QUANTIFYING EVIDENCE FOR PUNCTUATED EQUILIBRIA WITH PROBABILISTIC PHYLOGENETICS AND THE ECHINODERM FOSSIL RECORD

The apparently rapid taxonomic and morphological diversification of echinoderm higher taxa played an important but often forgotten role in Eldredge and Gould’s (E&G) classic paper on punctuated equilibria (PE). Citing the vast disparity between the large number of class-level taxa with a dearth of species-level diversity, E&G claimed “to many paleontologists, nothing is more distressing than the current situation in echinoderm systematics” (E&G, 1972, p. 110). In their view, the discrepancy between early echinoderm diversity and disparity was inconsistent with a “stately unfolding” pattern of clade diversification implied by gradualism, which they argued required 1) concordance between temporal patterns of diversity and disparity, and 2) an extrapolation that their common ancestor would have lived “uncomfortably” deep during the Precambrian. However, E&G surmised their theory resolved this apparent discrepancy because PE provided a framework for speciation patterns to be both rapid and episodic, and therefore “echinoderm evolution becomes more intriguing than bothersome” (E&G, 1972 p. 110). Over the last 50 years, the validity of PE has been intensely debated, yet only recently have these debates incorporated quantitative frameworks for testing alternative hypotheses, and few have explicitly use phylogenetic approaches in the spirit of Eldredge’s (1971) initial empirical insight into PE. In this talk, I develop a stepwise, quantitative approach to empirically test the predictions of punctuated equilibria using probabilistic phylogenetic methods, and apply this approach to six taxonomically and stratigraphically vetted species-level datasets of Paleozoic echinoderms. Specifically, I quantify evidence for three key predications of PE theory, including the frequency of “budding” speciation vs. anagenesis, the proportion of a species’ history in stasis vs. change, and whether character models featuring punctuated change concentrated at speciation events are favored over standard Mk model assumptions assuming more gradual, continuous change.