all ~6000 species using a 31-gene supermatrix and fossil node- and tip-dating approaches to establish a robust evolutionary timescale for mammals. Our findings link the causes of uneven modern species richness with ecologically-driven variation in diversification rates, including 24 detected rate shifts. Speciation rates are a stronger predictor of among-clade richness than clade age, countering claims of clock-like speciation in large phylogenies. Surprisingly, rate heterogeneity in recent radiations shows limited association with latitude, despite the well-known richness increase toward the equator. Instead, we find a deeper-time association where clades of high-latitude species have the highest speciation rates, suggesting that species durations are shorter outside than inside the tropics. At shallower timescales (i.e., young clades), diurnality and low vagility are both linked to greater speciation rates and extant richness. High turnover among small-ranged allopatric species may erase the signal of vagility in older clades, while diurnality may adaptively reduce competition and extinction. These findings highlight the underappreciated joint roles of ephemeral (turnover-based) and adaptive (persistence-based) diversification processes, which manifest as speciation gradients in recent and more ancient radiations to explain the evolution of mammal diversity.
Significance statement The over 6000 living species in the mammalian tree of life are distributed unevenly among branches so that similarly aged groups sometimes differ many fold in species richness (e.g., ~2500 rodent species versus 8 pangolins). Why differential bursts of species diversification occur, and how long they persist, has implications for sustaining biodiversity. Here we develop a robust evolutionary timescale for most extant species, recovering signatures of rate-variable diversification linked to ecological factors. Mammals with low dispersal or that are day-active show the fastest recent diversification, consistent with mechanisms of allopatric speciation and ecological opportunity, respectively. Speciation rates are surprisingly faster in extra-tropical than tropical lineages, suggesting that longer species durations for tropical lineages underpin the latitudinal diversity gradient in mammals.
