Trait-based extinction catches the Red Queen napping during the Cambrian.

Abstract

The tenuous balance between speciation and extinction governs the rise and fall of diversity within clades, from which have emerged the sweeping changes in Earth’s standing biodiversity since life’s origin (1, 2). In a clade where speciation and extinction are equally likely, net diversity remains constant; when the rate of speciation exceeds extinction, diversity increases exponentially; and when the chance of extinction outweighs speciation, then diversity falls, resulting ultimately in the demise of the clade. Metazoan life diversified rapidly in the Cambrian and Ordovician periods (542–443 Mya), after which short-term extinction events and bursts of radiation overshadowed long-term increases in diversity (3, 4). This pattern suggests that rates of speciation exceeded extinction during the first phase of metazoan diversification, after which the balance shifted back and forth. A long-standing question is whether clades that have successfully diversified have done so by evolving new traits that confer an advantage allowing them to outperform or outcompete other groups, either by winning battles in evolutionary arms races or otherwise adapting faster than the Red Queen’s pace of constant extinction expected in resource-limited environments (5, 6). In PNAS, Wagner and Estabrook (7) ask whether diversification was associated with the evolution of new traits and, if so, did the probability of speciation go up in clades that have the new trait, did the group’s rate of trait evolution go up, did extinction go up in the clades that do not have the trait, or some combination of these. They analyzed 319 trait datasets from metazoan phyla with good fossil records—arthropods, brachiopods, molluscs, echinoderms, and chordates—sampled from the early radiation in the Cambrian to the present day. They found that diversification was linked to trait evolution in all post-Cambrian clades, but not in the earliest trilobites, and that it was linked to an increase in extinction among the clades that did not evolve new traits.