Loss of macroevolutionary species fitness explains the rise and fall of clades.

How and why species diversity varies over geological timescales remains disputed. Debate revolves around the existence of equilibrium dynamics, the predominance of adaptive radiations and the relative importance of speciation and extinction in driving diversity trajectories. We analyse the evolutionary history of 27 radiations of plants, arthropods and vertebrates, 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. Here, lineages undergo speciation and extinction rates that diffuse continuously in time and generate fossils with rates that can vary with stratigraphy. 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 dependence. 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. The rise and fall of clades results from species-level fates.