TESTING PUNCTUATED EQUILIBRIUM IN THE OSTRACODE GENUS POSEIDONAMICUS

Punctuated Equilibrium makes two central claims: (1) fossil species tend to show little net change over time, and (2) changes in morphology are concentrated into pulses associated with cladogenesis. These claims are not on equal empirical footing. Paleontologists have documented enough cases of within-lineage trait evolution to conclude that gradual, directional change over paleontological timescales is not common. Instead, most traits show meandering or oscillating change, consistent with stasis as broadly construed. The second claim, that changes are concentrated at speciation events, requires measurement of anagenetic and cladogenetic changes in a phylogenetic context. Far fewer fossil studies bear directly on this issue, though those by Cheetham and colleagues on the bryozoan genus Metrarabdotos are notable exceptions. Here I present a case study of similar scope and structure with an analysis of the deep-sea ostracode genus Poseidonamicus. Evolution is modeled with anagenetic, cladogenetic and (optionally) geographic components. Repeated samples within lineages, when coupled with a phylogenetic hypothesis for the clade, allow estimation of the separate contribution of these components. Simulations indicate that parameters of this model can be estimated successfully via maximum likelihood. Analysis completed thus far with data from Poseidonamicus indicate that parameter estimates can be sensitive to hypotheses about species limits and the nature of geographic variation. Nevertheless, these model fits are not consistent with purely cladogenetic change: for the shape features analyzed, the anagenetic component of change is always substantial.