杏吧原创

I WELL remember the summer I read The Structure of Evolutionary Theory. It was 2002, Stephen Jay Gould had just died, but he had left behind this magnum opus, a substantial work that summarised and synthesised his view of evolution. I purchased a copy right away. As soon as my teaching duties ended for the summer, I settled down to enjoy the wisdom of a master of science writing, and drink deeply of his ideas.

I got a rude shock. Not only was the book huge, but Gould鈥檚 rhetorical excesses had been left unchecked 鈥� the parenthetical asides (and parentheses inside parentheses), the long-winded expositions explaining the rationale for what he was about to say, the repetitiveness. The book would have benefited greatly from an editor鈥檚 ruthless hand. I struggled with it for months, but I was well rewarded: there are great and provocative ideas buried in the wordiness, as well as long passages of lucid description. My regret, though, was that the book would not be read by many, that the daunting task of ploughing through more than 1300 pages of evolutionary detail would leave it to gather dust on people鈥檚 bookshelves.

Now, in a brilliant move, Belknap Press has posthumously extracted a single chapter 鈥� number nine 鈥� from The Structure of Evolutionary Theory and published it as a stand-alone book, Punctuated Equilibrium. It鈥檚 a testimony to the density of the work that a single chapter is sufficient to make a complete and thorough book on its own. The publisher has simply cut away the first 745 pages and the last 318 of the original. What鈥檚 left is a text that is sharply focused on the theory for which Gould and his colleague Niles Eldredge are best known. It works beautifully.

Even in this more digestible form, Punctuated Equilibrium is far from light reading. Gould documents the evidence for his controversial theory and its implications in impressive detail. The book is rich in data and dense in theory, representing a powerful summary of the arguments. In short, punctuated equilibrium is the idea that evolution in many (but not all) lineages has been marked by long periods of stasis, interspersed with relatively brief periods of more rapid change when new species arise. Gould estimated that the period of rapid change when a new species is born accounts for about 1 per cent of its total lifespan. If the average stable lifespan of a species is a few million years, then its period of development 鈥� the punctuation 鈥� would have lasted a few tens of thousands of years. While that is not long on a geological scale, it is still longer than the history of human civilisation.

Gould and Eldredge proposed punctuated equilibrium as a palaeontologist鈥檚 view of the history of life: they were describing the palaeontological data available at the time, pointing out that there was no geological evidence to support Charles Darwin鈥檚 belief that species evolved gradually. Time has shown them to be correct, and their observations are now accepted by most biologists as an accurate account of evolutionary history.

This was not always so, of course. When they introduced the theory in 1972, it provoked howls of protest. Critics pronounced them immodest and presumptuous 鈥� not an unfair accusation, since Gould was both. Their idea was considered trivial, heretical or unsupported by evidence, or dismissed as a descriptive account of palaeobiology without theoretical importance. It was also appropriated gleefully by the creationists, who claimed (they still do) that punctuated equilibrium was a post-hoc rationalisation for the absence of transitional forms in the fossil record, and that palaeontology actually supported their belief in the sudden, divinely inspired appearance of species.

鈥淐ritics pronounced the ideas immodest and presumptuous鈥�

The confusion was compounded by the fact that even some of the grand figures of biology misunderstood it. Zoologist and evolutionary biologist Ernst Mayr equated it with the German geneticist Richard Goldschmidt鈥檚 鈥渉opeful monster鈥� hypothesis, in which novel forms arise by great leaps in a single generation. You can read an entertaining account of these early controversies in the book鈥檚 appendix, entitled: 鈥淎 largely sociological (and fully partisan) history of the impact and critique of punctuated equilibrium鈥�. Setting aside the graphs and tables usually found in an appendix, Gould describes how his theory was gradually taken up by the wider public. He also gives his views on those of his colleagues whose opinions, he wrote, 鈥渋n my judgment are not based on logical or empirical argument, but rather on personal feelings spanning the gamut from appreciation to bitter jealously and anger鈥�. The book is worth buying for the appendix alone.

While the descriptive element of punctuated equilibrium is largely uncontroversial today, the theory鈥檚 implications still kindle some fiery reactions. Gould didn鈥檛 hesitate to fan the flames. He argued that punctuated equilibrium鈥檚 most important contribution was that it defined species as distinct, individual elements, 鈥渆stablishing the basis for an independent theoretical domain of macroevolution鈥�. This is quite different to the Darwinian theory of gradualism, which states that species accumulate change over time until at some blurry point they have become sufficiently unlike previous forms to be dubbed new species. Punctuated equilibrium, on the other hand, regards a species as a discrete entity with a beginning and end.

Gould鈥檚 thinking had a significant impact on theories of macroevolution 鈥� evolution that takes place at or above the level of species. Macroevolutionary theory sets out to explain the large-scale patterns of evolutionary history. Speciation 鈥� the formation of new species 鈥� is a macroevolutionary concept, as is extinction. Population genetics and microevolutionary theory can explain the processes within a single lineage, but they are not sufficient to account for the varying degrees of success of different species derived from a common ancestor. For instance, we cannot explain why chimpanzees are close to extinction while humans swarm in their billions by comparing the fitness of alleles.

Gould, in his typically immodest way, suggested that the theory of punctuated equilibrium could tell us about much more than the rate of evolution, and that it pointed to a whole new hierarchy of evolutionary phenomena. He proposed that the discipline of evolutionary biology should be expanded to accommodate new ideas that he, in part, had established. Inevitably that raised hackles. Yet critics and proponents must read his ideas. This sharp, detailed extract from his last great work offers an essential summary.