PUNCTUATED EQUILIBRIUM: THE STATE OF THE EVIDENCE

The central claim of Punctuated Equilibrium is that species don’t tend to change much once they appear in the fossil record, and that the changes that do occur are disproportionately associated with speciation. When Eldredge and Gould made this suggestion 50 years ago, there were almost no datasets of sufficient quality to test it, but the controversy that ensued spurred paleontologists to begin to collect data that could be used to test the predictions of this model. The goal of this presentation will be to review that state of this accumulated evidence with the particular aim of evaluating the central claim of Punctuated Equilibrium as articulated above. The large literature documenting and modeling phenotypic change within lineages has demonstrated that stasis is commonly observed, though other patterns of change do occur with substantial frequency. Whether speciation is associated with pulses of morphological change is rather harder to judge, even in the present day. Studies that combine time-series of traits within lineages with phylogenetic divergences are the best way to make progress on this issue, but such studies are surprisingly few. Understanding the microevolutionary process that underlie paleontological patterns will always be challenging, but the originally proposed mechanism for punctuated equilibrium – the breakdown of genetic homeostasis at speciation – has not fared well. Stabilizing selection is likely to be operating on traits that show stasis, but other processes, including genetic constraint may contribute to observations of stasis as well.