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Did Michael Behe State Exaptation has been “Shown” to Produce Irreducible Complexity?

Casey Luskin

A questioner recently wrote me and asked if Michael Behe has retracted his arguments for irreducible complexity after the Kitzmiller v. Dover case. Specifically, he wondered if Behe’s examples of irreducible complexity “have been shown to be explicable by known evolutionary mechanisms, something Behe conceded under cross examination.”

In response, I explained that no, Michael Behe has not backed away from his arguments for irreducible complexity since the Dover trial, nor did he ever say that evolutionary mechanisms have been shown to plausibly produce irreducible complexity. In fact, I’ve heard Behe give two lectures this year endorsing his classic examples of irreducible complexity, and he’s posted multiple articles defending the concept in recent years. For a small sampling, please see:

In any case, I’ve heard this claim before that somehow Behe retracted his arguments for irreducible complexity at, or since, the Dover trial–but strangely nobody ever explains exactly where or when this took place. It’s all rumor and innuendo.

The “known evolutionary mechanisms” which have supposedly been “shown” to explain irreducible complexity pertain to exaptation (also called “co-option”)–an idea which Behe mentioned and critiqued in Darwin’s Black Box (see pp. 40, 66-67). In this process, parts are said to be borrowed (or “exapted” or “co-opted”) from elsewhere in an organism and retooled to perform some new function

Whether at the Dover trial or elsewhere, Behe has never stated that exaptation has been “shown” to plausibly explain irreducible complexity. Here’s Behe’s testimony during cross-examination at the Dover trial where he shows he’s very skeptical of exaptation as a viable explanation:

Q: You say, Even if a system is irreducibly complex and thus could not have been produced directly, however, one cannot definitively rule out the possibility of an indirect, circuitous route, right?

Behe: Yes.

Q: And by indirect, you mean evolution from a pre-cursor with a different function than the system being studied?

Behe: Yes, different function, perhaps different number of parts, and so on.

Q: And one example of that is what’s discussed in, among evolutionary biologists, as the concept of exaptation, correct?

Behe: Yeah — well, before I say, yes, I’d just like to say, the word exaptation is oftentimes used in loose sense, but, yes, that’s generally correct.

Q: And that is a concept that people in the field of evolutionary biology consider to be a valid concept, a valid description of the way more and more complex systems get developed?

Behe: Let me say —

Q: I’m not asking you to agree with it. I’m asking you, is that what an evolutionary biologist proposes?

Behe: Again, let me make clear what we’re talking about here. Some evolutionary biologists certainly think that exaptation is real and that it’s important and so on. But simply saying that this part over here was exapted from that part over here does not give an explanation of how random mutation and natural selection could have gotten it from one state to the other.

(Michael Behe, Kitzmiller v. Dover testimony, Day 12 AM session, pp. 66-67.)

Behe goes on to say:

Q: Okay. Now you go on in this passage and say, 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 Darwinian’s criterion of failure has been met and skyrockets toward the maximum that science allows? What you’re saying there is, you know, it could happen, I’m not ruling it out, but it’s really improbable?

Behe: Yes, it’s improbable.

Q: Okay. And you haven’t — and based on that, you conclude that intelligent design is a much more probable explanation?

Behe: Not just based on that, based on the purposeful arrangement of parts.

Q: Fair enough. And you haven’t actually quantified this, have you?

Behe: Not explicitly, but as a biochemist who understands what it takes to, for example, for a protein to function, for two proteins to bind specifically to each other, and so on, I rely on my experience of that in arriving at this conclusion.

(Michael Behe, Kitzmiller v. Dover testimony, Day 12 AM session, pp. 67-68.)

So Behe is highly skeptical of the viability of exaptation / co-option to build even modestly complex biological features. He wouldn’t say it’s a “known” process which has been “shown” to build complex systems. In his response to Judge Jones’s ruling, Behe explains this point in more detail:

(8) 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. (19:88-95 (Behe)).
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:

we 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.

Likewise, in Science and Evidence for Design in the Universe, Behe notes that appeals to exaptation are highly speculative and essentially put evolutionary theory in an untestable, unfalsifiable position:

[O]ne needs to relax Darwin’s criterion from this: “If it could be demonstrated that any complex organ existed which could not possibly have been formed by numerous, successive, slight modifications, my theory would absolutely break down.” to something like this:

If a complex organ exists which seems very unlikely to have been produced by numerous, successive, slight modifications, and if no experiments have shown that it or comparable structures can be so produced, then maybe we are barking up the wrong tree. So, LET’S BREAK SOME RULES!

Of course people will differ on the point at which they decide to break rules. But at least with the realistic criterion there could be evidence against the unfalsifiable. At least then people like Doolittle and Miller would run a risk when they cite an experiment that shows the opposite of what they had thought. At least then science would have a way to escape from the rut of unfalsifiability and think new thoughts.

(Michael Behe, “Answering Scientific Criticisms of Intelligent Design,” in Science and Evidence for Design in the Universe, Proceedings of the Wethersfield Institute, Vol 9:146-147 (Ignatius Press, 2000).)

In sum, Behe has not backed away from his strong arguments about irreducible complexity in the wake of the Dover trial. For more articles defending Behe from scientific errors in the Dover ruling, please see:

Casey Luskin

Associate Director, Center for Science and Culture
Casey Luskin is a geologist and an attorney with graduate degrees in science and law, giving him expertise in both the scientific and legal dimensions of the debate over evolution. He earned his PhD in Geology from the University of Johannesburg, and BS and MS degrees in Earth Sciences from the University of California, San Diego, where he studied evolution extensively at both the graduate and undergraduate levels. His law degree is from the University of San Diego, where he focused his studies on First Amendment law, education law, and environmental law.



Irreducible ComplexityJudge John E. JonesKitzmiller v. Dover Area School DistrictMichael Behe