Plus ça change — a model for stasis and evolution in different environments

The Plus ça change model proposes that morphological stasis is the usual response to widely fluctuating physical environments on geological timescales (until thresholds are reached). The lineages that survive in a more changing environment on geological timescales are those that are relatively inert to each environmental twist and turn, in contrast to more sensitive lineages in less changing environments. Generalists in this long-term sense are species with properties that enable them to survive throughout wide environmental fluctuations over geological timescales; they are thus related to but distinct from ecological eurytopes. The model predicts a tendency for continuous, gradualistic evolution on land in the tropics and in the deep sea, and for more stasis (and occasional punctuations) in shallow waters and temperate zones. As the vast majority of the fossil record comes from dynamic shallow marine environments, it is not surprising that many fossil lineages show approximate stasis and occasional punctuations. Punctuated equilibrium could be being mistakenly perceived as the overwhelming pattern in the history of life because the environments in which gradualism predominates are rarely preserved in the fossil record. The evolutionary pattern in a lineage at a single locality is predicted to be stasis when there is greater environmental fluctuation, and net directional change and/or more widely fluctuating morphologies when environments are more narrowly fluctuating. Given the Quaternary climate upheavals, relatively little evolution may be occurring worldwide at present (except for evolution induced by humans). Such a model is bound to have many exceptions; it is only about tendency and relative frequency. Though few case histories are yet known, many existing data sets probably contain clues that can be used in testing the model. Other relevant aspects of perception are also discussed, including descriptive biases and evolutionary reversals.