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More Articles on Evolution

The Scope of Natural Selection

The author replies to H. Allen Orr's review of "Darwin's Dangrous Idea" (Boston Review, Summer 1996).

Daniel C. Dennett

When Professor Orr published his hostile review of Darwin's Dangerous Idea in the biology journal Evolution last February, I was not pleased. His review was full of falsehoods and misconstruals, but I had no recourse; that journal, like most academic journals, does not permit authors to respond to reviews. Luckily for me, Orr was so eager to warn the world of my errors that he restated his attack, with embellishments, in Boston Review, which has invited me to respond. Months have passed, the damage has been done, but at least I get to set the record straight.

I am grateful to Orr for embellishing his attack for the benefit of his lay readers, since these additions vividly expose his own errors and confusion, which were somewhat masked in the more professional version. In what follows I will concentrate on his criticism of my understanding of his field: biology. I trust that the readers of Boston Review have seen the flaws in his philosophical arguments without my help, but a non-biologist might well suspect that, on his home ground, Orr's complaints are as authoritative and devastating as he makes them out to be. They are not.

Before getting to the meat of his criticism, Orr warns readers that my book is:

marred with factual errors, some scientific and some historical. Population genetic theory, for instance, does not prove that evolution by random change is faster than evolution by natural selection, as Dennett claims.

I did indeed misspeak (p. 126), but the result was ambiguity, not error. The issue is complicated: it depends on whether you're measuring the (average) speed of departure from a starting point in genetic space, or the speed of attainment of some particular evolutionary product. I meant the former, as the context ought to have made clear (I was in the process of observing that there has been plenty of time in evolutionary history for the sheer genomic diversity observed to accumulate). But I did couch my claim in terms that permitted Orr to put the false reading on it. Score one half point for Orr. He goes on:

And it was Darwin's theory of sexual, not natural, selection that he called an 'awful stretcher.'

I made no such claim. I entitled a chapter section "Natural Selection-an Awful Stretcher." The vivid epithet comes from Darwin, but it was I who applied it to natural selection. Orr, having already stretched too far in his effort to find an historical error to impute to me, topples over when he tries to elaborate: "Darwin branding natural selection a stretcher is about as plausible as Chomsky branding Universal Grammar a howler." What? Darwin branded sexual selection an awful stretcher in the very act of endorsing it-it is, after all, a special kind of natural selection, not an alternative! Here's what Darwin said (as I noted, when I introduced the term): "It is an awful stretcher to believe that a peacock's tail was thus formed; but, believing it, I believe in the same principle somewhat modified applied to man." Surely Darwin would have agreed, if asked, that natural selection was another awful stretcher he believed in. Orr's remark about Chomsky makes it appear that he thinks Darwin was dismissing sexual selection when he called it an awful stretcher, but I suspect he knows better and simply got carried away by his desire to rub it in about my nonexistent error.

These are the only two "factual errors" he comes up with, and he judiciously grants them the status of "peccadilloes." There are some factual errors in my book that others have pointed out, but since Orr does not brandish any of them in his essay, I think we can conclude that they eluded his scrutiny. Let's turn, then, to his main concern: my book's alleged deep misunderstandings of biology.

Orr concentrates on my criticism of Gould and Lewontin's famous attack on adaptationism, "The spandrels of San Marco and the Panglossian paradigm." Evolution-what Darwin called "descent with modification"-is an uncontroversial fact, as secure as any in science, but there is still controversy among biologists about just how many facts about evolution are explained by natural selection, the mechanism Darwin proposed as the most important force shaping evolution. To see what the issue is, consider the popular retort of software engineers when somebody finds a flaw in their program: "It isn't a bug, it's a feature." In other words, it's supposed to be that way; it was designed to work like that. Now is everything observable in the biosphere a "feature," an "adaptation," as an evolutionary biologist would call it? Are there no bugs, no undesigned bits, no historical accidents? Of course there are. Everybody agrees on that. Everybody does not agree on how important these non-adaptations are; the greater the role you give to natural selection, the more "adaptationist" you are, and the Gould/Lewontin essay was an attempt to swing opinion the other way.

Orr begins by usefully perpetuating some of the misunderstandings I exposed in my discussion of Gould and Lewontin's central architectural metaphor for a non-feature: a "spandrel"-one of the curving wedges of masonry that serve as the transition from the domes of the Basilica di San Marco in Venice to the arches that hold them up. (I find this useful because some critics have wondered if any biologists have actually been confused on this score; my answer is yes-see, e.g., H. Allen Orr.) This is what he says:

Although spandrels are often decked out with mosaics, no one would seriously argue that spandrels are there because they provide such swell surfaces for mosaics. Instead, spandrels are there because they have to be-they are, it turns out, the inevitable by-product of putting a dome on rounded arches.

"Inevitable by-product"? This is either flat false or true but irrelevant. If by "spandrel" Gould and Lewontin mean the particular structure used in San Marco (properly called a pendentive) then what they say is false; pendentives are one of many options, but they are probably the optimal engineering solution to the problem of supporting the dome-what I would call a Forced Move. In this sense, spandrels (pendentives) are adaptations par excellence. If, on the other hand, Gould and Lewontin mean by "spandrel" just "whatever you put in that place between the dome and the arches," then spandrels are trivially inevitable-you have to put something there. Gould himself has recently1 opted for this reading, but as I had already pointed out in my book, pp. 272-3, this interpretation also disqualifies spandrels for their role as lead metaphor in Gould and Lewontin's biological argument. In this sense, architectural constraints present a problem, not a solution, in biology as much as in building, and that is where natural selection comes in.

This has been much discussed recently, and the architectural engineer Robert Mark, in a fascinating article in the July issue of American Scientist, shows that my own amateur engineering (and architectural history) led me to underestimate the structural requirements of putting a large dome on arches. The pendentives are in fact not just aesthetic adaptations, as I had claimed, but also structural adaptations. So the San Marco "spandrels" are a doubly poor choice for eponymous non-adaptations. The mosaics on the pendentives are aesthetic design choices enabled by prior structural design choices. An example of one adaptation exploiting a prior adaptation hardly strikes a telling blow against adaptationism.

But why are we quibbling about these fine points of architectural history? Why, indeed, do I pay such obsessive attention to Gould and Lewontin's attack on adaptationism? Orr expresses his bafflement:

Why, after all, should a man hoping to export Darwinism from biology be so obsessed with defending the minutiae of adaptive storytelling within biology? I would not think the legitimacy of, say, cultural Darwinism hangs critically on whether selection is very important in biological evolution (as everyone, including Gould and Lewontin, believe) or is almost exclusively important (as Dennett believes).

First, Orr strains to create a gulf that does not exist between me and the run of biologists. What, we may ask, is the difference between "very important" and "almost exclusively important"-the presumably dissenting view he attributes to me? It is Gould and Lewontin, in fact, who have had trouble clarifying their charge. Everybody agrees, as Orr says, that selection is very important, so just what are Gould and Lewontin saying? That it is not as important as some people think? If that had been their message, I would agree with John Maynard Smith that their paper was "on the whole, welcome." Orr quotes this verdict with approval, and so did I (p. 278), noting, however, that this was not their message-or at least it has not been Gould's version of their message.

I think Orr's professed perplexity must be disingenuous. I make it crystal clear why I have to go to all this trouble clarifying these minutiae: Gould has persistently misrepresented the import of the Gould/Lewontin paper outside biology, and many have been taken in. My task was to show the non- biologists that they have been seriously misled by Gould about this. In my book I list four propositions that are widely believed by non-biologists to have been demonstrated by Gould. The first two are relevant here:

If you believe: 1) that adaptationism has been refuted or relegated to a minor role in evolutionary biology, or (2) that since adaptationism is 'the central intellectual flaw of sociobiology' (Gould, 1993a, p. 319), sociobiology has been utterly discredited as a scientific discipline . . . then what you believe is a falsehood. (p. 265)

Well, are these truths or falsehoods? They are widely believed. Many non-biologists are under the weird misapprehension, thanks to Gould's rhetoric, that one is under no obligation to provide an adaptive account of the evolution of a complex competence or organ-such as human language. In some misguided quarters, indeed, adaptationist explanations of anything are automatically suspect! For more than a year before the publication of my book and on several occasions since then, I have repeatedly requested that he clarify his position on these propositions. Steve Gould is undeniably a Great Communicator. If these propositions are not what he meant, if over-eager readers have misunderstood him, he should find it both obligatory and easy to correct these widespread misapprehensions. If he meant them, he should either defend them against my charge that they are false, or concede that he has misled his readers. He has not accepted my invitation to clarify his position, so it falls to me to explain to the world, at whatever length it takes, why these are not the take-home messages from Gould and Lewontin's article.

The centerpiece of Orr's critique of my defense of adaptationism is his feigned astonishment that I omit any discussion of Motoo Kimura's neutral theory, "the most serious and famous of all challenges to selectionist story-telling." Here he plays a cute rhetorical trick: Boy oh boy, Dennett must be really out of it to have overlooked Kimura's work! Why on earth did he omit this? Was it because he never heard of it? Naw. Or because Kimura's book was too difficult for him to read? Naw. Or because he "doesn't want to let the cat out of the bag" about the fact that biologists have found out that "non-Darwinian" evolution is common? Naw. Having relieved himself of these snide but indefensible suggestions-as he himself so generously acknowledges-he arrives at the obvious truth: "Dennett is interested in Design . . . and the neutralist/selectionist controversy has nothing to say here." Right. It wasn't relevant. Kimura's theory is about typographical change, visible at the molecular level of the genome. As Orr says, "most evolution at the molecular level [my emphasis] is not caused by natural selection, but by 'genetic drift.'" Before Kimura, theorists hugely underestimated the possible role of the accumulation of random mutations that are (as Orr notes) functionally equivalent. Kimura's work is wonderful, surprising, and still controversial; it claims (among other things) that the diversity observed at the molecular level could be much more the result of random genetic drift than many theorists-"selectionists"-had thought, but this aspect of selectionism has little or nothing to do with adaptationism, which is why I left it out. Oh, I could have had some fun with the fact that there are indeed some biologists- Gould and Lewontin come to mind-who have often suggested to the lay public that Kimura's neutral theory is a rival to natural selection as an account of morphologic evolution (the shaping of complex phenotypic features), but as Orr as much as admits, this is not taken seriously among biologists.

From all Orr's hooting one might get the impression, by the way, that I had nothing to say on these topics in my book, but in fact, I discuss the role of random genetic drift (pp. 125-6) and the ubiquitous possibility of typographically different but functionally equivalent genetic recipes for proteins (pp. 195-6, p. 287). Where Orr mentions functionally irrelevant differences in varieties of hemoglobin, I mention functionally irrelevant differences in lysozyme. Same points made, right down the line. In fact I also make all the other points about adaptation he goes on to explain for his lay readers, saying "Dennett never confronts these legitimate worries." (All of them? Yes. If you want to test my claim, you can go back and match page 30 of Orr's review with my pages 199, 248-249; I'll spare readers the tedious details.)

One last point about Orr's understanding of central concepts in his own field before we turn briefly to the evolution of culture. I say that natural selection is an algorithmic process, and hence is "substrate neutral"-it doesn't matter what material the algorithm is executed in, so long as the recipe is followed. Orr says, correctly:

This substrate neutrality argument is supremely important to Dennett. It-and nothing else-explains why selection can be lifted from its historical base in biology. It is what makes Darwinism so dangerous.

He goes on: "But Dennett slips here." It is Orr who has slipped, falling flat on his face with his proposed counterexample to my claim that natural selection is a substrate neutral process: "blending inheritance."
Orr says:

Evolution would quickly grind to a halt, for instance, if inheritance were blending, not particulate. With blending inheritance, the genetic material from two parents seamlessly blends together like different colored paints. With particulate Mendelian inheritance, genes from Mom and Dad remain forever distinct in Junior. This substrate problem [sic] was so acute that turn-of-the-century biologists-all fans of blending inheritance-concluded that Darwinism just can't work. Modern evolutionary genetics was born in 1930 when Sir Ronald Fisher cracked this problem: Population genetics shows that particulate Mendelian inheritance saves the day. It is just the kind of substrate [sic] needed for evolution by natural selection to work.

Orr is right that Mendel saved the day for Darwin (a point of history I discuss in somewhat greater detail in my book), but this was a triumph for substrate neutrality! Mendel-and Fisher, too, for that matter-was clueless about what the substrate was. (It's DNA, of course, but that wasn't discovered until 1953, by Crick and Watson.) Mendelian genes are a paradigm case of substrate neutrality; they are pure data structures, whose material composition is irrelevant just so long as they obey the combinatorial rules Mendel laid down. The field of population genetics thrives to this day in almost complete independence of any concern with the nitty-gritty biochemistry of the substrate- just the way software engineering is conducted by people who need know little about the electronics or physics of their silicon substrate. The problem with blending inheritance is the code, not the substrate; it would be just as hopeless a system to use in evolutionary software engineering as it would be in carbon-based biological evolution.

Orr has apparently missed this fundamental point about the abstract nature of evolutionary theory, and having missed it, he compounds his confusion when he turns to my exploitation of Richard Dawkins' concept of memes in defense of a Darwinian theory of cultural evolution. His "basic problem" with any attempt at such a theory is that we are "very ignorant of how humans hold ideas in their heads. . . So how can we possibly conclude that the process 'must be' Darwinian?" Well, population geneticists were very ignorant of how organisms held genes in their bodies until recently, but this did not stop Fisher and his colleagues from establishing (in a substrate neutral way) the soundness of the modern synthesis, did it? Orr comes close to contradicting himself here, and doesn't notice.

But then his whole discussion of memes is inattentive to detail. Orr says: "The fitness of memes is strangely tautological. While we can often point to ecological reasons why certain genes are fitter than others, a meme is deemed 'fit' only because it is common." This is obviously false, belied by many of the examples discussed in my book. He goes on: "'Elvis is alive' is certainly a fit meme, but it is neither true nor helpful." Has he been paying attention at all? This independence of meme fitness from truth and utility (to us) is the single most important feature of the meme's-eye view of cultural evolution. He continues: "Last, Dennett confesses [sic] that memes often show
a Lamarckian, not Darwinian, style of evolution, in that acquired traits get passed on," ignoring the fact that I explain why the "charge" that cultural evolution is Lamarckian misses the mark entirely (p. 355).

Given this obliviousness, his "astonishment" that I would point out how difficult-and maybe impossible-it will be to establish a predictively powerful science of memetics counts for little or nothing. There are many uncontroversially evolutionary phenomena in biology that also defy formal scientific inquiry by being either computationally intractable or bereft of accessible data. Ancient memes, like ancient genes for soft body parts, leave almost no fossil traces. The genetic contributions to many complex abilities in many species are both practically impossible to research and utterly indubitable. For instance, does Orr doubt that there is an evolutionary account-probably forever inaccessible, alas-of the nest-building behaviors in birds? He interprets my cautious discussion of the prospects for memetics as "backing off" from stronger claims about memes, but this retreat is a figment of his imagination. (Aficionados will recognize this trick as an instance of the Gould two-step, which I exposed several years ago in the New York Review of Books.)

Orr's other criticisms of memes follow the same pattern as his earlier criticisms of my biology. Once again, he reiterates points I make myself about important problems to be solved, but he treats these problems as clearly insoluble, without giving any arguments or acknowledging that he is following me. He claims that I have overlooked-or deliberately suppressed-trenchant criticisms of memetics from within biology, but he doesn't mention any. I deal with all the criticisms that I have encountered in the literature. If he knows of others, he should have referred to them specifically.

One final point: Orr's failure to understand substrate neutrality leads him to miss the otherwise obvious point that in any theoretical analysis of the evolution of a cultural phenomenon-say, morality-there can be complex interplay between genetic and memetic sources of design; substrate neutrality means that the work of evolution can be accomplished in either medium (at different rates, under the same or different selection pressures) and combined in the final product. He apparently thinks that any account of the evolution of morality must either be all genetic or all cultural. He asks, at one point: "Is it obvious that genetic changes are required for such a thing? [No. It is false.] Where are Dennett's trusty memes when we need them? [Right under his nose.]" Orr might want to look at some of the literature I cited on these topics, such as the recent theoretical work by Philip Kitcher, an alumnus of Lewontin's lab. It's substrate neutral, all the way.

I will not comment on Orr's critical treatment of the more purely philosophical parts of my book, beyond noting that the charitable interpretation would be that he doesn't understand the difference between serious criticism and the mug's game of quoting out of context. (Those who are curious may consider as Exhibit A his abuse of my discussion of "bait-and-switch" in
evolutionary theory.)

One does not lightly undertake the task of dislodging heroes from their pedestals so that their ideas can be critically assessed in the same arena with the ideas of ordinary mortals. So I expected to be treated fairly roughly by their fans, especially in their home town. Gould and Lewontin and Chomsky have so far all chosen to leave the counter-attack to others, my criticisms being too far beneath their notice, one gathers, to merit any detailed public response. The attacks I have seen to date-of which Orr's is the best, by the way-have been long on sneering and short on substance. It's been surprisingly easy to take, since my task has been far from thankless. Indeed, the thanks I have been receiving from biologists around the world has been most gratifying.

Endnotes
1 After an address at Dartmouth, in April 1996, in response to a question from me.

Originally published in the October/ November 1996 issue of Boston Review



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