The dramatic effects of the mass extinctions recorded in Earth's geologic record have led many geologists to view them as a distinct class of events -- so powerful that they swamp out normal evolutionary processes. Others have argued the opposite, that over geologic time, mass extinctions have had little effect on global diversity trends.
University of Cincinnati geologist Arnold Miller argues in a review paper in the August 21 issue of Science that the truth is somewhere in the middle of those extreme views. Miller's evaluation of global marine diversity patterns through the Phanerozoic, in light of recent research, indicates that mass extinctions are simply the most globally extensive of a continuum of abrupt transitions that combine to affect overall global diversity. The Phanerozoic covers the pasts 540 million years.
That view is in contrast to Stephen Jay Gould and others who have viewed mass extinctions as an over-arching tier of evolutionary processes, distinctly different than the processes which occur in the intervening time periods.
"While I agree that mass extinctions are the largest and most important of a class of 'catastrophes,' I would argue that they are not fundamentally different as an evolutionary mechanism from what causes diversity change in the intervals between them," said Miller.
"The so-called background intervals are also built of catastrophes that are more local or regional in scope, but can have the same profound effect on a local biota that a mass extinction has on a global biota."
As an example of an abrupt regional change, Miller discusses the diversification of a group of organisms known as "biological bulldozers," which dig into deposited sediments to find food.
The geologic record includes cases where the bulldozers replaced another group known as ISOSS (immobile suspension feeders on soft substrates). Previously, researchers believed the biological bulldozers out-competed the ISOSS organisms. Miller suggests an alternative explanation.
"Many deposit-feeders favor muddy, nutrient-rich sediments as a substrate to occupy. Just such sediments were provided during key intervals as a consequence of eroding areas uplifted during mountain-building. Quite apart from the action of the bulldozers, many of the incumbent immobile types would not have liked this kind of habitat.
So I am suggesting, as an alternative to the above 'competitive' scenario, that the transition happened because of major, physical changes to habitats."
In fact, previous research reported by Miller and others on the Ordovician Period showed that diversification of certain groups was favored in areas where active mountain-building took place.
As for those who argue that mass extinctions are actually minor contributors to global diversity patterns, Miller has used his database of Ordovician fossil occurrences to demonstrate that regional and local bursts of diversification occur at different times around the world.
Over time, the bursts add up to present what Miller believes is a misleading picture of diversification trends a smooth, gradual change over geologic time.
"The global pattern does not match any localized pattern," said Miller. "The global diversity trend is misleading for what it says about the rates of transition and perhaps even for what causes them."
Miller's research is supported by NASA's Program on Exobiology.