What explains patterns of biodiversity across the Tree of Life? New research is revealing the causes of the dramatic variation in species numbers across branches of the Tree of Life

A major challenge in biology is to explain why some groups have thousands or millions of species whereas others have few. Here, I review the causes of this variation. New studies reveal that higher species numbers in many major groups are explained by higher diversification rates (and traits that accelerate these rates). These traits span most of biology (e.g. genomics, ecology, morphology). Rather than simply testing individual traits, research should now focus on comparing how much variation in diversification rates is explained by different types of traits. For example, is local-scale ecology (e.g. microhabitat, diet) more important than large-scale climate (e.g. occurring in tropical vs. temperate regions)? Are traits based on particular values (e.g. smaller body sizes) more important than those based on rates of change (e.g. faster size evolution)? I review recent results on the relative importance of different traits for driving diversification, and present a framework for future research.