The Plus ça Change Model: Explaining stasis and evolution in response to abiotic stress over geological timescales

Summary

The Plus ça change model addresses the surprising finding that morphological stasis seems to be the usual response to widely fluctuating physical stresses over geological timescales (until thresholds are reached). The lineages that survive in an environment of fluctuating sea levels, climate, substrate, salinity and so on appear to be 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 fluctuations in the physical environment for millions of years. 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. Punctuated equilibrium may be mistakenly perceived as the usual pattern in the history of life because the environments in which gradualism predominates are rarely preserved in the fossil record. Given the Quaternary climate upheavals, relatively little evolution may be occurring worldwide at present compared with, say, 3 million years ago (except for evolution associated with human activity).