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Friday, January 25, 2008

Macromutations and Punctuated Equilibria

Olivia Judson has published a piece in The New York Times where she discusses "Hopeful Monsters" [The Monster is Back, and It’s Hopeful]. The basic idea here is that there can be single mutations that have a large effect on the phenotype of an organism. Think of a four-winged fruit fly, for example, that loses one set of wings.

The first organism to exhibit this new phenotype is a "Hopeful Monster." If it has aselective advantage, then the mutation will be passed down to its progeny and in a short time the species will be transformed in a single jump.

The term "Hopeful Monster" is associated with the views of Richard Goldschmidt. These views were thoroughly discredited in the 1940's at the time when the Modern Synthesis was being forged. The sudden leap in evolution due to macromutations was called "saltation" and we now know that this is not a common mechanism of evolution.

That does not mean that saltations never happen. Stephen Jay Gould wrote an essay in 1980 called Return of the Hopeful Monster in which he gave some examples of sudden changes. One of them is the acquisition of fertility by the axolotl of Lake Xochimilco. This amphibian never transforms into a normal-looking salamander but reproduces as a tadpole with external gills. The idea that mutations affecting development can have such effects is entirely in line with Gould's interest in the subject. His first book was Ontogeny and Phylogeny (1977).

Another example is the lobster Scyllus in which the second pair of antennae have been transformed into uropods in a manner highly reminiscent of homeotic mutations in Drosophila melanogaster (Dawkins, 1996 p. 252). Dawkins (1996 p. 103) also mentions the evolution of snakes, which have hundreds of vertebrae.
The number of vertebrae in different species of snakes varies from about 200 to 350. Since all snakes are cousins of each other, and since vertebrae cannot come in halves or quarters, this must mean that from time to time, a snake is born with at least one more, or one fewer, vertebra than it its parents. These mutations deserve to be called macro-mutations, and they have evidently been incorporated in evolution because all these snakes exist.
I think it's safe to say that the concept of macromutations and saltations is not ruled out in evolution although it is certainly rare. Gould makes the point in his essay that this kind of evolution, while dramatic, is Darwinian.

Jerry Coyne does not like macromutations or hopeful monsters and he has written a strong rebuttal of Olivia Judson's article. Carl Zimmer published it on his blog [Hopeless Monsters--A Guest Post from Dr. Jerry Coyne].

I don't want to get into the details of the Judson article. I think Coyne makes some very valuable points about the specific examples used in the article and I agree that the general tone of the New York Times piece is wrong. Macromutations are not common.

I want to make another point. Coyne says,
The idea of macromutational hopeful monsters, or "saltations," had a prominent resurrection in 1980 when Stephen Jay Gould, as part of his and Niles Eldredge's theory of punctuated equilibrium, proposed that macromutations could explain the "jumps" in the fossil record. After getting a severe drubbing from geneticists, Eldredge and Gould retreated in 1993, claiming that they never suggested the idea of saltations.
The idea that Gould's example of hopeless monsters was connected to punctuated equilibria is not correct. This is the same mistake that Greg Laden makes in his discussion of the topic [Hopeful Monsters and Hopeful Models]. Greg says,
The second reason is that the fossil record seems to have the property whereby many species stay roughly similar for long periods of time, then suddenly, there is lots of evolutionary change. You've heard of this, it's called "punctuated equilibrium." If hopeful monsters ... also called saltational (dancing, leaping) evolution ... occurred generally, we might postulate that these moments of dramatic change, these punctuations, are periods in time where for some reason a lot of hopeful-monstering was going on all at once. That would be cool.
Let's be very clear about what punctuated equilibria are and what they aren't. The pattern of punctuated equilibria show that speciation by splitting (cladogenesis) is associated with morphological change. The actual speciation event is relatively rapid (in geological time) and the end result is a morphologically distinct sibling species where the changes were not evident in the population before the split. The most common explanation is that variants in the larger population were enriched in a small founder population that went on to speciate. It's an example of random genetic drift, or possibly selection, but no new mutations have occurred.

What kind of changes are we talking about here? Very small changes. So small, in fact, that it often takes an expert to recognize them in the fossil record. We're talking about the differences between snails in the same genus, or different species of trilobites, or changes in the surface marking of diatoms. We aren't talking about saltations when we look at punctuated equilibria patterns. People who think that the normal pattern of punctuated equilibria represent big leaps in evolution are confusing two different aspects of evolution.

Daniel Dennet wrote a book in 1995 where he tried very hard to destroy the reputation of Stephen Jay Gould. One of the chapters in his book was "Punctuated Equilibria: A Hopeful Monster". Dennet lays out the case for confusion between saltation and puncturated equilibria as follows; "So it seemed to many biologists that Gould was arguing that punctuated equilibrium was a theory of Goldschmidtian speciation through macromutation" (Dennett 1995 p.288). When Gould (and Eldredge) denied any such thing these biologists "scoffed in disbelief. " After all, they knew what Gould had said. Dennet goes on to say,
But did they? I must admit that I thought they did until Steve Gould insisted to me that I should check all his various publications, and see for myself that his opponents were foisting a caricature on him.
Dennett checked, and found that Gould was right. To his credit, he reports that this claim about saltation being part of punctuated equilibria is wrong. Dennett concludes on p. 289-290.
"Punctuated equilibrium is not a theory of macromutation" (Gould 1982, p.88). Confusion on this score still abounds, however, and Gould has had to keep issuing his disclaimers [as has Eldredge, LAM]: "Our theory entails no new or violent mechanism, but only represents the proper scaling of ordinary events into the vastness of geological time" (Gould 1992b p.12).

So this was the false-alarm revolution that was largely if not entirely in the eyes of the beholders.
I'm quoting Dennett here instead of quoting Gould and Eldredge1 directly because Dennett is one of Gould's fiercest opponents. If Gould's worst enemy can see the truth then why is this myth still being propagated?

I'll close with one of my favorite quotations from Eldgredge (1995 p.99).
Nonetheless, we were accused of being saltationists. Steve Gould wrote two consecutive essays in Natural History in 1977. Among other things, Steve speculated that the recent (sic) discovery of regulatory genes—genes that turn other genes on and off—raised the possibility that mutations in the regulatory apparatus might occasionally have the sort of effect Goldschmidt had in mind with his notion of 'macromutations.' These macromutations had the large-scale effects of the sort he posited for his 'hopeful monsters.' Nowhere in either article did Steve mention punctuated equilibria.

But it was enough, it seems, that he, champion of a new model positing bursts of relatively rapid change, would, a few years later, discuss Goldschmidt in favorable terms. Mayr was one of the first to level the charge that punctuated equilibria was nothing but old saltationism in new guise. Our debt to Mayr's concept of species and speciation, so central to the idea of punctuated equilibria, eventually induced him to do an about face. Mayr came to prefer taking credit for punctuated equilibria rather than seeing it linked to his old nemesis Goldschmidt.

Dawkins, R. (1996) Climbing Mt. Improbable W.W. Norton & Company, New York.

Dennett, D. (1995) Darwin's Dangerous Idea. Simon & Schuster, New York.

Eldredge, N. (1995) Reinventing Darwin. John Wiley & SOns, Inc., New York


T Ryan Gregory said...

Thank you for posting that, Larry. It was getting to me too to see the conflation of punk eek with saltationism. It's especially curious to see claims that Mayr ruled out saltationism, and presumably by extention punk eek, when he considered himself (with just cause) as an early originator of the latter.

I believe I was the first author to develop a detailed model of the connection between speciation, evolutionary rates, and macroevolution (Mayr, 1954). Although long ignored, my new theory of the importance of peripatric speciation in macroevolution is now widely recognized. "Mayr's hypothesis of peripheral isolates and genetic revolution must of necessity be a centerpiece of the punctuated equilibria theory; it is the theory, for all practical purposes" (Levinton, 1983:113). I once more presented my theory in great detail (Mayr, 1963:527-555). Under these circumstances it is most curious that the theory was completely ignored by paleontologists until brought to light by Eldredge and Gould (1972).

Mayr, E. 1992 Speciational evolution or punctuated equilibria. In The Dynamics of Evolution, ed. A Somit and S. Peterson. Cornell University Press, Ithaca, NY, pp.21-48.

T Ryan Gregory said...

Incidentally, it could very well be that Mayr did an about face as Eldredge argues, but one does not with interest what he said in 1954:

"...“rapidly evolving peripherally isolated populations may be the place of origin of many evolutionary novelties. Their isolation and comparatively small size may explain phenomena of rapid evolution and lack of documentation in the fossil record, hitherto puzzling to the palaeontologist."

Larry Moran said...

The problem with Mayr is that he said so many, often contradictory, things that he can take credit for almost anything.

If his idea was so important then why does has have to go back and quote mine his own work to find the key sentences? Shouldn't he have published an entire paper on this topic if it was that much of an insight?

Timothy V Reeves said...

I assume 'punctuated equilibria' and the like goes down a treat with the ID community.

T Ryan Gregory said...

Hi Larry,

Fair enough. My point is simply that Mayr was anti-saltationist but also claimed that he laid the foundation for punctuated equilibria, such that they are obviously distinct items.