“If [Ernst] Mayr’s characterization of the synthetic theory [of evolution] is accurate, then that theory, as a general proposition, is effectively dead, despite its persistence as textbook orthodoxy”.—Gould, 1980
“If Steve Gould’s characterization of punctuated equilibrium involves the evolutionary mechanisms that he and Niles Eldredge proposed, then that theory, as a general proposition, is effectively dead, despite its persistence in Gould’s writings.”—Coyne, this post
Punctuated equilibrium (PE) was first proposed in a paper by Niles Eldredge and Steve Gould (“E&G”; reference below) in 1972, the year before I entered graduate school. When I entered Harvard in 1973 it was a huge deal, heavily promoted by Gould, a professor in the Museum of Comparative Zoology, as a replacement for the view of evolution most people held (“neo-Darwinism). Not a little of the theory’s popularity came from Gould’s nonstop promotion of it as well as his extraordinary ability to write popular science.
At first the theory was largely about the pace of evolution. Instead of imperceptibly gradual change in a species over time (the view Darwin proposed, though Darwin did note in the Origin that evolution could also be rapid), Eldredge and Gould proposed that the pace of evolution was jerky, with big changes occurring relatively rapidly over evolutionary time little evolution happening the rest of the time. (This of course concerned only morphology, and was tested largely using hard parts—the parts most often preserved in fossils.)
Most of us microevolutionists were willing to go with the data, and some fossil data did seem to show an episodic pattern of morphological change. But of course there was argument about this, for what constitutes “big” versus “small” change in fossils? Further, the fossil record is often incomplete, so missing strata can make a gradual change look like a near-instantaneous big change.
Nevertheless, the theory that the pace of evolution could vary widely sat comfortably within the neo-Darwinian paradigm, which predicts that change will be rapid when natural selection is strong, and small when selection is weak or nonexistent. I remain open about the prevalence of the pattern, for I don’t know all the data.
But, over time, PE became more than a hypothesis about the relative rate of evolutionary change in fossil lineages. It morphed into a theory of evolutionary process—a theory that was pretty much “non-neo-Darwinian” and also much more controversial. And while the pattern may be right, the processes proposed by E&G are so wrong that I’d call them “definitively falsified”.
Over time, the following six assertions became part of Gould and Eldredge’s theory, and were proposed by the pair themselves:
a.) The claimed observation that most of the times species in the fossil record didn’t change (i.e., exhibited “stasis”) was not due to weak selection or an absence of selection, nor was it due to “stabilizing selection”: the kind of selection in which the average character in a population is the most fit, and extremes are selected against. That is the classic explanation for a lack of evolutionary change over time. These explanation were rejected by E&G in favor of two other explanations:
1.) Organisms have “developmental constraints”: there may be selection, say, to make individuals of a species bigger, but the species doesn’t get bigger because it either lacked genetic variation for bigger size or, alternatively, attaining a bigger size would have negative effects on the average fitness of the species (for example, if food is scarce, getting bigger might lead to faster starvation).
2.) Gene flow among populations of a species means that no population could change in response to local selection pressures because there was a constant influx of genes from other populations that didn’t experience such selection. This constant mixing of genes from populations undergoing different forms of selection averaged out to no net change in the appearance of a fossil species.
b.) Punctuated change in morphology can occur only when the genome is somehow “shaken up”, and this shake-up occurs during speciation events—when one lineage branches into two or more lineages. Absent such splitting events, a species stays static.
c.) The genomic discombobulation that somehow releases a species from its stasis—that is, loosens the developmental constraints—occurs when, as supposedly happens during most speciation events—a small peripheral population undergoes a form of “genetic revolution”, a kind of speciation in which reproductive barriers arise during an interaction between natural selection and genetic drift (random changes in the proportion of gene variants that are most prevalent in small populations.) At the time of this theory, several evolutionists, including Sewall Wright and Hampton Carson, had proposed that some types of evolutionary change require genetic drift in small populations. Without those population “bottlenecks”, these proponents said, species don’t change much. E&G drew on these ideas to buttress the episodic nature of evolution. One problem here is that there was and is little evidence that this kind of drift-associated change occurs, and almost no evidence that it’s ever associated with the appearance of a new species. Evolutionists have repeatedly put species through extreme bottlenecks—as few as two individuals—and have never seen that lead to even the beginning of reproductive isolation. (Reproductive isolation is the sine qua non of speciation to evolutionists.)
d). These claims all combine in the following way to lead to a punctuated evolutionary pattern. A big, widespread species is resistant to evolutionary change for the reasons mentioned above. Then, a small peripheral isolate population, cut off from the rest of the species, forms. Being small, it undergoes genetic drift, which releases the evolutionary constraints and allows the isolate to undergo rapid and substantial evolution. Eventually, the isolate rejoins the main population, but by that time it’s evolved reproductive isolation from the other populations and is thus a new species. For reasons unexplained, the isolate quickly replaces the other populations. And voilà!—one sees a big change in the fossil record as the small and changed population supplants its ancestral species.
e.) But there’s another way that big morphological change can occur rapidly, too—one that was promoted by Gould: macromutation. This is the notion that changes in an animal’s appearance, behavior, physiology, and so on, don’t need to occur in small, incremental steps (the “Darwinian” pattern) but can occur via mutations that make big jumps, creating “hopeful monsters” (“saltations”). This idea was popularized by Richard Goldschmidt in the 1930s, and was revived by Gould in PE. Gould, for example, said this in a 1982 paper in Science.
I envisage a potential saltational origin for the essential features of key adaptations. Why may we not imagine that gill arch bones of an ancestral agnathan moved forward in one step to surround the mouth and form proto-jaws? (Gould, 1980)
When called out for the absence of adaptations based on such huge mutations, Eldredge and Gould backtracked, claiming that PE was “never meant as a saltational theory”. As you see, and this is true of other parts of PE, Gould in particular waffled about what the mechanisms of episodic fossil change really were.
f.) One of the most important parts of PE, worked out largely by Gould, was the claim that major features of adaptive evolution, and evolutionary trends in general, like the increase in body size in many lineages (“Cope’s Rule”) was due to species selection. This is a process of differential speciation and extinction that is said to occur not within species (that’s just classical Darwinism), but among species. A further claim was that the changes within species had little to do with selection itself (they may have resulted from drift)—or at least little to do with the process of differential speciation and extinction.
So, for example, an increase in body size among a group of mammals over time would be explained by species selection this way: each species attains its average body size either by drift or by forms of selection that have no relationship with the persistence, speciation rate, or extinction of species. But it may happen that, for other reasons, the biggest species either speciate faster or go extinct more slowly. Over time, then, we’d see a pattern among lineages of an increase in body size, but this has nothing to do with classical Darwinian selection on gene forms.
The problem with this is that species selection cannot account for complex adaptations like jaws so easily. Each feature of an adaptation would have to evolve by a process of differential extinction or speciation, and evolving a complex adaptation would take a gazillion years. That’s because species selection is much slower than individual Darwinian selection since the former relies on replacement of species over evolutionary time, while the latter relies on the rapid replacement of gene forms within a species, which can occur over a few thousand generations or fewer. Further, the evidence for species selection as a general explanation of evolutionary trends is very thin. In his last big book, The Structure of Evolutionary Theory, a 1400-page, poorly written monster that I actually read (and wish I hadn’t), Gould winds up admitting that he can’t adduce a single good example of species selection. However, in the last chapter of my book with Allen Orr, Speciation, we do make a case that a limited form of species selection may operate in nature and can explain evolutionary trends but not adaptations themselves. Species selection is just not as ubiquitous as Gould thought.
So apart from a) and the presence of genetic drift, virtually every part of PE is “non-neo-Darwinian”: processes that aren’t considered widely as part of the modern evolutionary synthesis. That doesn’t mean that they’re wrong, but when examined closely, the evidence for these ancillary assertions is virtually nonexistent. Although to G&E, PE represented a Kuhnian “paradigm shift”, closer examination shows that these components (peak shifts, connection of morphological change with speciation, restriction of response to selection by developmental constraints, saltation, widespread species selection etc., etc.) are individually not common, and in tandem seem impossible to form the basis of a convincing theory. Despite that, Gould claimed that PE put neo-Darwinism to rest (see his quote at the top of the article).
Now I could write in detail why the assertions above are dubious, and why PE as a mechanism of evolutionary change is almost certainly wrong, bu that case has already been made. It was first made by three of my colleagues, Brian Charlesworth, Russ Lande, and Monty Slatkin, in a 1982 paper in Evolution that pretty much put the mechanism of PE to rest. You can read that paper below; it’s a classic not of modern evolutionary genetics, and also a paradigm of close examination and debunking of a popular theory (click on screenshot for the pdf). The debunking involved a massive mustering of evidence from genetics, population-genetic theory, laboratory experiments, field experiments, artificial selection, and geology. The last bit of their conclusions says this:
We have also demonstrated, as has Orzack (1981), that punctuationists have often severely distorted the neo-Darwinian theory of evolution. Punctuationists are mainly criticizing oversimplified versions of neo-Darwinism (which are currently popular in some fields) rather than the original statements of this theory and the evidence which has been used to support it. Furthermore, some of the genetic mechanisms that have been proposed to explain the abrupt appearance and prolonged stasis of many fossil species are conspicuously lacking in empirical support. Thus, we do not feel logically compelled to abandon neo-Darwinism in favor of the theory of punctuated equilibria.
This paper of Charlesworth et al. was expanded and brought up to date by the new “Perspectives” paper of Hancock et al. in Evolution (reference below, pdf here).
And here is the abstract, supporting the conclusions of Charlesworth et al. (my emphasis)
The Modern Synthesis (or “Neo-Darwinism”), which arose out of the reconciliation of Darwin’s theory of natural selection and Mendel’s research on genetics, remains the foundation of evolutionary theory. However, since its inception, it has been a lightning rod for criticism, which has ranged from minor quibbles to complete dismissal. Among the most famous of the critics was Stephen Jay Gould, who, in 1980, proclaimed that the Modern Synthesis was “effectively dead.” Gould and others claimed that the action of natural selection on random mutations was insufficient on its own to explain patterns of macroevolutionary diversity and divergence, and that new processes were required to explain findings from the fossil record. In 1982, Charlesworth, Lande, and Slatkin published a response to this critique in Evolution, in which they argued that Neo-Darwinism was indeed sufficient to explain macroevolutionary patterns. In this Perspective for the 75th Anniversary of the Society for the Study of Evolution, we review Charlesworth et al. in its historical context and provide modern support for their arguments. We emphasize the importance of microevolutionary processes in the study of macroevolutionary patterns. Ultimately, we conclude that punctuated equilibrium did not represent a major revolution in evolutionary biology – although debate on this point stimulated significant research and furthered the field – and that Neo-Darwinism is alive and well.
So the best you can say about the mechanism of PE, a claim I’ve heard many times, was that it furthered the field of paleobiology—brought paleontology to the “high table of evolutionary biology”, as someone asserted. Well, while it did stimulate debate about the relative frequency of rapid versus gradual change in the fossil record, the falsity of its claims about mechanism was already known to evolutionary geneticists when PE was first proposed! Charleworth et al. simply collected all the theoretical and empirical work that showed the falsity of the mechanism.
I remember debating this issue with Steve Gould in our conference room at Harvard, asking him to explain the details of PE’s mechanism. Gould got more and more exercised, and wound up tarring me by telling me that I was just a “hidebound gradualist.” I still wear that label with pride.
Later, Brian Charlesworth and I had several exchanges criticizing PE in the journal Science (see Coyne and Charlesworth references below).
It apparently wasn’t enough for E&G to point out a pattern in the fossil record that might have been real (I still don’t know how ubiquitous “jerky” evolution is). No, they wanted to go further—to be Kuhnians and tear down the wall of evolutionary theory, erecting the new paradigm of PE in its place. Well, such an endeavor is fine, but the new paradigm hasn’t worn well, and in fact was stillborn when first proposed.
I’m not sure whether paleobiologists still teach punctuated equilibrium as a viable theory, but if you hear that claim, remember this: PE as a pattern in the fossil record may well be correct, but as a mechanism of evolutionary change is “not even wrong.”
Addendum: I don’t want to go through the Charlesworth et al. and Hancock et al. papers in detail, as you can read them for yourselves. But if you have specific questions about the mechanism of PE that I can answer briefly, put them in the comments.
1982 A Neo-Darwinian commentary on macroevolution. Evolution 36: 474– 498., , and .
Coyne, J. A. and B. Charlesworth. 1996. Mechanisms of punctuated evolution (technical comment). Science 274:1748-1749. (includes response by Elena et al.)
Coyne, J. A. and B. Charlesworth. 1997. Punctuated equilibria (technical comment). Science 276:338-340.
Eldredge, N. and S. J. Gould. 1972. Punctuated equilibria: An alternative to phyletic gradualism. Pp. 82-115 in T. J. M. Schopf, ed. Models in Paleobiology. Freeman, Cooper, San Francisco.
Gould, S. J. 1980. Is a new and general theory of evolution emerging?. Paleobiology 6 119-130.
Hancock, Z.B., Lehmberg, E.S. and Bradburd, G.S. (2021), Neo-darwinism still haunts evolutionary theory: A modern perspective on Charlesworth, Lande, and Slatkin (1982). Evolution. https://doi.org/10.1111/evo.14268