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Excerpt: An Obstacle to Darwinian Evolution

Photo: Tap dancing, Iowa State College, 1942, by Jack Delano, Public domain, via Wikimedia Commons.

Editor’s note: We are delighted to celebrate the publication of the new book A Mousetrap for Darwin: Michael J. Behe Answers His Critics. What follows is an excerpt, drawn from “Irreducible Complexity: Obstacle to Darwinian Evolution,” in Debating Design: From Darwin to DNA, eds. William Dembski and Michael Ruse (New York: Cambridge University Press, 2004).

Author’s note: Whenever professors get together to talk, somebody eventually says, “Hey, let’s write a book on this!” (We get to add it to our CVs.) For Debating Design, the philosopher of biology Michael Ruse and design theorist William Dembski gathered contributions in 2004 from some true academic luminaries: geneticist Francisco Ayala, philosopher of science Elliott Sober, complexity theorist Stuart Kauffman, physicist Paul Davies, theologians John Polkinghorne and Richard Swinburne, and more. Also included were Brown University biologist Kenneth Miller and myself, taking shots at each other.

Rather than showing how their theory could handle the obstacle, some Darwinists are hoping to get around irreducible complexity by verbal tap dancing. At a debate between proponents and opponents of intelligent design sponsored by the American Museum of Natural History in April 2002, Kenneth Miller actually claimed (the transcript is available at the website of the National Center for Science Education1) that a mousetrap isn’t irreducibly complex because subsets of a mousetrap, and even each individual part, could still “function” on their own. The holding bar of a mousetrap, Miller observed, could be used as a toothpick, so it still had a “function” outside the mousetrap. Any of the parts of the trap could be used as a paperweight, he continued, so they all had “functions.” And since any object that has mass can be a paperweight, then any part of anything has a function of its own. Presto! — there is no such thing as irreducible complexity! Thus the acute problem for gradualism that any child can see in systems like the mousetrap is smoothly explained away.

A Brazen Equivocation

Of course the facile explanation rests on a transparent fallacy, a brazen equivocation. Miller uses the word “function” in two different senses. Recall that the definition of irreducible complexity notes that removal of a part “causes the system to effectively cease functioning.” Without saying so, in his exposition Miller shifts the focus from the separate function of the intact system itself to the question of whether we can find a different use (or “function”) for some of the parts. However, if one removes a part from the mousetrap I pictured, it can no longer catch mice. The system has indeed effectively ceased functioning, so the system is irreducibly complex, just as I had written. What’s more, the functions that Miller glibly assigns to the parts — paperweight, toothpick, key chain, etc. — have little or nothing to do with the function of the system of catching mice (unlike the mousetrap series proposed by John McDonald), so they give us no clue as to how the system’s function could arise gradually. Miller explained precisely nothing.

With the problem of the mousetrap behind him, Miller moved on to the bacterial flagellum — and again resorted to the same fallacy. If nothing else, one has to admire the breathtaking audacity of verbally trying to turn another severe problem for Darwinism into an advantage. In recent years it has been shown that the bacterial flagellum is an even more sophisticated system than had been thought. Not only does it act as a rotary propulsion device; it also contains within itself an elegant mechanism to transport the proteins that make up the outer portion of the machine, from the inside of the cell to the outside.2 Without blinking, Miller asserted that the flagellum is not irreducibly complex because some proteins of the flagellum could be missing and the remainder could still transport proteins, perhaps independently. (Proteins similar — but not identical — to some found in the flagellum occur in the type III secretory system of some bacteria.3) Again he was equivocating, switching the focus from the function of the system to act as a rotary propulsion machine to the ability of a subset of the system to transport proteins across a membrane. However, taking away the parts of the flagellum certainly destroys the ability of the system to act as a rotary propulsion machine, as I have argued. Thus, contra Miller, the flagellum is indeed irreducibly complex. What’s more, the function of transporting proteins has as little directly to do with the function of rotary propulsion as a toothpick has to do with a mousetrap. So discovering the supportive function of transporting proteins tells us precisely nothing about how Darwinian processes might have put together a rotary propulsion machine.


  1. Forum on ‘Intelligent Design’ Held at the American Museum of Natural History (April 23, 2002).”
  2. Shin-Ichi Aizawa, “Flagellar Assembly in Salmonella typhimurium,” Molecular Microbiology 19 (January 1996): 1–5.
  3. Christoph J. Hueck, “Type III Protein Secretion Systems in Bacterial Pathogens of Animals and Plants,” Microbiology and Molecular Biology Reviews 62 (1998): 379–433.