MASS EXTINCTIONS AND MACROEVOLUTION: BLOOD, SWEAT, AND TIERS

Steve Gould saw the major mass extinctions as a third macroevolutionary tier with its own “predominating causes and patterns,” so that these rare, intense events tend to derail evolutionary patterns shaped by organismic selection, speciation, and species sorting. This view does not challenge the efficacy of natural selection, but the smooth extrapolation of short-term, population processes as a full explanation of macroevolutionary phenomena. The apparently unimodal size-frequency distribution of extinction intensities among stratigraphic stages need not undermine the proposed evolutionary role of mass extinctions. As noted by several authors, removing the secular decrease in background extinction rates, by omitting Cambrian stages or via residuals analysis, leaves the Big Five extinctions as a discrete cluster outside the “background” distribution. More importantly, nonlinearities or thresholds in the biotic effects of extinction events can negate any apparent continuity in extinction intensities by imposing novel macroevolutionary regimes. Such effects are poorly understood – and thresholds almost certainly differ among clades – but are evident in the shift of extinction selectivities during mass extinctions. These shifts can include changes in the hierarchical level of survival-enhancing geographic ranges, with species-level ranges becoming unimportant relative to those of clades, whereas the opposite is true at other times. Mass extinctions mainly appear to fit a “nonconstructive selectivity” model, with survivorship difficult to relate to most aspects of organismic phenotypes, but still following discernable rules. Many evolutionary trends end at extinction events, but others persist, with or without setbacks. Large-scale or high-level trends tend to be buffered whenever extinction is random at the species level, and the result can be a ratcheting effect where net morphological changes accrue even when species losses are high, whereas selective extinctions more severely clear regions of morphospace. Sorting of clades and morphologies in the post-extinction recovery phase is also important: not all survivors rise to, or maintain, prominence in the post-extinction world, but little is known of ecological, biogeographic or evolutionary processes that operate during recoveries.