Absolute Diversification Rates in Angiosperm Clades

The extraordinary contemporary species richness and ecological predominance of flowering plants (angiosperms) are even more remarkable when considering the relatively recent onset of their evolutionary diversification. We examine the evolutionary diversification of angiosperms and the observed differential distribution of species in angiosperm clades by estimating the rate of diversification for angiosperms as a whole and for a large set of angiosperm clades. We also identify angiosperm clades with a standing diversity that is either much higher or lower than expected, given the estimated background diversification rate. Recognition of angiosperm clades, the phylogenetic relationships among them, and their taxonomic composition are based on an empirical compilation of primary phylogenetic studies. By making an integrative and critical use of the paleobotanical record, we obtain reasonably secure approximations for the age of a large set of angiosperm clades. Diversification was modeled as a stochastic, time-homogeneous birth-and-death process that depends on the diversification rate (r) and the relative extinction rate (epsilon). A statistical analysis of the birth and death process was then used to obtain 95% confidence intervals for the expected number of species through time in a clade that diversifies at a rate equal to that of angiosperms as a whole. Confidence intervals were obtained for stem group and for crown group ages in the absence of extinction (e = 0.0) and under a high relative extinction rate (epsilon = 0.9). The standing diversity of angiosperm clades was then compared to expected species diversity according to the background rate of diversification, and, depending on their placement with respect to the calculated confidence intervals, exceedingly species-rich or exceedingly species-poor clades were identified. The rate of diversification for angiosperms as a whole ranges from 0.077 (epsilon = 0.9) to 0.089 (epsilon = 0.0) net speciation events per million years. Ten clades fall above the confidence intervals of expected species diversity, and 13 clades were found to be unexpectedly species poor. The phylogenetic distribution of clades with an exceedingly high number of species suggests that traits that confer high rates of diversification evolved independently in different instances and do not characterize the angiosperms as a whole.