For maybe a century after Darwin proposed his theory of evolution by natural selection, the conventional wisdom was that this occurred through gradualism; that is, over the course of generations, a given species will slowly change and one day you look around and it's a different species. And the species it was before, is gone.
Problem: The fossil evidence for this was pretty thin. The response was that the fossil record just had too many gaps to properly catch the intermediate stages. In 1972, Gould and Niles Eldridge instead proposed the explanation of punctuated equilibrium. This argued that most species tended to remain the same for long periods, and then part of the population, usually isolated in some ways, shift to a new species in a short (in geologic terms) time period. The original species still exists alongside the newcomer, for a time.
This idea was apparently met with much harrumphing for at least a decade or two, but by 2007, when this book was published, it was gaining wider acceptance as the evidence to support it mounted. This book, which seems to be Gould covering the history of the concept, the arguments against it and the studies that investigated those claims, is strongest when it sticks to discussing actual research done to test the ideas of punctuated equilibrium and gradualism against the fossil record. He cites a lot of studies, done with a variety of phyletic groups, and he's usually able to summarize the findings in a clear manner, with accompanying charts to help the reader visualize. He makes it clear that he and Eldridge were never arguing that gradualism doesn't happen, only that it's far less common a driver of speciation than punctuated equilibrium. So when a study does show signs of speciation through gradualism, or at least, doesn't show signs it was through punctuated equilibrium, he makes sure to mention it.
One bit I found interesting was, Gould feels the idea of punctuated equilibrium has gotten people to place more importance on the notion of "stasis." Before, when it was all about gradualism, a species showing no change in the fossil record was treated as no data at all. If 39 of 40 species showed no change within a fossil formation and 1 showed gradualism, the paper would only discuss the 1 and ignore the 39. Gould and Eldridge argued lack of change is data, and needs to be considered as such. It's like how, if you decide to test the effect of substance x on substance y, your null hypothesis is probably, "there will be no effect." If that's the result you get, that isn't nothing. Now you know x does not affect y. If you were thinking x was responsible for a particular reaction or property observed in y, well something else must be responsible.
Likewise, one of the things I learned in college biology classes was the environment an organism (or species) exists in exerts selective pressure on the individual (or species.) Certain traits are going to convey an advantage for survival and reproduction. But then we have these species that their physical characteristics remain largely static (meaning the range of values remain largely the same) for millions of years. Why wasn't the environment exerting a pressure that pushed those characteristics one way consistently? A lot of the studies show the values fluctuate across that range over time, but stay around the same mean, until a sudden, relatively quick shift marks a speciation event.
All that said, the book can be a tough read. Largely because Gould won't say in 3 words what he can say in 40. There are sometimes multiple commas, a semi-colon and even a parenthetical, all in one sentence. It's exhausting and a little befuddling, in that the sentence becomes just a jumble of words where I've entirely lost the point he was driving at. He's worst about that when focused more on theory or controversy, which is part of why I prefer the sections where he describes actual research. Having data to explain seems to focus his writing. Either way, this does not feel like a book written for a general audience.
'We can distinguish the punctuations of rapid anagenesis from those of branching speciation by invoking the eminently testable criterion of ancestral survival following the origin of a descendant species. If the ancestor survives, then the new species has arisen by branching. If the ancestor does not survive, then we must count the case either as indecisive, or as good evidence for rapid anagenesis - but in any instance, certainly not as evidence for punctuated equilibrium.'
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