VOLATILITY AND THE PHANEROZOIC DECLINE OF BACKGROUND EXTINCTION INTENSITY

The well-known decline of global background extinction intensity was caused by the sorting of higher taxonomic groups. Two factors were responsible. First, probabilities of familial origination and extinction in these groups (taxonomic orders) were highly correlated. Groups whose families had high probabilities of origination and extinction tended to have highly volatile diversity paths and, consequently, short life spans. Second, orders with high probabilities of familial origination and extinction were rarely replaced by new high-turnover orders. Thus, because high-turnover orders tended to become extinct without replacement, the global background extinction intensity declined. Since familial origination and extinction probabilities are correlated, global background origination intensity inevitably declined as well. As a consequence of these processes, virtually all groups of organisms now living have low probabilities of familial origination and extinction. Simulations of branching evolution were used to obtain the expected relationships among probabilities (of origination and extinction), volatilities, and longevities for the entire range of possible probabilities, and these relationships were compared to those obtained from the empirical record. In the simulations, only the probabilities of origination and extinction were specified, so volatilities and clade longevities were determined entirely by the probabilities. The similarity between results obtained by simulation and those obtained by analysis of the empirical record further supports the inference that the observed decline of background extinction (and origination) intensity can be explained largely by the loss of high-probability groups to induced volatility.