Elizabeth Vrba, a South African paleontologist, coined the term “turnover pulse.” The idea is that most species most of the time are in an equilibrium with their physical and biotic environments, and not changing much. But every once in a while an environmental disturbance comes along, resulting in large scale extinctions, and a pulse of speciation. Most evolutionary change, on this theory, occurs during these pulses. Vrba saw evidence of a pulse 2.5 million years ago among South African antelopes (affecting specialized more than generalized feeders). The pulse was associated with a shift toward cooler weather, and a shrinking of forests and expansion of grasslands. Part of the turnover pulse hypothesis is that the same pulses should affect many different species at the same time: there is arguably a turnover among hominins as well, giving rise to a later generation of savannah-adapted australopithecines.
Vrba was one of a circle of paleontologists pushing the idea of punctuated equilibrium ¬– that during most of their existence species don’t change much (stasis), and evolutionary change is concentrated in the times and places when a new species branches off from another. Steven Jay Gould and Niles Eldredge were others pushing the idea. There are various possible explanations for why we might see a combination of stasis and punctuated equlibrium (assuming the theory is true in the first place). Environmental controls, as suggested by Vrba are one possibility. Or maybe species’ developmental systems are tightly integrated and resistant to change. Or (for sexually reproducing species) the need to find a similar-enough mate might foster stasis most of the time.
We’ve seen some enormous episodes of mass extinction before on Logarithmic History. The turnover pulse hypothesis implies that the same phenomenon on a smaller scale is behind most evolutionary change. Not everybody buys this though. Punctuated equilibrium is a hugely contentious topic. And plenty of evolutionary biologists think that arms races among (and within) species keep evolution running along pretty constantly even without environmental changes. We’ll see some of the same issues – punctuation versus gradualism, ecological forcing versus internal dynamics driven by arms races – when we take up human history.