Tuesday, October 23, 2007

Why Five Fingers?

 
Josh Rosenau is settling into his new job at the National Center for Science Education (NCSE). Part of his mission is to educate us in the ways of evolution and so far he's doing a great job. NCSE has always had a correct perspective on evolution, as far as I'm concerned, even though some of the people who used to work there tended to favor adaptationism.

Here's Josh's latest from his blog Traveling from Kansas [The Panglossian Paradigm, or as science moves forward, creationists move back]. Note that the opinions on his blog do not necessarily reflect those of NCSE.
For really confused students, I draw on a point Stephen Jay Gould made in Eight Little Piggies (in the essay by the same name), that the number of fingers we have is entirely contingent on history. While one can try to construct an explanation for the superiority of 5 fingers, paleontological history shows that there were potential ancestors of the tetrapod clade (which we are part of) which had as many as eight rays per fin. If they had succeeded, 8 fingers would be the norm, and the Simpsons would look very odd with only 4. As Gould says of historical contingency: "Other configurations would have worked and might have evolved, but they didn't--and five works well enough."

In the essay, Gould is building on a point he made most forcefully in an essay he wrote with Richard Lewontin, "The Spandrels of San Marcos and the Panglossian Paradigm: A Critique of the Adaptationist Program." The point was that biologists were too quick to insist that every feature was adaptive and a result of natural selection. Spandrels are triangular structures produced when two round arches meet. They are necessary byproducts of joining rounded and flat surfaces. Nonetheless, in many churches they are richly decorated and the entire artistic vision for a space can be shaped by the spandrels. One might, Gould points out, be lead to think that the spandrels are there in order to be used for paintings, and not that they are necessary by-products nicely dressed up. The worldview he criticizes treats anything, whether spandrels or five fingers, as the product of intense selection, a perfect solution to the problems it faces.
There's lot more where that came from so get on over to Travelling from Kansas for more information on the "correct" worldview.

By coincidence, today's Scientific American question is Why do most species have five digits on their hands and feet?. While there's a bit of catering to an adaptationist perspective the answer to the question is ...
Is there really any good evidence that five, rather than, say, four or six, digits was biomechanically preferable for the common ancestor of modern tetrapods? The answer has to be "No," in part because a whole range of tetrapods have reduced their numbers of digits further still. In addition, we lack any six-digit examples to investigate. This leads to the second part of the answer, which is to note that although digit numbers can be reduced, they very rarely increase. In a general sense this trait reflects the developmental-evolutionary rule that it is easier to lose something than it is to regain it. Even so, given the immensity of evolutionary time and the extraordinary variety of vertebrate bodies, the striking absence of truly six-digit limbs in today's fauna highlights some sort of constraint.
Remember the take-home lesson (mostly from Josh's article). Living organisms are not well designed in spite of what the creationists and the adaptationists would have you believe.



17 comments:

  1. You appear to be conflating Josh Rosenau (of Thoughts from Kansas) and Jason Rosenhouse (of Evolutionblog). In doing so, you've created "Jason Rosenhau" -- some sort of portmanteau creature.

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  2. While looking for features that ‘self evidently’ don’t have a strong selection pressure behind them, digit number is apparently (repeat, apparently) one of them. However, is it very difficult to come up with a ‘killer example’ of a feature that is ‘clearly’ indifferent to selection pressures as the problem seems so open ended; organisms and evolutionary scenarios are such complex cans of worms that there is plenty of potential for a worm to pop out of the conceptual woodwork that we have never seen or thought of before. So who knows, five digits might yet be a favored configuration. In evolution irrefutability is the name of the game.

    But what about this one: the preponderance of right handed mollusk shells? If we ‘reflected’ the evolutionary scenario that lead up to this phenotype would the probabilities at each ‘event juncture’ remain unchanged? That is, is the left-handed shell equally probable to the right-handed shell? Generalizing: is evolution insensitive to reflections?

    To me this seems to be a killer argument in favor of a feature insensitive to selection pressure (namely right-handedness and left-handedness), but that may be because I simply don’t know enough about the effect of reflections on chemistry.

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  3. You all do know that some people are born with six fingers and toes?

    As in Ishbibenob of the Philistines, if memory serves me correctly, though no doubt I have misspelled his name--here's hoping he doesn't mind--he also was said to be rather a large fellow...

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  4. …. but once a world with a particular ‘handedness’ has evolved, even if it is equally probable to the world with the opposite ‘handedness’, this ‘handedness’ will then become an environmental feature which in turn opens up the possibility that it will have a selection pressure on the handedness of other organisms. Oh, I’m confused now.

    PS Lee Merril: Anne Boleyn was supposed to have six fingers.

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  5. Wave that copy of Gould & Lewontin! Wave it wide and high!

    But of course five digits--no more, no less--is adaptive! It leads directly to base-10 arithmetic, which is clearly optimal. Also, do you know how hard it would be to play the saxophone without exactly 10 fingers? No pentadactyly, no John Coltrane; QED.

    "Living organisms are not well designed in spite of what the creationists and the adaptationists would have you believe."
    Yes, that's right; our cover is blown. We adaptationists are in secret league with creationists to spread hither and yon the false doctrine that living organisms are well designed. *eyes rolling uncontrollably*

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  6. steve dimilo says,

    Yes, that's right; our cover is blown. We adaptationists are in secret league with creationists to spread hither and yon the false doctrine that living organisms are well designed. *eyes rolling uncontrollably*

    Perhaps there are some adaptationists who have escaped the trap. But have you read what Dawkins says it in the preface to The Blind Watchmaker?

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  7. Thanks for the kind words, though I'll note that there's no 'h' in my last name.

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  8. Joshua Rosenau says,

    Thanks for the kind words, though I'll note that there's no 'h' in my last name.

    Okay. So I got your first name wrong and your last name wrong. I'm an idiot.

    Do you have a middle name? :-)

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  9. In evolution irrefutability is the name of the game.

    IANAB, but I will respectfully disagree here. See, my idea on this is that adaptionism should be one research strategy among others. Any hypothesis must still be predictive and tested in the end. Otherwise it will be alongside a non-adaptionist hypothesis or two, and the conclusion is still that "we don't know". [Unless someone can claim that one or another is preferred by parsimony. But I don't see how to do that comparison here.]

    the preponderance of right handed mollusk shells?

    Is there? Do you have any references?

    Here is a post describing chirality, its distribution, and its causes in snails, and it links to an interesting post on diverse mechanisms for symmetry breaking. (And yes, here is one group of snails described where 5 of 22 species are sinistral. But it looks like the mechanisms are symmetric.)

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  10. "But have you read what Dawkins says it in the preface to The Blind Watchmaker?"

    Yep. What's more, I agree with him. The complexity of organisms is empirical. Organisms are pretty good at what they do. This is the "appearance of being well-designed" Dawkins is talking about (I think; I am anyway).

    Maybe we could quibble over the meaning of the adverb "well." To me it means not perfection, not even optimality in many cases, but "design" good enough to have permitted successful reproduction of organismal complexity in an unbroken 3+-billion-year-long chain of ancestors, including the successful tracking of umpteen-million major changes of environment. I do not deny that contingency, luck, and chance have likely played multiple roles in the evolutionary history of extant (and, perhaps moreso, extinct) organisms, but I think that the dominant evolutionary engine between these unpredictable events has always been natural selection, which, despite its (acknowledged) constraints, and by selecting from variation produced by several mechanisms, has in fact produced organisms that are pretty much "well designed." Again, not in every detail, not perfectly, and not in every identifiable trait, but in general, in the sense that organisms and their parts are good at what they do.

    I study organisms in a comparative context and I see differences and similarities that seem adaptive everywhere. No (Sanders), I don't usually know the genetic or epigenetic underpinnings of the measurements I make, much less the mutation-by-mutation, population-by-population, environment-by-environment history of their evolution. What I do see (quite clearly, it seems to me) is an impressive match between phenotypic and environmental variation, and as far as I know natural selection is, in fact, the only known mechanism that can effect such a match.

    So there's my adaptationist's manifesto. I do think that organisms are well-designed, especially given what we know about how evolution works (e.g. the mechanisms Dr. Moran prefers to emphasize).

    What I objected to was being grouped with creationists!

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  11. "In evolution irrefutability is the name of the game"

    UGH.

    I'd NEVER say that

    Maybe it's because I'm not adaptationiste

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  12. oops. that was me at 1:46:00 PM.

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  13. Well done Torbjorn, keeping on my toes as usual! I got this business about the preponderance of right-handed shells from Stephen Jay Gould’s collection “Dinosaur in a Haystack” chapter 16. Since then I have spent many years looking for sinisteral shells and haven’t found one. So why the preponderance? Is it really a killer example of a feature insensitive to selection? Thanks very much for your links – I’ll study those and see if I can get a handle on the subject.

    In evolution irrefutability is the name of the game. I through that one out because I guessed somebody would catch it! ‘Refutability’ might in some cases look easy, but philosophically speaking it isn’t. ‘All swans are white’. Does a single black swan refute this statement absolutely? No, only probabilistically. Because we live in an open-ended world there are all sorts of conditions (of varying degrees of likelihood) that can be suggested to explain the observation away. Evolutionary theory, being such a complex object, also provides the opportunity for ‘laundering’ out contra-evidence, but the more fanciful the laundering becomes the less likely it is. ‘Refutable’ should read as ‘probably refutable’.

    Now Alex (Sanders): I’m not an adaptationist either but that’s because I’m an ignorant amateur – I simply don’t know enough to refute the wider options of ‘pluralism’. Hence I rely on the comments of knowledgeable people like you for guidance.

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  14. But have you read what Dawkins says it in the preface to The Blind Watchmaker?

    Is that supposed to be our sacred text or something?

    TL: Is there [a preponderance of right-handed snails]? Do you have any references?

    Here's Gould himself saying it's overwhelming. But it's one of those "widely known" things where it's hard to find a definite ref. Unless you would be willing to, you know, actually look at some shells :)

    TVR: Is it really a killer example of a feature insensitive to selection?

    No way.

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  15. Thanks Windy for breezing in and bringing this link to my attention. Once my puritanical sensibilities had recovered from the shockingly explicit sexual material in this article I inferred a not unexpected conclusion: that is, that selection of handedness only occurs once a particular handedness is already in the environment. In other words: the handedness of a mollusk has a bearing on its reproduction chances because of the handedness of its mates. Thus, handedness has the potential to self reinforce – that is, to use a vogue term, it is ‘non-linear’. ‘Non-linearity’ could conceivably also apply to number of digits. For example, 5 digit mates might shy away from the ‘strangeness’ of potential mates with more or less digits. (In this connection it is perhaps significant that ‘left handedness’ is labeled with the term sinistral!)

    But the question I really had in mind is this: Are either sinistral or dextral shells intrinsically favored by the environment when the playing field is ‘level’, that is before the self reinforcement sets in? Well the ‘killer argument’, for what its worth, is no, because it seems that physics is invariant under reflections, and so I deduce that all the probabilities for sinistral and dextral worlds are the same. Hence, I infer that the preponderance of dextral shells is just an accident, an accident that has become locked in by ‘non-linearity’. Where’s my faux pas here?

    Incidentally talking about being willing to look at some shells: Today, as it happened, I had an afternoon out by the sea and I came across a shop selling sea shells; all the species in the window that I saw were dextral! Does that count as rigorous science? Can I publish?

    But on second thoughts, does any of this really say anything profound at all: It seems quite likely that morphospace has lines of equi-probability running through it. So, as with the tossing of a coin where the sequence HTTH is equally probable to the sequence HHTT, then on this basis particular biological forms are ‘accidental’ in as much as what ‘selects’ them from the outset is an ‘accident’ which may then self reinforce. Could this explain the universality of five digits and the preponderance of dextral shells, whose intrinsic survivability seems neither here nor there?

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  16. windy:

    But it's one of those "widely known" things where it's hard to find a definite ref. Unless you would be willing to, you know, actually look at some shells :)


    Thanks! Interesting. And no, I haven't thought too much about my own observations. Because, you know, my sampling of shells would probably not be covering. :-P

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