Variations in the origination and extinction rates of species over geologic time often are linked with a range of factors, including the evolution of key innovations, changes in ecosystem structure, and environmental factors such as changes in climate and physical geography. Before hypothesizing causality of a single factor, it is critical to demonstrate that the observed variation in diversification is significantly greater than one would expect due to natural stochasticity in the evolutionary branching process. We use a likelihood ratio test to compare taxonomic rate heterogeneity to a birth-death model, using data on well-supported sister pairs of taxa and their species richness. Simulations indicate that this metric has greater power and more accurate Type I error rates than previous relative rate tests using sister taxa. We assess the likelihood that the distribution of extant species among angiosperm genera and families could be the result of stochastically constant diversification rates. Results strongly support the conclusion that there is significantly more heterogeneity in diversification at the species-level within angiosperms than would be expected if the process occurred at a constant, stochastically varying rate. This result is consistent in independent data sets of genus pairs (S Li = 203.543, d.f. = 146, p = 0.0012) and family pairs (S Li 77.527, d.f. = 44, p = 0.0013) and is not affected significantly by degrading pairs to simulate inaccuracy in the assumption of simultaneous origin of sister taxa. When we parse taxon pairs into higher groups of angiosperms, results indicate less non-stochastic rate heterogeneity within rosids and basal eudicots than within asterids and eumagnoliids. These results provide strong support for the hypothesis that species-level rates of origination and/or extinction have varied non-stochastically within angiosperms, and that the magnitude of heterogeneity varies among major groups within angiosperms.

Key words: angiosperms, diversity, evolutionary rates, likelihood, sister pairs