“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.
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, , and . 1982 A Neo-Darwinian commentary on macroevolution. Evolution 36: 474– 498.
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
Gould was such a good popularizer of evolutionary biology (although he seemed to become overly “erudite” in later years). Yet, it’s a pity that The Structure of Evolutionary Theory was indeed a poorly written “monster”: glaringly repetitious, self-aggrandizing, and poorly organized. The scuttlebutt was that Gould refused to let any editor change a word.
I read the book shortly after its publication, and was especially taken aback by Gould’s criticisms of his detractors, especially Dawkins and Dennett (whose criticisms I believe he ungenerously described as “pathetic”). I happened to give a seminar at the U. of Rochester in 2003 and had the good fortune to meet with Professor Orr (a PCC(E) offspring), who had just written an excellent, critical review of the book. I joked that we might be among the handful of people in the world that persevered and read the whole thing! (A. year ago I gave my copy to a junior colleague.)
The term “punctuated equilibrium” was introduced by Eldredge and Gould (note the order) in 1972, but the statement of the ‘jerky’ pattern and the idea that rapid evolution via allopatric speciation explained it was in Eldredge’s 1971 paper in Evolution “The allopatric model and phylogeny in Paleozoic invertebrates.” Eldredge always objected to the notion that it was Gould’s idea; I once heard him say, “The next guy who asks me about Gould’s theory, I’m gonna punch him in the nose!”
GCM
A while ago I stumbled across an essay by Huxley (around the 1870s) that seemed to advocate a “pace of evolution can change” version of PE, saying that one would expect different rates of morphological change in different populations and at different times, caused by different selection pressure.
And Darwin in fact anticipated the irregular pace of evolutionary change in the Origin, even though he had a puny fossil record to work with:
But I must here remark that I do not suppose that the process ever goes on so regularly as is represented in the diagram, though in itself made somewhat irregular, nor that it goes on continuously; it is far more probable that each form remains for long periods unaltered, and then again undergoes modification. (Ch. 4, “Natural Selection”)
It is a more important consideration … that the period during which each species underwent modification, though long as measured by years, was probably short in comparison with that during which it remained without undergoing any change. (Ch. 10, “On the imperfection of the geological record”)
Wasn’t it Eldredge who coined the term ‘ultra Darwinians’ for George C Williams et al? People neglect Williams it seems to me.
Also, interesting division between biologists & palaeontologists. I suppose they see from opposite ends…
In a more general note, I find it worrying that the charisma and PR skills of individual scientists can shake good paradigms and shape opinions in the scientific community for decades, regardless of the quality (and the very existence) of evidence supporting their pet theories.
‘twas ever thus!
Aye! And add to that institutional prestige, bias in grant funding based on name and reputation (ergo size and resources of graduate student research group), and bias among top journal reviewers of papers favoring name recognition.
Even “blinded” grant review or journal article review is often easy to unblind because peer reviewers who are experts in the field can recognize the work. It’s human nature really.
I still don’t fully understand PE. I vaguely recall Dawkins arguing against it in some of his books, read a lifetime ago. I believe he talked about the evolution of evolvability, like the first to hit upon the basic insect morphology than then unleashed the plethora of insect bodies we see today. I could be mistaken, or misremembering, but then I don’t have the steel trap mind of most readers here, I’m closer to a mind like a plastic sieve. Anyway, I’ll attempt to read the attached papers, even if I probably need a dumbed down kids’ version of the argument, maybe written in red crayon, and I certainly appreciate posts like this. Even with my limited evolution education and understanding, it is enjoyable to sit at the grownups table and listen in.
Well, I wrote my summary to help readers understand the tenets of PE, and I guess I didn’t fully succeed. The papers are for those who want to go on to understand why the tenets are untenable.
FWIW – I found it to be most helpful. Really enjoyed this post.
Any lack of understanding is on my part, not due to your writing or any lack of clarity.
Actually Dr. Coyne, I think you succeeded quite well with your summary. I found it most helpful and accurate after reading the papers.
Chapter Nine of The Blind Watchmaker.
Ah, thank you. It’s been 15+years since I read it, I suppose I should be surprised that was rattling around my empty head.
Glad to see this review and analysis of PE. Thanks so much!
I’m a fan of two ideas relevant here: 1) sudden changes in morphology in the fossil record at any locality simply reflect the fact that ordinary Darwinian (gradual) events took place “off stage” (another location); 2) given fluctuations in population size, deleterious mutations fixed by drift in small populations often will be “corrected” by mutations at other sites when the population expands. In separate isolates, the problems and the fixes will be different, leading to incompatibility in hybrids, I.e., speciation.
I wonder what the creationists made of the PE theory.
They loved it.
GCM
Their primary response was to first mischaracterize it as a mere restatement of Goldschmidt’s Hopeful Monster hypothesis –conflating, IMO, basic PE with Gould’s speculations on potential macromutational mechanisms (for which Gould is somewhat culpable)– and to then argue that the only reason evolutionists such as Gould, Eldredge, Stanley etc. were putting the hypothesis forward was due to the supposed lack of intermediate fossils in the geologic record; misinterpreting Gould et al. as seeking to explain fossil gaps between higher taxonomic groups –i.e. between fish and amphibians etc.– rather than those between lower groups, which, ironically, creationists dismiss as mere changes within a “kind”.
Thanks for this review of PE!
As a geologist I’m more familiar with the fossil record and what patterns paleontologists have seen in it. It does seem to be the case that many species remain morphologically unchanged over their time on Earth (‘stasis’), and that new species often appear suddenly. Their sudden appearance could potentially be due to the fact that most moment-to-moment time is not recorded by any given sedimentary sequence. The Eldridge (1971) paper makes the argument that allopatric speciation may be a more general answer, and my impression was that it was a Neo-Darwinian explanation: Any parent species population is likely to have a number of peripheral isolates living on the edges of the home range of the species, and these may be subject to genetic drift, the founder effect, and different selection pressures. One of these isolates may evolve traits that allow them to spread back into the home range and become an abundant species that is well-represented in the fossil record. (The new species often does NOT displace the parent species, but the two live side-by-side until one or the other go extinction.) This appearance would appear sudden in the fossil record, even when sampled over a broad area.
Thus, up to this point, isn’t PE simply the application of Neo-darwinism to the interpretation of the fossil record? I never go into the other stuff that you summarized because it seems speculative or, in the case of species-level selection, I don’t understand it.
I’ve always disliked the term “punctuated equilibrium” because I don’t know what equilibrium is being disturbed. The peripheral isolates evolve relatively fast or slow but presumably gradually, but usually off-stage since their small populations are unlikely to be well-sampled by the fossil record. When conditions happen to be right for one of the isolates, they expand their range as a new species, but almost always the other closely related species in the fossil record persist apparently disturbed.
The pattern may be real in some cases, but we need evidence that it’s due to allopatric speciation rather than invasion. Remember, though–all the excitement about PE was because it was floated as an ALTERNATIVE TO A NEO-DARWINIAN EXPLANATION, not just a neo-Darwinian explanation based on allopatric speciation for a fossil pattern. So no, you’re incorrect in saying that PE is the application of neo-Darwinism to the interpretation of the fossil record. The big excitement about the theory, and the way Gould sold it, was due to its “non-Darwinian” and Kuhnian aspects as a Paradigm Changer.
I agree, punctuated equilibrium was indeed sold as a paradigm changer.
I remember attending a graduate seminar as a beginning graduate student, an entire semester on punctuated equilibrium, species selection, macroevolution, etc. Around 1986, at Cornell. I remember the divisive debates among the faculty, some who were convinced the Modern Synthesis and standard evolution were dead, some who argued that apparent patterns of punctuated equilibrium in the fossil record were mostly artifacts. I remember how emotional the debates were. I remember the ex-cathedra proclamations when Gould visited to give a seminar.
The current debates about Extended Evolutionary Theory reminds me alot of those debates 35 years ago. Too little bending-over-backward to prove yourself wrong
From Hancock et al.’s introduction:
I wonder if changes in genomic structure and sequence might be preferred to mutations. Few of us think of phenomena like retroviral elements as mutations, although they obviously contribute to phenotypic variation.
Perhaps, though I’d maintain that insertions and the like are also random with respect to fitness.
I am not a scientist, but I always saw PE as a combination of how evolution looks different to a paleontologist as opposed to how it looks to the ‘fly guys’, and a sop to Gould’s ego. He seemed determined to get his name alongside the ‘greats’ in the pantheon of evolutionary theory, which, despite his popular works and vast erudition, was not going to happen from a bunch of papers on snails. Unfortunately, the quarrel was misunderstood by the creationists as exposing a major theoretical flaw in the theory of evolution.
I think one of the ways in which Jerry’s summary is so good is that it emphasizes this distinction between the pattern (how things look to palaeontologists vs. other biologists) and the process (which is where the sop to ego came from). PE was originally a paleontological pattern of stasis. It seems that, when challenged to explain why that pattern could not arise via good old neo-Darwinian processes, Gould then retreated to ideas about other processes, especially developmental constraints, genetic revolutions, and macromutations. In that telling of the story, the retreat was necessary in order to retain the status of PE as a paradigm shift. But my reading of that history could be wrong.
Thanks for this, professor! Fascinating and a perennial challenge of how to scientifically educate the public. How long does it take for scientifically discredited theories to leave our consciousness and the scientific record? How often do the erroneous textbooks and pop-sci books get updated to convey not just new theories (which sell books) but the obituaries of deceased ones (which don’t)?
At least now with the internet and the ability to maintain up-to-date “living documents” of our knowledge base, the bad ideas and defunct theories can be pruned more quickly. I think this reckoning has been even harder in philosophy (e.g. Freud) but even there has begun to improve.
Daniel Dennet in his book “Darwin’s Dangerous Idea” tries to indirectly explain why Gould takes the PE route with such vigor, and hints at Gould being religiously an anti-adaptionist. He quotes Gould:
“But modern punctualism – especially in its application to the vagaries of human history – emphasizes the concept of contingency: the unpredictability of the nature of future stability, and the power of contemporary events and personalities to shape and direct the actual path taken among myriad possibilities.”
Gould does is not a big fan of mindless algorithms, preferring that agency drive destiny. Dennett alludes – and not vaguely – that his Marxism was driving his science.
Was punctuated equilibrium mostly about speciation? I have the vague impression that it was also about how the giraffe got a long neck or the elephant got a trunk, that is, evolution within a species. I remember reading several Gould books back in those days, he was quite prolific.
Coming at the controversy long after it had played out, my introduction to it was Dawkins’ The Blind Watchmaker, Mayr’s What Evolution Is, and Prothero’s Evolution that introduced me to it and none of it seemed out of line with the modern synthesis. It must have been completely different being part of the conversation at the time, especially with some of the more grandiose statements from Gould about it. I’m not sure why anyone would glob onto the need for major mutations when the data from the fossil record isn’t that granular. Periods of incremental chance would happen in the blink of an eye – geologically-speaking.
Jerry, you are conflating two separate issues; (1) is the Modern Synthesis effectively dead, and (2) are punctuated equilibria a common feature of the fossil record. Your post implies a direct connection between these two issues when you say that punctuated equilibria don’t exist therefore “long live the Modern Synthesis.”
In Gould’s 1980 paper, which you quote, he explained that the Modern Synthesis as defined by Ernst Mayr is pretty much confined to “the accumulation of small changes, guided by natural selection.” There is general agreement on this point – the Modern Synthesis was an adapationist view of evolutionary theory.
Gould then goes on to describe three challenges to the Modern Synthesis; challenges that effectively kill it, in his opinion. One of them is hierarchical theory and one other is the challenge to the gradualism assumption. It’s that second challenge that includes punctuated equilibria. The first one is still being debated as the question of whether microevolution is sufficient to explain macroevolution. (I don’t think it is; microevoltuon can’t explain a lot of speciation events and it can’t explain mass extinctions. I’m not sure about species sorting in spite of the fact that you think it’s impossible.)
However, the third challenge is Neutral Theory and the role of random genetic drift. Gould argues that much of evolution involves the fixation of neutral or nearly-neutral alleles by random genetic drift and this view of evolutionary theory is inconsistent with the adapationist view of evolution promoted by the founders and promoters of the Modern Synthesis. He’s right, of course, the Modern Synthesis died in the late 1960s and it wasn’t punctuated equilibria that killed it. It was Kimura, King and Jukes.
Maybe you should discuss Gould’s 1982 paper in Science? That’s the one with this title: “Darwinism and the Expansion of Evolutionary Theory.”
Sorry but I disagree with you strongly. Gould’s explanation for the commonality of a punctuated pattern in the fossil record was that the neo-Darwinian explanation didn’t really apply to the fossil record. Ergo neo-Darwinism (i.e., the Modern Synthesis) was wrong as a general explanation of evolution.
Frankly, I think you’re the one who’s confused here. For example, you say that hierarchical theory is not part of puncutated equilibrium. You’re wrong. Species selection was definitely part of the theory (by the way, you say that there were three challenges but list only two). And I don’t think that species selection is impossible; as I say in the text, Orr and I show that a form of species selection is indeed possible and has probably operated in nature.
I don’t want to go after a lot of the claims you make without support, like saying “micreoevolution can’t explain a lot of speciation events” without giving one example. And who ever claimed the microevolution can explain mass extinction? Microevolution has nothing to say about the ability of a species to survive an asteroid, for crying out loud.
Sorry, but I find your criticism not only rude (telling me what papers I should discuss, for example) but uninformed. And you have a way of writing that exudes hostility; I wish you’d knock that off.
I recall first hearing about PE in a bbc documentary. Reading the views of an evolutionary biologist about it now was both entertaining and informative, thanks.
This provides circumstantial evidence only, but useful for amateurs: ask yourself what would evolution look like if we have only an imperfect fossil record? Second question: how complete do you think the fossil record is?
It is to be expected that an imperfect, incomplete fossil record would appear to show large leaps in development. Instead of ‘missing links’ Gould suggested it was a feature, not a bug. Imagining complex mechanisms for evolution to occur in fits and starts is like Ptolemy using epicycles to explain findings in planetary motion he did not understand. We may applaud the cleverness of the imagination, but that doesn’t mean it is correct.
Half a century ago there was plenty of confusion about the so-called Cretaceous–Paleogene extinction event. I’m sure many paleontologists thought the Modern Synthesis had played down everything non-gradual in evolution. This helped selling the PE as a paradigm shift.
Of course there’s nothing in PE that couldn’t be explained by natural selection and genetic drift. After all, the consequences of cataclysmic events also work through selection and drift.
Later on, Eldredge sort of drifted away (pun intended) from Gould and concentrated on an interesting concept of ecological hierarchies.
Nothing sensible to add, just to note my appreciation for the post.
People writing accessible-to-many posts on their area of expertise is What The Internet Is For, in my view.
Thanks for this post, I’ve read some of Gould’s work and about PE in general but have struggled understanding it’s relevance (if any) to modern evolutionary genetics.
Other than just appreciating this post the following quote made me think of a paper I’ve read recently with a very provocative title (https://link.springer.com/article/10.1007/s11229-021-03158-9):
“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.”
A large part of the linked paper references a hypothesis about particle physics that was widely disagreed with, ended up being wrong, and the proponent (William H. Bragg) even expressed skepticism of it in private letters. So the authors then make the case that the debate over this hypothesis still stimulated Nobel-prize winning research and thus was worth publishing as extensively as it was. I wonder what others’ opinions are on whether or not PE was “worth it” in causing this debate or if the necessary facts demonstrating its falsity were there from the start.
Just found this post which damns Gould’s character more than his science: https://www.lesswrong.com/posts/BahoNzY2pzSeM2Dtk/beware-of-stephen-j-gould
Is it a fair assessment or has was case against him overstated?
I am a retired judge and a lawyer friend and I have had an ongoing and intense argument regarding the specific significance of mutation in evolution. I argue that, at least theoretically, adaptation is the key elemental force and evolution would occur even with the total absence of a mutation factor. He argues that mutation is the sine qua non and major generator in the evolutionary process. I will note that, as lawyers, we share a not-uncommon professional deficiency in reason, he more than me, however. He agrees to abide by your determination. I am not so sure.
WEIT is my go-to source in most aspects of evolution, religion, free speech and on and on and I thank you JAC for your incredible production. Moreover, while I have been a WEIT reader for many years and share your love of cats and ducks, I must confess to having a d*g.
Evolution by natural selection requires the presence of both genetic variation that arises through mutation (here I include stuff like horizontal gene transfer or insertion of transposable elements) and a differntial effect of those variants on reproduction (i.e., selection). Without either variation or differential reproduction of genes, evolution grinds to a halt. So your claim that adaptation would occur without mutation isn’t correct. When existing variation is used up, there would be no more evolution. In fact, we’d never have changed since the first Ur-species!
Thanks for your kind words. But your friend is wrong too, for you can’t single out either mutation or selection as THE sine qua non of adaptive evolution, as both are required for adaptation. Does this mean that I win?
Yes, Jerry, you win, even though I struggle with your parenthetical inclusions amplifying on “mutation” and where adaptation goes with giraffe’s necks and elephant trunks not the product of mutation but the necessary development of survival.
Without mutations, you don’t get the genetic variation that builds necks or trunks.
The lure of various “alternative” evolutionary models seems to arise from the objection to the role of “meaningless”, stochastic processes (described in Bowler’s book “The Eclipse of Darwinism” and the more recent “Revisiting the eclipse of Darwinism” see https://link.springer.com/article/10.1007%252Fs10739-004-6507-0
This was one of the more challenging science posts, but I am crawling up here finally to lodge my “sub” vote…. and done! Well written, glad to have given it attention!
I wonder if susceptibility to disease, and moreover its opposite, creates small populations (of survivors) after which rapid (and nearly random) evolution is more likely.
Thanks for sharing Dr. Coyne
What I found frustrating about Gould’s presentation of punctuated equilibrium is that the definition was a moving target, so tests and critiques were like attempts to hold water in one’s hands.
Namely, some of Gould and Eldredge’s writings, PE required exotic genetic mechanisms like macromutation, “genetic revolutions” etc, whereas in other instances Gould and his collaborators would return to conventional Neo-Darwinian explanations for the patterns they describe: stabilizing selection and/or habitat tracking for stasis and disruptive+ frequency dependent selection for the speciation-associated punctuation events. Which version was presented often depended on target audience and who was being argued against.
The exotic mechanisms proposed by Gould and Eldredge were entirely speculative and largely without empirical or theoretical support, but there are two components of PE that have some scientific substance and merit.
The first is their claim that most important directional changes in phenotype (at least those relevant to trends seem among rather than within species, such as increases in body size among many Cenozoic mammal lineages) are associated with speciation events, whereas the changes that occur prior to speciation are undirected and tend to get “washed out” local adaptation and temporal fluctuations in selection pressure. This is an interesting empirical question that can be tested in a variety of ways using paleontological data. While most studies have focused on documenting stasis events in fossil species, and alternative method would be to determine whether the magnitude of phenotypic change depends more on the number of nodes in a phylogeny (i.e. the number of speciation events) or on lineage longevity.
The other worthwhile idea to come out of PE, as you mention, is species selection. Species selection cannot account for adaptive changes in phenotype, but it can account for other evolutionary phenomena, such as the question of why some clades are more species-rich than others. The biology of some organisms predisposes them to reproductive isolation and niche specialization. Species selection may explain why non-dispersing specialists are more species-rich than highly dispersing ecological generalists, assuming these characteristics are (within the clades) neutral with respect to individual selection.
The main problem with species selection is that, like group selection, it operates on too long of a time scale to counter individual selection, so it is only effective in driving evolutionary changes that are basically neutral with respect to individual selection.
In this piece you derided Gould’s comment “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?” But whether or not this sort of thing is particularly common in evolutionary history, do you think it would contradict neo-Darwinism in any way? I doubt Gould was suggesting here that functional jaws appeared in one mutation, just that gill arches suddenly placed nearer the mouth might have some minor benefits e.g. making it slightly less likely prey would escape back out from the mouth cavity before being swallowed, and then a many-generational process of mutation/selection would be needed to turn them into more functional hinged jaws (with accompanying changes in the brain to give the fish the instincts to use the muscles that open and close them when feeding).
Neo-Darwinism doesn’t allow for the sudden appearance of new complex adaptations with accompanying giant leaps in fitness, but it needn’t conflict with the idea of occasional mutations that cause large changes in morphology that are associated with some very minor new functional benefit and which then puts the population on an evolutionary path to developing that structure to be much more finely tuned for the new function. This wouldn’t be all that different from Gould’s talk about spandrels becoming adapted to new purposes, like if Maniraptoran dinosaurs originally evolved long stiff arm feathers purely for display like a peacock’s tail, but this had the inadvertent effect of giving their young a slight extra amount of lift when scrambling around in trees, and this new function was then fine-tuned by mutation/selection.