Volatility and the Phanerozoic decline of background extinction intensity

Abstract

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.

Biotic Transitions in Global Marine Diversity

“…Gilinsky (19) found empirical support for this prediction. A clade with a higher turnover rate is more volatile and is therefore more likely to become extinct as a result of stochastic fluctuations in diversity.…”

“…For example, comparative familial extinction and origination probabilities exhibited by orders of trilobites-the major taxonomic contributors to the Cambrian fauna-varied by as much as a factor of 4 (19). Indeed, Ordovician trilobites might be appropriately viewed as comprising two distinct macroevolutionary biotas (21).…”

Survival without recovery after mass extinctions

“…Taking the Paleozoic extinctions alone, distributions are significantly different when the immediate postextinction intervals are compared with the other nonextinction stages, despite the lower totals (P ϭ 0.005). The immediate postextinction biotas in the Paleozoic included a pool of more volatile taxa (6), and this may have rendered the short-term sorting of DCWs more detectable at the ordinal level. However, the fact that Paleozoic taxa tended to be shorter ranging does not explain their significant concentration in the three postextinction stages relative to the rest of the Paleozoic.…”

Testing for Ecological Limitation of Diversification: A Case Study Using Parasitic Plants

“…Rabosky (2009b) used a simulation approach to test three possible explanations for why a positive clade age-to-clade diversity relationship might break down: (1) extreme variation in diversification rate among lineages; (2) clade volatility (Gilinsky 1994), in which extinction rate covaries with speciation rate; and (3) density-dependent diversification (Sepkoski 1978;Nee et al 1992), in which diversification rates decrease as a function of the number of species within a clade. In Rabosky's simulations, a positive clade age-to-clade diversity relationship broke down only under density-dependent simulations.…”