ENVIRONMENTAL STASIS AND VOLATILITY: DRIVERS OF ECOLOGICAL-EVOLUTIONARY PATTERN AND MACROEVOLUTIONARY PROCESS

The pattern of coordinated stasis (Brett and Baird 1995), based originally on the mid-Paleozoic marine record of eastern North America, describes morphological and ecological stability in basin-wide faunas over extended intervals of geologic time (10⁵ to 10⁶ yrs). These “ecological-evolutionary subunits” (EESUs) are separated by episodes of rapid faunal change (10³ to 10⁴ yrs) that include extinction, migration and speciation as well as ecological reorganization. Akin to Vrba’s (1982) Turnover Pulse Hypothesis, the pattern extends the predictions of punctuated equilibrium in single lineages to groups of co-occurring lineages, and indicates a major role for environment in driving both stasis and synchronous change.

This empirical pattern, if common, places important constraints on where and when evolutionary change occurs. The absence of significant change in a majority of lineages over long time intervals and across their biogeographic range indicates that phyletic change is negligible during most of geologic time. A combination of stabilizing selection, habitat tracking, and incumbency likely maintains morphological stasis during these times, despite significant oscillations in sea level and environmental shift. As many taxa exhibit basin-wide stasis, speciation is probably occurring in small, locally isolated populations that are seldom preserved in the rock record, and mainly during brief times of regional environmental change, consistent with punctuated equilibria.

Newly developed high-resolution chronostratigraphy reveals the tempo and environmental framework within which long-term stasis and rapid evolutionary and ecological change occurs in the mid-Paleozoic. EESUs persist longest during intervals of comparatively low-amplitude variation in climate, sea level, and the carbon cycle, as reflected in the stable isotope records. Comparatively abrupt, clustered, fluctuations in climate, often associated with hypoxia and sea-level rise, may drive regional or global benthic assemblage turnovers, migrations, and speciation. Associated biozonal durations are short, suggesting similar evolutionary and biogeographic responses in the nekton as well. These patterns suggest that volatility in the global ocean-climate system is a primary driver of macroevolution.