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Thursday, June 07, 2007

Charles Darwin Was a Gradualist

 
Punctuated Equilibria refer to patterns of evolution characterized by long periods of stasis interrupted by shorter periods of evolutionary change. The changes are associated with speciation events where a new, changed, species splits off from the parental, unchanged, species. This is speciation by cladogenesis (splitting) as opposed to anagenesis, where a single species changes gradually into something different without splitting.

Stephen Jay Gould wrote a massive book on macroevolutionary theory that featured punctuated equilibria and its consequences such as species sorting. The original book was called The Structure of Evolutionary Theory. It's 1433 large pages long. Few people have read the entire thing. I am one.

Recently, the publishers of The Structure of Evolutionary Theory (Belknap Press of Harvard University Press) have brought out a new book by Stephen Jay Gould—a remarkable achievement since Gould died in 2002. As it turns out, the new book Punctuated Equilibrium, is just chapter nine of the large book; a 279 page chapter called Punctuated Equilibrium and the Validation of Macroevolutionary Theory. PZ Myers reviewed this book for New Scientist a few weeks ago [Punctuated Equilibrium by Stephen Jay Gould].

One of the significant impacts of Punctuated Equilibrium is the emphasis on episodic change rather than the gradualism that was so commonly believed to be the main pattern in evolution. Eldredge and Gould explain why gradualism was such a prominent part of evolutionary theory before punctuated equilibrium came along. Gould has extended that explanation in Structure and in numerous essays.

Gould points out that Charles Darwin was a firm believer in gradualism. Indeed, it was an essential component of Darwin's defense of evolution and his promotion of natural selection. Darwin's gradualism arose from his commitment to the uniformitarianism of Lyell and his emphasis on the vast age of the Earth. According to Gould there are several different meanings of gradualism but the one that conflicts with punctuated equilibria is the idea that change must be gradual at geological scales.

This is why Darwin said that nature does not proceed by leaps and it's why Darwin postulated that gaps in the fossil record were due to lack of data.

PZ Myers noted this in his review when he said,
Gould and Eldredge proposed punctuated equilibrium as a paleontologist's view of the history of life: they were describing the paleontological data available at the time pointing out that there was no geological evidence to support Charles Darwin's belief that species evolved gradually. Time has shown them to be correct, and their observations are now accepted by most biologists as a accurate account of evolutionary history.
Now, PZ is no dummy. He's been around the blogosphere and newsgroups long enough to know that calling Darwin a gradualist is like waving a red flag in front of a bull. There's a large group of Darwin apologists out there who will do anything to prove that Charles Darwin was right about everything, even if is has to be proved retrospectively (i.e., somehow we didn't notice that Darwin believed in punctuated equilibrium until 1972).

The first salvo was fired in the letters column in this week's issue of New Scientist. Wayne Bagguley writes,
I am shocked that someone as knowledgeable as P.Z. Myers is promoting the age-old myth about "Charles Darwin's belief that species evolved gradually" without periods of stasis. In On the Origin of Species Darwin wrote "Although each species must have passed through numerous transitional stages, it is probable that the periods, during which each underwent modifications, though many and long as measured by years, have been short in comparison with the periods which each remained in an unchanged condition. These causes, taken conjointly, will to a large extent explain why—though we do not find many links—we do not find interminable varieties connecting together all extinct and existing forms by the finest graduated steps."
Stephen Jay Gould is no dummy. As an expert on Darwin and the history of biology you can be sure that he's heard these complaints before. You can be certain that Gould has addressed them numerous times.

The longest, and best, defense of Darwin's gradualism can be found in chapter 2 of Structure. This is a 76 page chapter titled "The Essence of Darwinism and the Basis of Modern Orthodoxy." I'm sure PZ Myers has read this chapter and that's why he said what he said. Gould makes the point that,
SInce Darwin prevails as the patron saint of our profession, and since everyone wants such a preeminant authority on their side, a lamentable tradition has arisen for appropriating single Darwinian statements as defenses for particular views that either bear no relation to Darwin's own concerns, or that even confute the general tenor of his work.... I raise this point here because abuse of selective quotation has been particularly notable in discussion of Darwin's views on gradualism. Of course Darwin acknowledged great variation in rates of change, and even episodes of rapidity that might be labelled catastrophic (at least on a local scale); for how could such an excellent naturalist deny nature's multifariousness on such a key issue as the character of change itself? But these occasional statements do not make Darwin the godfather of punctuated equilibrium ....
You'll have to read Gould's chapter to see the case he makes because it's much too involved to summarize here. I do note one pithy comment to the effect that,
I will not play 'duelling quotations' with 'citation grazers,' though a full tabulation of relative frequencies could easily bury their claims under a mountain of statements.
He then proceeds to tabulate dozens of quotations that demonstrate Darwin's gradualism!

17 comments :

  1. Hi Larry,

    I know you've been around the block on talkorigins about this many times, but here, perhaps, is a new argument:

    This whole debate is pretty bogus. The word "gradual" did not mean "constant-rate-ism" back in Darwin's day. The word "gradual" is related to "graduation" and "graduated", as in "graduated cylinder." In other words, "gradual", for Darwin, meant stepwise. Darwin referred to the "gradual" rise of the west coast of South America -- after watching an earthquake that hauled coral reefs up to dry land.

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  2. 97.8% of the hoorah about this topic rests on careless use of fuzzy words. Darwin, as I read him, was an incrementalist, which is equivalent to what Nick calls step-wise. The genuine questions revolve around things like the size of the increments (steps) and the rate at which they accumulate via selection or drift. Subsidiary questions are things like how detectable the increments (and accumulation of increments) will be in various kinds of datasets -- fossil, molecular, what have you.

    I agree with Nick: It's a bogus issue.

    RBH

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  3. "According to Gould there are several different meanings of gradualism but the one that conflicts with punctuated equilibria is the idea that change must be gradual at geological scales."

    Hmm, okay, but that didn't help this non-biologist much.

    But apparently the idea of punctuated equilibria doesn't have different meanings, but claims that virtually all change takes place during speciation. "Punctuated equilibrium (also called punctuated equilibria) is a theory in evolutionary biology, which states that most sexually reproducing species will show little change for most of their geological history. [...] Eldredge and Gould proposed that the gradualism predicted by Charles Darwin was virtually nonexistent in the fossil record, and that stasis dominates the history of most fossil species." ( http://en.wikipedia.org/wiki/Punctuated_equilibria )

    The difference between refusing that (appropriately scaled) gradually change "must" or "can" take place is huge, and I certainly hadn't appreciated the refusal of other speciation mechanisms than the allowed subset. It must be much harder to show that all change is punctuated instead of some cases.

    Hmm, confusing, how does ring species fit into this all-or-nothing picture? Also, why the PE 'hardline' confinement to a particular subset of all proposed speciation mechanisms? I know, I know, RTF manual (book) and learn something for once! :-)

    "97.8% of the hoorah about this topic rests on careless use of fuzzy words."

    I'm leaning towards 97.9 % here. ;-)

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  4. I have also read TSOET (never ever again), and it drives home one very clear opinion that I've had since Spandrels: Gould builds weight for many of his arguments out of rhetoric, rather than generous consideration of all the data. And TSOET has a lot of rhetoric.

    Exaptation is a decent and helpful concept, as is Spandrels. But unlike those two, PE has very little predictive power; there are many mechanisms that can produce PE-like patterns. And reading those sections of TSOET particularly carefully as well as his other PE-related articles and letters, it's clear that he struggled and struggled with trying to bring PE forth into something more than simply a description of a pattern. But he failed, and I wish that he would have been able to accept that.

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  5. RBH writes,

    I agree with Nick: It's a bogus issue.

    Which issue is bogus? Is it the idea that punctuated equilibria conflict with the old fashioned view of evolution or is it the issue that Darwin wasn't a gradualist (in the modern sense of the word)?

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  6. Torbjörn Larsson says,

    But apparently the idea of punctuated equilibria doesn't have different meanings, but claims that virtually all change takes place during speciation.

    Punctuated equilibrium is a "pattern" where most change takes place during cladogenesis. The proponents of PE recognize that there are many examples of anagenesis and other patterns that do not look like PE.

    The question in evolutionary theory is whether most evolutionary change is associated with punctuated equilibria patterns or whether gradual anagenic speciation is the dominant mode of speciation (or something else).

    If it's PE then there are consequences. One of them is the recognition that parental species tend to live on after the split. This has implications for theories of speciation that require adaptation. Another is what to do with all the species. You can't just keep doubling the number of species forever. What causes some species to go extinct? (This leads us into species sorting.)

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  7. Darwin did explicitly state that there would be no fixed, constant rate. That is a little different from endorsing an idea like PE, though. It's hard to see Darwin as a punctuationist because, for one thing, while he thought the earth was old, on the order of millions of years, his scale was off--his estimated timing for the duration of the entire planet was closer to the lifespan of a single species. Gould estimated that 99% of a species lifespan was spent in approximate morphological stasis, a time on the order of several million years, and that speciation and the punctuational transformation were coupled, and took on the order of tens of thousands of years. The argument is not just about changing tempo of evolutionary change, but how rates are correlated with species transitions. Speciation is the locus of evolutionary change--that is not a simple Darwinian view.

    Now I agree that TSoET is loaded with rhetoric ("rhetoric", by the way, is not an evil word—it refers to using communication to persuade and convince. It's kinda like "framing" nowadays. It's not intrinsically bad unless it is used to misrepresent), and that Gould shot himself in the foot by not getting an editor to trim down some of the bloated excesses of his verbiage. But good grief, did you actually read chapter 9 of the book? It's a hard slog not just because Gould uses three words where one would do, but because despite that the information density is incredibly high. For instance, start on page 822, the section titled "Sources of Data for Testing Punctuated Equilibrium". Keep reading to page 874. It's a data review. It made my head hurt, there's so much information and so many citations packed into that relatively short (hah!) section. And it's not the only place where he backs up his argument with a deep delving into the literature.

    Basically, though, if you're thinking PE just means evolution is sometimes fast, sometimes slow, you don't know what PE means. It's a theory that says that there are processes that drive relatively rapid change in a lineage that establish a species as a discrete entity, and that we need to consider multiple levels of evolution beyond just populations. The book has a hard core of data (very hard!), and it also has lots of explanation and interpretation by Gould. Both are indispensable for understanding the concept. If it were just rhetoric, it would be philosophical masturbation, but it is not. However, the complement to that is that if it were just tables and charts with no theory and no philosophy, it would be just as dead and useless. Gould tries to provide both, that necessary framework for understanding the data, and the data itself.

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  8. anonymous says,

    Gould builds weight for many of his arguments out of rhetoric, rather than generous consideration of all the data. And TSOET has a lot of rhetoric.

    The word "rhetoric" has several meanings. I assume that you are not being complimentary so you must mean that Gould uses words in an affected and exaggerated way. That's one of the meanings of "rhetoric."

    When you say that Gould uses rhetoric instead of carefully examining all the the data you are dead wrong. If anything, Gould does the opposite. Unlike many other science writers (e.g., Dawkins) Gould goes out of his way to consider alternative explanations and data that does not agree with his ideas. Part of the reason why TSET is so dense is that the case for macroevolutionary theory is laid out in painstaking detail. That's the exact opposite of what you say.

    The other reason why some people have trouble reading the book is because the level of language is very high. Gould uses a lot of big words and he assumes that his readers are familiar with English literature. It's definitely not for high school students.

    PE has very little predictive power; there are many mechanisms that can produce PE-like patterns

    This is correct. Punctuated equilibria is actually an observation related to the history of life so it doesn't make predictions.

    On the other hand, the mechanism that gives rise to PE is part of macreovolutionary theory so it can be tested in many cases. Gould presents various mechanisms and then tells us which ones he prefers. I happen to agree with him on the mechanism of change during cladogenesis but disagree about species sorting.

    And reading those sections of TSOET particularly carefully as well as his other PE-related articles and letters, it's clear that he struggled and struggled with trying to bring PE forth into something more than simply a description of a pattern. But he failed, and I wish that he would have been able to accept that.

    Of course he wouldn't have accepted that! The whole point of the book was to try and explain why evolution looks the way it does when you examine the fossil record. The pattern cries out for an explanation and Gould struggled with evolutionary theory all his life.

    I think he made considerable progress. If you have a better explanation for the punctuated equilibrium pattern then please share it with us.

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  9. Isn't the explanation for the pattern just supposed to be allopatric speciation as observed in the fossil record? If Gould had just said that instead of "punctuated equilibria" I think this would have saved a lot of time.

    E.g. "equilibria" implies process, not pattern IMO, and the name invites the following:

    1. People think PE is about something other than speciation, e.g. the origin of phyla

    2. The implication that there is some kind of internal homeostasis keeping a species where it is at (in fairness Gould may have got this from Mayr, who seems to have theorized that "genetic revolutions" occurred in isolated populations), instead of e.g. ecological niches.

    3. "Punctuated" sounds sudden to many people, a very hairy topic since "sudden" can mean tens of millions of years, in paleontology.

    (whine, whine)

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  10. Larry asked which issue is bogus, that PE conflicts with the old fashioned idea of evolution or that Darwin wasn't a gradualist (in the modern sense of the word). Since part of my argument is that "gradualist" is itself a deceptive term and that "incrementalist" is more accurate, I can't directly speak to that question.

    The central argument of PE, at least in its original form, rested on the argument that the fossil record for speciation is genuinely punctuate -- new species really do appear "suddenly" in geological terms. To account for that claimed-to-be-genuine pattern, PE had two main components: (1) Most speciation is cladogenesis, what Eldredge and Gould consistently called "allopatric" -- their consistent usage was to contrast "allopatric" with "phyletic gradualism" -- and most (all?) speciation occurs relatively rapidly in small isolated sub-populations. They write

    In summary, we contrast the tenets and predictions of allopatric speciation with the corresponding statements of phyletic gradualism previously given:

    (1) New species arise by the splitting of lineages.
    (2) New species develop rapidly.
    (3) A small sub-population of the ancestral form gives ries to the new species.
    (4) The new species originates in a very small part of the ancestral species' geographic extent -- in an isolated area at the periphery of the range.

    These four statements again entail two important consequences:
    (1) In any local section containing the ancestral species, the fossil record for the descendant's origin would consist of a sharp morphological break between the two forms. This break marks the migration of the descendant, from the peripherally isolated area in which it developed, into its ancestral range. Morphological change in the ancestor, even if directional in time, should bear no relationship to the descendant's morphology (which arose in response to local conditions in its isdolated area). Since speciation occurs rapidly in small populations occupying small areas far from the center of ancestral abundance, we will rarley discover the actual event in the fossil record.

    (2) Many breaks in the fossil record are real; the express the way in which evolution occurs, not the fragments of an imperfect record. (p. 96; remainder snipped)


    In contrast to PZ's claim, Eldredge and Gould did not invoke of any sort of 'special' mechanisms that drive relatively rapid change beyond isolation of small sub-populations. In the two examples of fossil series they give, one from Eldredge's work with trilobites and one from Gould's with snails, they are at pains to show the incremental change through time in the small sub-populations.

    I read Eldredge's "Time Frames: The Evolution of Punctuated Equilibria" years ago, and IIRC he added no novel mechanisms to PE. (His account there of discovering the small isolated transitional series of trilobites was interesting, though.) However, I have not read TSoET; Gould may have identified some of those specific special processes there. But the core of PE seems to me to be captured in the quotations from the 1972 paper I gave above. It is a fundamentally incrementalist view of evolution which is argued to account for the punctuate fossil data at the species level of analysis. They were simultaneously evolutionary incrementalists and paleontological punctuationists.

    RBH

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  11. "The question in evolutionary theory is whether most evolutionary change is associated with punctuated equilibria patterns or whether gradual anagenic speciation is the dominant mode of speciation (or something else)."

    Thanks for your time! This makes sense since it roughly reconciles Wikipedia's entry with your description here, and with the other commenters. The PE picture of what happens in a specific speciation seems fine for this layman.

    My uneasiness was that it is usually very hard to show a "no-go" theorem (for alternative mechanisms, here). A normal testable model I have no problem what so ever with.

    "Another is what to do with all the species. You can't just keep doubling the number of species forever."

    I am sure that makes sense in the PE framework, because as long as populations are finite as they must (and so the total "population"), I don't really see the idealized problem for sexual species. The smallest remnant population with N=1 will surely go extinct. :-)

    But I believe I see the interest of what happens in real life under different models.

    PZ:

    Thanks for the review, I will have to read another FM. :-)

    RBH:

    Um, okay, but changed population size (from the geographical split) and codependent new characteristics could perhaps drive the "New species develop rapidly"? Incremental, but a different pace.

    I think I get the morphological break. IIRC there is currently indications of a speciation in a sea here in Sweden. I believe it was perch that has divided into two habitats from intensive predation from pike. The vegetation hider is stubby, while the free swimmer is lank. And they don't breed with each other.

    If the pike would disappear, it could certainly look like two different species in the fossil record when they migrate into each others habitat. Or at least for a while, if they can interbreed.

    (I also note that since the two populations have the same source of forcing due to being parapatric (?), the morphological changes are correlated, which is not what your quote from G&E describes. Perhaps I get the allopatric barrier part too.)

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  12. Errm, I did not claim that there were 'special' mechanisms. If there were, I'd like to know what they are.

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  13. Above I wrote

    They [Eldredge & Gould] were simultaneously evolutionary incrementalists and paleontological punctuationists.

    Earlier this evening I wandered into the college bookstore and there was Chapter 9 of TSoET, "Punctuated Equilibrium", all 350+ pages of it, smiling at me from the shelf. So I bought it and started reading. This is what I found on page 27:

    The theory of punctuated equilibrium does not engage this important meaning [of gradualism in the sense of the insensibility of intermediates] for two reasons: first, our theory does not question the operation of natural selection at its conventional organismic level; second, as a theory about the deployment of speciation events in macroevolutionary time, punctuated equilibrium explains how the insensible intermediacy of human timescales can yield a punctuational pattern in geological perspective -- ... (p. 27).

    As I said, "... simultaneously evolutionary incrementalists and paleontological punctuationists".

    Of course, following the dash in the quotation above Gould has to go on:

    ... thus requiring the treatment of species as evolutionary individuals and precluding the explanation of trends and other macroevolutionary patterns as extrapolations of anagenesis within populations.

    I'm not at all sure how that first phrase about "species as evolutionary individuals" after the "thus" actually follows. I guess I'll have to read the rest of the book/chapter.

    RBH

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  14. It's rubbish to state that there is no fossil evidence showing the 'gradual' evolution of at least some species; there is.

    All evolution occurs gradually, there are no saltational leaps between generations (or at least they are very very rare indeed and do not significantly contribute).

    Darwin pointed out that the periods of evolutionary change that occur are smaller than the periods where little or no change occurs. This is, essentially, what PE is all about.

    There are large periods of stasis but the evolution that occurs is still gradual, no matter how rapid it may be geologically. What's more, different creatures evolve at different rates at different times. Darwin knew this and PZ Myers, Gould and Eldredge are all wrong to claim otherwise.

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  15. Anyone who thinks that Darwin thought that change was gradual and constant is ignorant, as demonstrated by the following passage from TOOS:

    "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."

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  16. Was Charles Darwin a gradualist? If yes is there actual proof of him being a gradualist? If not is there actual proof?

    Any and all information would be helpful. Thank you.

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