The rise, decline and fall of clades

How and why species diversity varies over geological time scales remains disputed. We analyzed the evolutionary history of 27 radiations with phylogenetic information incorporating extinct and extant species under a new ‘fossilized birth-death diffusion’ model that provides a detailed characterization of past diversification and resulting diversity dynamics. Clade diversity trajectories follow rise and decline dynamics, with fast accumulation following recurrent speciation while slowdowns and losses are modulated primarily by changes in extinction. Diversity dynamics do not appear to be governed by clade-level processes expected from adaptive radiations or diversity equilibria. Rather, these patterns emerge from dynamics at the species-level, where lineages tend to become increasingly vulnerable to extinction and less likely to speciate with time. Those species that counteract this trend create and maintain biodiversity through deep time.