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Keywords:

  • Punctuated equilibrium;
  • speciation;
  • Poseidonamicus ;
  • anagenesis;
  • cladogenesis

Summary

  1. The punctuated equilibrium model predicts that phenotypic change is concentrated into pulses associated with speciation, with little change otherwise accruing in established lineages. Palaeontological tests of this model have generally lacked an adequate phylogenetic and modelling framework, whereas tests relying on extant populations lack direct constraints on the evolutionary dynamics within lineages.
  2. This study extends a modelling approach developed in comparative studies and applies it to a clade with a rich fossil record, the deep-sea ostracode genus Poseidonamicus. Using a phylogenetic framework and an independent set of shape traits plus body size, a model was fit that allows estimation of anagenetic (within-lineage) evolution, cladogenetic (speciational) change and geographic variation within species.
  3. Maximum-likelihood parameter estimates suggested dominantly speciational change for only one or two shape traits, depending on model assumptions. For the remaining shape traits and body size, the contribution of anagenesis was always substantial. Confidence limits on these solutions were quite broad (although narrower when multiple traits were analysed jointly), with most traits consistent with both strongly anagenetic and strongly cladogenetic change.
  4. Whereas uncertainty about phylogenetic topology and species limits has little influence on the conclusions, assuming stasis instead of Brownian motion within lineages shifted support to solutions in which speciational change was more dominant, although several traits remained dominantly explained by anagenetic evolution.
  5. These results suggest that for the traits and taxa examined, anagenesis contributes substantially to long-term divergence. The uncertainty in the results highlights the analytical difficulty of decomposing anagenetic and cladogenetic sources of phenotypic evolution, even with fossil constraints. When model uncertainty is taken into account, the task of doing so using observations from entirely extant populations is even more daunting.