Why Darwin Matters_ The Case Against Intelligent Design - Michael Shermer [33]
By irreducibly complex I mean a single system composed of several well-matched, interacting parts that contribute to the basic function, wherein the removal of any one of the parts causes the system to effectively cease functioning. An irreducibly complex system cannot be produced directly (that is, by continuously improving the initial function, which continues to work by the same mechanism) by slight, successive modifications of a precursor system, because any precursor to an irreducibly complex system that is missing a part is by definition nonfunctional. Since natural selection can only choose systems that are already working, then if a biological system cannot be produced gradually it would have to arise as an integrated unit, in one fell swoop, for natural selection to have anything to act on.25
The human eye is a favorite example among Intelligent Design creationists because of its irreducible complexity—take out any one part and it will not work. How could natural selection have created the human eye when none of the individual parts themselves have any adaptive significance? Or consider the bacteria flagellum, Behe’s type specimen of irreducible complexity and intelligent design—the little tail that propels the cell is complex and composed of many parts, and the removal of any one of them would cause the system to cease working. The bacteria flagellum is not like a machine, it is a machine, and it has no antecedents in nature from which it could have evolved in a stepwise Darwinian manner.
Irreducible complexity leads to an inference of intelligent design, an inference that Behe immodestly claims “is so unambiguous and so significant that it must be ranked as one of the greatest achievements in the history of science.” He equates it with the discoveries of “Newton and Einstein, Lavoisier and Schrodinger, Pasteur, and”—with gumption—“Darwin.”26 There is even an emotional appeal to the struggle of Intelligent Design theorists for recognition of their scientific searches beyond the facile attribution of design to Darwin. “It is a shock to us in the twentieth century to discover, from observations science has made, that the fundamental mechanisms of life cannot be ascribed to natural selection, and therefore were designed. But we must deal with our shock as best we can and go on,” Behe pleads. “The theory of undirected evolution is dead, but the work of science continues.”27
Yes, the work of science does continue, and scientists in Behe’s field of biochemistry and microbiology have responded to his claims.
First, when Behe says that “any precursor to an irreducibly complex system that is missing a part is by definition nonfunctional,” he is committing the fallacy of bait-and-switch logic, says the philosopher of science Robert Pennock. He is reasoning from something that is true “by definition” to something that is proved through empirical evidence. Every time someone finds an example in nature that is simpler than Behe said it could be, Behe redefines irreducible complexity to that simpler level of complexity.28 In other words, irreducible complexity is what Behe says it is, depending on the example at hand.
Second, the evolutionary biologist Jerry Coyne, responding to Behe directly, identified a number of biochemical pathways that Behe has claimed are impossible to explain without an Intelligent Designer but which, in fact, “have been rigged up with pieces co-opted from other pathways, duplicated genes, and early multi-functional enzymes.” Behe, for example, claims that the blood clotting process could not have come about through gradual evolution. Coyne shows that in fact thrombin “is one of the key proteins in blood clotting, but also acts in cell division, and is related to the digestive enzyme trypsin.”29 In other words, thrombin evolved for one purpose and was later coopted for other purposes.
Third, Behe’s irreducible complexity is a more sophisticated version of an argument made against Darwin