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Perplexing: Michael Behe’s Critics Falsely Claim He Ignores Exaptation


In their review of Darwin Devolves for the journal Science, biologists Nathan Lents, Joshua Swamidass, and Richard Lenski seek to portray Behe as a careless scientist who refuses to acknowledge challenges to his ideas. The title of the review claims he “misrepresents theory and ignores evidence” and their conclusion further states that he dodges challenges:

Ultimately, Darwin Devolves fails to challenge modern evolutionary science because, once again, Behe does not fully engage with it. He misrepresents theory and avoids evidence that challenges him.

In their review they give various examples of Behe supposedly engaging in this unsavory behavior, which have already been refuted here at Evolution News. One illustration is their false claim that Behe in his new book ignores the idea of exaptation. Dr. Behe noted this in his reply. There is more that can be be said. 

The Accusation

Here is the accusation made by the reviewers: 

Missing from Behe’s discussion is any mention of exaptation, the process by which nature retools structures for new function and possibly the most common mechanism that leads to the false impression of irreducible complexity. Some Sphingomonas bacteria, for example, have evolved the ability to digest a wood preservative, pentachlorophenol, by recruiting two unrelated biochemical pathways (7). Neither pathway can do that job alone, yet there they are together. The feathers of birds, gas bladders of fish, and ossicles of mammals have similar exaptive origins.

Note that when they write “Missing from Behe’s discussion is any mention of exaptation,” the reviewers make a very specific, clear, strong, and unambiguous claim. They say that there is absolutely zero discussion of exaptation in Darwin Devolves. This is, in fact, a falsehood. 

Introducing Exaptation

Here’s where Behe first introduces the concept of exaptation in Darwin Devolves:

Two broad ways that evolutionary novelties have been envisioned to occur, writes Mayr, are by intensification of function and change of function. In a change of function, a structure that was used for one purpose is adapted to serve a different one; for example, early lungs in fish may have been converted to swim bladders. This is an example of what has been called the “principle of tinkering.” (Darwin Devolves, p. 81)

The “change of function” mode of evolution is a description of the exact idea of exaptation. If you don’t believe it, note that Behe gives an example of evolution by “change of function,” and it is one of the very same examples of exaptation given by the reviewers: the origin of swim bladders in fish! 

While Behe doesn’t use the technical term “exaptation,” the concept is not missing from his book. There are other instances in Darwin Devolves where Behe discusses evolution by “change of function” or the “principle of tinkering,” as he calls it:

Books by even the most distinguished neo-Darwinians, writing after the turn of the millennium and purporting to explain evolution, continue to rely on Darwin’s last theory as a bare postulate. No serious evidence is ever presented that selection acting on random variation can, say, convert lungs into swim bladders or produce feathers (or pork bellies), let alone such sophisticated systems as described in Chapter 2. All such assertions rest on the vaguest of concepts, like the previously mentioned “principle of tinkering” or “intensification of function,” or on the claim that the evolution of such marvels as the eye isn’t “unthinkable” (a standard whose laxity is hard to beat). (Darwin Devolves, p. 90)

Again note that here, Behe gives another example of exaptation, the origin of feathers, and this is one of the very same examples given by the reviewers. 

In a third instance, Behe refers to the “principle of tinkering,” which is his way of describing exaptation:

Yet, as we saw in the last chapter, neo-Darwinian accounts rely heavily on such vague notions as the “principle of tinkering” and the contention that profound transformations aren’t “unthinkable.” Leaving aside theological premises, why are so many smart evolutionary biologists so blasé about the evolution of extraordinarily intricate, detailed molecular machinery. (Darwin Devolves, p. 101)

That’s three separate places Behe refers to the concept of exaptation, if not by name then by substance and description. 

Not the First Time

Incidentally, this is not the first time that Behe has been wrongly accused of ignoring exaptation when his book (or other writings) discuss it. In fact, precisely the same thing happened with Darwin’s Black Box. 

In the debate following Darwin’s Black Box many critics also claimed that Behe ignored exaptation. But this wasn’t true then either because Behe’s first book also discussed the mechanism of exaptation in three different passages: 

(1) Even if a system is irreducibly complex (and thus cannot have been produced directly), however, one cannot definitively rule out the possibility of an indirect, circuitous route. As the complexity of an interacting system increases, though, the likelihood of such an indirect route drops precipitously. And as the number of unexplained, irreducibly complex biological systems increases, our confidence that Darwin’s criterion of failure has been met skyrockets toward the maximum that science allows. (Darwin’s Black Box, p. 40.)

(2) Because the cilium is irreducibly complex, no direct gradual route leads to its production. So an evolutionary story for the cilium must envision a circuitous route, perhaps adapting parts that were originally used for other purposes.” (Darwin’s Black Box, pp. 65-66.)

(3) For example, suppose you wanted to make a mousetrap. In your garage you might have a piece of wood from an old Popsicle stick (for the platform), a spring from an old wind-up clock, a piece of metal (for the hammer) in the form of a crowbar, a darning needle for the holding bar, and a bottle cap that you fancy to use as a catch. But these pieces couldn’t form a functioning mousetrap without extensive modification, and while the modification was going on, they would be unable to work as a mousetrap. Their previous functions make them ill- suited for virtually any new role as part of a complex system. In the case of the cilium, there are analogous problems. The mutated protein that accidentally stuck to microtubules would block their function as “highways” of transport. A protein that indiscriminately bound microtubules together would disrupt the cell’s shape–just as a building’s shape would be disrupted by an erroneously placed cable that accidentally pulled together girders supporting the building. A linker that strengthened microtubule bundles for structural supports would tend to make them inflexible, unlike the flexible linker nexin. An unregulated motor protein, freshly binding to microtubules, would push apart micrutubules that should be close together. The incipient cilium would not be at the cell surface. If it were not at the cell surface, then internal beating could disrupt the cell; but even if it were at the cell surface, the number of motor proteins would probably not be enough to move the cilium. And even if the cilium moved, an awkward stroke would not necessarily move the cell. And if the cell did move, it would be an unregulated motion using energy and not corresponding to any need of the cell. (Darwin’s Black Box, pp. 66-67.)

Behe doesn’t ignore the idea that features can evolve via a pathway where the function changes during the evolutionary process. He just found such explanations for them to be highly unpersuasive due to their great unlikelihood and lack of supporting evidence.

Kitzmiller v. Dover

So if Behe doesn’t ignore exaptation, then where do people get the idea that he ignores it? A lot of critics don’t realize that much of what they think is true about ID was imbibed while reading the highly inaccurate and error-filled Kitzmiller v. Dover ruling. There, Judge Jones promoted the myth that Michael Behe ignores exaptation:

Professor Behe excludes, by definition, the possibility that a precursor to the bacterial flagellum functioned not as a rotary motor, but in some other way, for example as a secretory system. … By defining irreducible complexity in the way that he has, Professor Behe attempts to exclude the phenomenon of exaptation by definitional fiat, ignoring as he does so abundant evidence which refutes his argument.

But don’t think that Judge Jones came up with those ideas on his own. He simply copied and pasted the above verbiage from an ACLU brief submitted to the court during the trial: 

He [Behe] excludes, by definition, the possibility that a precursor functioned in some other way — for example, in the case of the bacterial flagellum, as a secretory system. … By defining irreducible complexity in the way he has, Professor Behe attempts to exclude the phenomenon of exaptation by definitional fiat.

Thanks, ACLU

In any case, in his response to the Dover ruling, Behe again explained to the world that he does  not ignore or “exclude” mechanisms where the function changes during an evolutionary pathway — i.e., exaptation:

I certainly do not exclude that bald possibility merely by definition. In fact in Darwin’s Black Box I specifically considered those kinds of cases. However, I classified those as indirect routes. Indirect routes, I argued, were quite implausible:

“Even if a system is irreducibly complex (and thus cannot have been produced directly), however, one can not definitely rule out the possibility of an indirect, circuitous route. As the complexity of an interacting system increases, though, the likelihood of such an indirect route drops precipitously. (DBB, p. 40)” 

University of Rochester evolutionary biologist H. Alan Orr agrees that indirect evolution is unlikely: 

“[W]e might think that some of the parts of an irreducibly complex system evolved step by step for some other purpose and were then recruited wholesale to a new function. But this is also unlikely. You may as well hope that half your car’s transmission will suddenly help out in the airbag department. Such things might happen very, very rarely, but they surely do not offer a general solution to irreducible complexity. (Orr, H. A. Darwin v. intelligent design (again). Boston Review [Dec/Jan], 28-31. 1996)”

There is no strict logical barrier to a Darwinian precursor to a bacterial flagellum having functioned as a secretory system and then, by dint of random mutation and natural selection, turning into a rotary device. There is also no absolute logical barrier to it having functioned as, say, a structural component of the cell, a light-harvesting machine, a nuclear reactor, a space ship, or, as Kenneth Miller has suggested, a paper weight. But none of these has anything to do with its function as a rotary motor, and so none of them explain that actual ability of the flagellum. 

A bare assertion that one kind of complex system (say, a car’s transmission) can turn into another kind of complex system (say, a car’s airbag) by random mutation and natural selection is not evidence of anything, and does nothing to alleviate the difficulty of irreducible complexity for Darwinism. Children who are taught to uncritically accept such vaporous assertions are being seriously misled.

Now it’s 2019, over a decade since Kitzmiller v. Dover and over two decades since Behe first discussed the idea of exaptation or indirect evolution in Darwin’s Black Box. Yet we’re still finding critics in denial about the fact that Behe does address exaptation. It’s perplexing. 

Photo credit: Feathers, by Mack Fox (MusicFox) via Unsplash.