The punctuated equilibrium debate is one of the few controversies within evolutionary biology that has come to popular attention. No doubt this is largely a result of the popular writings and high profile of Stephen Jay Gould, the theory's leading proponent. The Dynamics of Evolution, coming twenty years after the paper by Gould and Eldredge that originally sparked the debate, is a retrospective evaluation of the theory and the surrounding debate, pitched at a level that makes it approachable by anyone with a general background in evolutionary biology. (Perhaps someone who has read the popular writings of Dawkins and Gould and wants to know what the status of the debate is.) The first six essays deal with the biological theory, and some of these will themselves be important works in the ongoing debate. The final six essays deal with implications for the social sciences (and the editors themselves are political scientists rather than biologists).

Punctuated equilibrium is the idea that "most" evolutionary change happens in geologically "brief" speciation events separated by long periods of "stasis". As the essays in this book demonstrate there is a great deal of uncertainty about what exactly this means and whether it is true or not (hence the scare quotes).

The opening essay by Ernst Mayr is a broad attempt to explain what the theory of punctuated equilibrium is and what the debate has been about. Mayr describes the different things that have been labelled "punctuated equilibrium", with a concern to clarify some points of uncertainty. His view is generally very favourable; apart from some of the more extreme ideas regarding saltationism (production of new species by large single mutations) which he sees Gould as having toyed with for a few years, he believes that most of the claims associated with punctuated equilibrium are in fact either true or in the process of being tested, and that the issues involved are significant. However he stresses that punctuated equilibrium does not "transcend" Darwinism in any way, and is in fact solidly in the Darwinian tradition.

In the second essay we have a discussion of punctuated equilibrium "straight from the horses mouth"; Stephen Jay Gould himself explains the origins of punctuated equilibrium, answers some of its detractors' arguments, and considers some of its wider implications. As the editors say in the introduction, so closely is Gould associated with punctuated equilibrium that this is almost an "apologia pro vita sua". His ideas are clearly less extreme than some of the other essays in this volume seem to think; though he claims that there have been no major changes in his ideas on the subject, this seems to be rather arguable.

Next up Steven Stanley presents some of the empirical evidence supporting punctuated equilibrium. This includes evidence for effective stasis in some lineages and evidence for extremely rapid speciation in others. It is extremely hard to evaluate this evidence, given the sampling problems inherent in choosing lineages to study that Stanley himself points out, but there seems little doubt that there is evidence for punctuated equilibrium in evolution and that further work will make it clearer exactly how important it actually is.

An essay by Eldredge, the other founder of punctuated equilibrium, is largely about hierarchical evolution rather than punctuated equilibrium. He also makes a few very general comments about possible metaphorical application in the social sciences (looking forward to the second section of the book).

The only really dissenting voice in this discussion is that of Antoni Hoffman. His essay is a general attack on the theory of punctuated equilibrium; he claims that the weak form (that rates of evolutionary change vary) is trivial and says nothing that wasn't known to Darwin, the strong from (macromutations and saltationism) is false, and the moderate form (widespread stasis in evolutionary lineages) is untestable. He does admit that punctuated equilibrium has had heuristic value in sparking debate and suggesting research. Again it is evident that there is confusion about whether (and how strongly) Gould actually pushed saltationism, but it is clear that he no longer does so; hence criticism of the "strong" version of punctuated equilibrium is now peripheral to the main debate. Given the palaeontological evidence presented by Stanley and Gould in this volume (and by others elsewhere), Hoffman's claim that the moderate version is untestable seems hard to sustain, although it is clear that testing it is more complicated than initially suspected.

The final essay in this section is by the philosopher Michael Ruse, who has changed his original negative opinion of punctuated equilibrium and now considers it to have many of the characteristics of a paradigm shift. He concentrates his attention on Gould's work, and has some particularly interesting things to say about the relationship between punctuated equilibrium and Gould's position on other issues (such as his Marxism, his aversion to sociobiology, his Jewishness, etc.). Ruse's conclusion is that the main influence on Gould's thinking is not any of these in particular, but a general background in European philosophy, and in particular in a biological tradition stretching back to Goethe that stresses form rather than function.

The second section, titled "Implications for the Behavioural Sciences", begins with an essay by Kenneth Boulding on "Punctuationism in Societal Evolution". This is basically a collection of comments on broad similarities between biological and social systems, with some reference to "punctuationism". It is far too vague to be interesting; most of what Boulding says is either trivially true or so sweepingly general that truth is rather irrelevant. A couple of the statements about biological evolution are worded in such a way as to make me slightly queasy; it is not entirely clear that the author understands the Central Dogma and how its existence means there are fundamental differences between biological evolution and societal evolution.

Susan Cachel's "Punctuated Equilibrium and Evolutionary Anthropology" is rather peripheral to punctuated equilibrium, being an attack on the misuse of cladistic systematics in palaeoanthropology. (She thinks an approach stressing evolutionary ecology, morphological change, etc. is more useful than one based purely on classification and construction of phylogenetic trees.) At any rate, though punctuated equilibrium is often coupled with cladistic systematics, it seems to me that the two are very uneasy bedfellows, and so her whole argument has little to do with the punctuated equilibrium debate.

Allan Mazur looks at the evolution of human social behaviour. He argues for a methodology based on progressive changes throughout the primate order. (Which seems to assume some ideal of progress as well as gradualist change.) He doesn't think there is enough evidence to decide whether punctuated events are important in human evolution or not. On a similar note Brian Gladue argues that the punctuated equilibrium debate is irrelevant to psychobiology, with homosexuality as a specific example.

The last two essays are on links between the punctuated equilibrium debate and politics. Glendon Schubert looks at parallels between catastrophism in evolution and in politics, with human effects on biological systems (imminent ecological disaster) as a link. Roger Masters looks at links between biological and political theories in Aristotle, Empedocles and Lucretius, and at Hobbes, Rousseau and Marx. His conclusion is that, since amongst them they hold all possible combinations of views on gradualism/punctuationism, it seems unlikely that either scientific position can be invalidated because of its proponents political beliefs. However I remain entirely unconvinced that even a perfect correlation between gradualist (or punctuationist) political beliefs and the corresponding evolutionary ideas would in fact have any consequences for biological theory, which is what Masters seems to be saying.

In general I was a bit disappointed by the essays in the second section. I would have liked to have seen something on parallels between species and cultures (the controversy over whether species are "real" entities in evolutionary theory is arguably considerably better formulated than arguments about the reality of cultures in anthropology), and between evolution and cultural change, perhaps from an anthropological viewpoint. It also seems to me that a punctuated model of human evolution could have important consequences; even if there is not enough data to say anything definite a bit of wild theorising wouldn't have gone amiss. (For example, is the "aquatic ape" theory compatible with punctuated speciation of Homo sapiens?)

Some of the essays in The Dynamics of Evolution were a bit disappointing, but all of them have something of interest to say and some of them are likely to be extremely important. Anyone interested in the punctuated equilibrium debate will want to read this book.

September 1993

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