Punctuated Equilibria refer to patterns of evolution characterized by long periods of stasis interrupted by shorter periods of evolutionary change. The changes are associated with speciation events where a new, changed, species splits off from the parental, unchanged, species. This is speciation by cladogenesis (splitting) as opposed to anagenesis, where a single species changes gradually into something different without splitting.
Stephen Jay Gould wrote a massive book on macroevolutionary theory that featured punctuated equilibria and its consequences such as species sorting. The original book was called The Structure of Evolutionary Theory. It's 1433 large pages long. Few people have read the entire thing. I am one.
Recently, the publishers of The Structure of Evolutionary Theory (Belknap Press of Harvard University Press) have brought out a new book by Stephan Jay Gould—a remarkable achievement since Gould died in 2002. As it turns out, the new book Punctuated Equilibrium, is just chapter nine of the large book; a 279 page chapter called Punctuated Equilibrium and the Validation of Macroevolutionary Theory. PZ Myers reviewed this book for New Scientist a few weeks ago [Punctuated Equilibrium by Stephen Jay Gould].
One of the significant impacts of Punctuated Equilibrium is the emphasis on episodic change rather than the gradualism that was so commonly believed to be the main pattern in evolution. Eldredge and Gould explain why gradualism was such a prominent part of evolutionary theory before punctuated equilibrium came along. Gould has extended that explanation in Structure and in numerous essays.
Gould points out that Charles Darwin was a firm believer in gradualism. Indeed, it was an essential component of Darwin's defense of evolution and his promotion of natural selection. Darwin's gradualism arose from his commitment to the uniformitarianism of Lyell and his emphasis on the vast age of the Earth. According to Gould there are several different meanings of gradualism but the one that conflicts with punctuated equilibria is the idea that change must be gradual at geological scales.
This is why Darwin said that nature does not proceed by leaps and it's why Darwin postulated that gaps in the fossil record were due to lack of data.
PZ Myers noted this in his review when he said,
Gould and Eldredge proposed punctuated equilibrium as a paleontologist's view of the history of life: they were describing the paleontological data available at the time pointing out that there was no geological evidence to support Charles Darwin's belief that species evolved gradually. Time has shown them to be correct, and their observations are now accepted by most biologists as a accurate account of evolutionary history.Now, PZ is no dummy. He's been around the blogosphere and newsgroups long enough to know that calling Darwin a gradualist is like waving a red flag in front of a bull. There's a large group of Darwin apologists out there who will do anything to prove that Charles Darwin was right about everything, even if is has to be proved retrospectively (i.e., somehow we didn't notice that Darwin believed in punctuated equilibrium until 1972).
The first salvo was fired in the letters column in this week's issue of New Scientist. Wayne Bagguley writes,
I am shocked that someone as knowledgeable as P.Z. Myers is promoting the age-old myth about "Charles Darwin's belief that species evolved gradually" without periods of stasis. In On the Origin of Species Darwin wrote "Although each species must have passed through numerous transitional stages, it is probable that the periods, during which each underwent modifications, though many and long as measured by years, have been short in comparison with the periods which each remained in an unchanged condition. These causes, taken conjointly, will to a large extent explain why—though we do not find many links—we do not find interminable varieties connecting together all extinct and existing forms by the finest graduated steps."Stephen Jay Gould is no dummy. As an expert on Darwin and the history of biology you can be sure that he's heard these complaints before. You can be certain that Gould has addressed them numerous times.
The longest, and best, defense of Darwin's gradualism can be found in chapter 2 of Structure. This is a 76 page chapter titled "The Essence of Darwinism and the Basis of Modern Orthodoxy." I'm sure PZ Myers has read this chapter and that's why he said what he said. Gould makes the point that,
SInce Darwin prevails as the patron saint of our profession, and since everyone wants such a preeminant authority on their side, a lamentable tradition has arisen for appropriating single Darwinian statements as defenses for particular views that either bear no relation to Darwin's own concerns, or that even confute the general tenor of his work.... I raise this point here because abuse of selective quotation has been particularly notable in discussion of Darwin's views on gradualism. Of course Darwin acknowledged great variation in rates of change, and even episodes of rapidity that might be labelled catastrophic (at least on a local scale); for how could such an excellent naturalist deny nature's multifariousness on such a key issue as the character of change itself? But these occasional statements do not make Darwin the godfather of punctuated equilibrium ....You'll have to read Gould's chapter to see the case he makes because it's much too involved to summarize here. I do note one pithy comment to the effect that,
I will not play 'duelling quotations' with 'citation grazers,' though a full tabulation of relative frequencies could easily bury their claims under a mountain of statements.He then proceeds to tabulate dozens of quotations that demonstrate Darwin's gradualism!