TESTING FOR SPECIATIONAL PULSES WITHOUT A PHYLOGENY: APPLICATION TO OSTRACODE SIZE, SHAPE, AND SEXUAL DIMORPHISM

The central claims of the punctuated equilibrium model are that species do not change much once they appear in the fossil record, and that trait changes are concentrated in pulses associated with speciation. The best tests of this model, such as the seminal studies by Alan Cheetham and colleagues, use ancestor-descendant sequences of populations and a phylogeny, to separately estimate anagenetic and speciational components of evolutionary change. Here we consider the more common situation in which a resolved phylogeny is not available. We analyze a dataset of adult carapace size and shape, and the sexual dimorphism of these two measures, estimated from the cytheroid ostracode fauna of the Late Cretaceous of the United States Coastal Plain (95 species in 34 genera). A subset of species was sampled more than once during their history, providing information on anagenetic evolution. We fit Bayesian regression models to estimate trait variance among populations within species, and among species within genera. The former represents anagenetic evolution alone, whereas the latter also includes changes occurring at speciation. The ratio of these two variance components therefore will reflect the relative amounts of speciational compared to anagenetic change. Results suggest that carapace size has experienced more speciational change than shape, and, similarly, that size dimorphism has had more speciational change than shape dimorphism. We also applied a separate approach to capture speciational change by measuring observed differences between pairs of species inferred to have a sister-species relationship. These differences revealed that changes in size and shape associated with the speciation between sister species were within the normal range of within-species variation. However, there were several changes in sexual dimorphism between sister taxa that were well outside within-species limits, suggesting occasional and large speciational pulses, possibly spurred by sexual selection. Finally, we clarify how our taxonomy-based approach can be related to explicit phylogenetic models of speciational change, potentially leading to more straightforward interpretations about the relative dominance of speciational versus anagenetic evolution.