QUIT TELLING EVERYONE I’M DEAD! PUNCTUATED EQUILIBRIA REMAINS A VALID OBSERVATION IN MODERN ANALYSES USING A PERSISTENCE-OF-ANCESTOR CRITERION

The hypothesis of punctuated equilibria (PE) has in important respects transitioned over the past half-century from unexpected by neontologists, to intuitively obvious and trivial, to its current status of widespread but partial acceptance and deep misunderstanding. While stasis is widely acknowledged, the overall theory is commonly described in both technical and popular literature as incorrect, moribund, or irrelevant, and two “classic” examples have recently been challenged: the Neogene bryozoan Metrabdotos and Pleistocene landsnail Poecilozonites, although only the Poecilozonites critique properly conceptualizes the hypothesis. We suggest that, despite 50 years of discussion, at least one reason for this lack of acceptance and understanding lies in persistent lack of agreement of key terms and concepts, especially “tempo”, “mode”, “model”, and “mechanism”. Clarification of these leads us to emphasize the two central empirical tests of PE: the prevalence of stasis and the persistence of ancestors.

Punctuated equilibria is a hypothesis stating that the majority of fixed morphology changes occur in association with cladogenesis (speciation), and that the dominant pattern observed within lineages follows a stasis model. Whether an unbranching lineage shows stasis, a directional trend, or a random walk is a question about model, whereas whether distinct forms arise by anagenesis or cladogenesis is a question about mode (topology of lineages), and questions regarding rates of change are questions of tempo. Numerous studies have demonstrated a numerical dominance of stasis over alternative models.

We examine the question of whether most morphological distinct lineages are the result of cladogenesis by examining several of our own recently completed phylogenies of Cenozoic gastropods as well as all species level phylogenies published in Paleobiology and Journal of Paleontology in the past 3 years which include stratigraphic range information. Persistence of ancestor is used as the criterion for discrimination between punctuated anagenesis and cladogenesis. PE is well supported in most cases examined: most distinct species-level lineages form by cladogenesis, as demonstrated by persistence of ancestors. PE is not dead and should be at the center of macroevolutionary understanding.