Species selection and driven mechanisms jointly generate a large-scale morphological trend in monobathrid crinoids

Abstract

All evolution attributable to natural selection, at any level, is due to a causal covariance between fitness and phenotype. Over macroevolutionary time scales, species selection is one of many possible mechanisms for generating large-scale morphological trends. For species selection to sort morphology, a correlation between morphology and taxonomic diversification rate must be present. Other trend mechanisms (driven mechanisms, e.g., a bias in the direction of speciation) produce a systematic change in the mean phenotype over time. All mechanisms can co-occur. Here I demonstrate (1) an inverse correlation between diversification rate and calyx complexity that demonstrates the effect of species selection on morphology. Genera with simple calyces tend to increase in diversity, whereas genera with complex calyces have a net decrease in diversity; and (2) the presence of a driven trend mechanism in monobathrid crinoids where descendant genera tend to be simpler than their ancestors. The separate effects of these two classes of trend mechanisms can be combined by using the Price's Theorem, which partitions the contribution to the overall change in calyx complexity over time accurately among selection and driven mechanisms. Price's Theorem provides significant conceptual and methodological clarification of the contribution of multiple and interacting hierarchical mechanisms in generating large-scale trends.

Persistent predator–prey dynamics revealed by mass extinction

“…Camerates exhibit an abrupt decline beginning at the Tournaisian-Viséan boundary (19,20,24). Whereas the overall camerate curve could not be distinguished from a random walk (runs test P = 0.07) because of the Tournaisian-Viséan inflection point ( Fig.…”

“…Camerates tended toward greater numbers of small, thickened plates and increased spinosity during the Devonian (6,20,24). In contrast, camerates with simplified, unspined calyces made of larger plates were favored in the later Paleozoic (20). The reversal is coincident with a Tournaisian-Viséan inflection point for camerate diversity and has been shown to result at least partially from species-level selection (Fig.…”

“…2 and SI Appendix, Supporting Methods and Discussion), the decline could not be directly tested owing to the small number of data points. However, it has long been known that camerates experienced high extinction rates in the later Mississippian and were reduced to just a few lineages by the Pennsylvanian (19,20,25). This trend has been attributed to environmental changes favoring advanced cladids (12).…”

“…Crinoid defensive structures consist of calcified plates forming a calyx (29). Camerates tended toward greater numbers of small, thickened plates and increased spinosity during the Devonian (6,20,24). In contrast, camerates with simplified, unspined calyces made of larger plates were favored in the later Paleozoic (20).…”

“…Predation has been raised as a possible driver of these crinoid trends but not tested quantitatively (1,9,20,21). It is known that vertebrates did consume crinoids at least intermittently during the Devonian-Carboniferous, on the basis of stomach contents, coprolites, and damage to both crinoids and other benthic invertebrates (12,17,22,23).…”