For the next several posts, I'll be writing on Paul Draper's article, "Irreducible Complexity and Darwinian Gradualism: A Reply to Michael J. Behe", Faith and Philosophy 19:1 (2002), pp. 3-21. Here's the first installment.
Michael Behe is a biochemist at Lehigh University. He uses his knowledge of biochemistry for the basis of the first stage of his two-stage design argument. In the first stage, he argues that certain biochemical structures (e.g., the bacterial flagellum and the blood-clotting cascade) could not have arisen through the gradualistic processes of evolution. In the second stage, he argues that if such structures could not have arisen in stepwise Darwinian fashion, then it's very probable that they were produced by one or more intelligent designers. Draper's article focuses on the first stage of Behe's argument.
II. Irreducible Complexity
The central notion of Behe's argument is what he calls 'irreducible complexity'. Behe defines an irreducibly complex system as one "composed of several well-matched, interacting parts that contribute to the basic function, wherein removal of any of the parts causes the system to effectively cease functioning" . He uses a mousetrap to illustrate the notion of an irreducibly complex system. A standard mousetrap has five parts: a hammer, a spring, a holding bar, a catch, and a base. The mousetrap is composed of several well-matched, interacting parts that contribute to the basic function. Furthermore, it appears that the mousetrap would effectively cease functioning (i.e., it would lose it's ability to catch mice) with the removal of any of the five parts listed above: it needs the hammer to clamp down on the mouse; it needs the spring and holding bar to arm the hammer; it needs the catch to detect the mouse; and it needs the base to secure the other parts. Therefore, the mousetrap appears to be a helpful illustration of the notion of an irreducibly complex system. As we will see, Behe argues that certain biochemical structures are irreducibly complex, and that such structures pose a serious challenge to evolution.
III. Evolutionary Pathways
Before we state Behe's argument against Darwinian gradualism, we need to briefly discuss what are ostensibly the only two sorts of evolutionary pathways for creating biological systems: direct and indirect. A gradualistic evolutionary pathway leading to a function F of a biological system is direct if it produces F by continuously improving it without changing F itself, and without changing the system's mechanism. And a gradualistic evolutionary pathway leading to F is indirect if it does so by changing the system's function or mechanism .
IV. Behe's Argument
With the notions of direct evolutionary pathways, indirect evolutionary pathways, and irreducible complexity before us, we can now state Behe's argument against Darwinian gradualism: There are irreducibly complex biochemical systems (e.g., the bacterial flagellum and the blood-clotting cascade). Now if Darwinian gradualism is true, then each such system is created via either a direct or an indirect evolutionary pathway. But no irreducibly complex biochemical system can be created via a direct evolutionary pathway, for any precursor to an irreducibly complex system is by definition non-functional. And while it's possible in principle to create an irreducibly complex biochemical system via an indirect pathway, the probability of this happening is too low to be plausible for systems that are very complex (and there are such systems). Therefore, it's very probable that Darwinian gradualism is false.
V. Key Aspects of Behe's Argument, and an Important Implication
As Draper points out, Behe's argument against direct evolutionary pathways to irreducibly complex biochemical systems differs from his argument against indirect pathways: direct pathways are ruled out by the irreducibility of the complexity with respect to function, and indirect pathways are ruled out by the complexity of such systems. Direct pathways to all irreducibly complex systems are therefore ruled out as logically impossible, while indirect pathways to irreducibly very complex systems are ruled out as too improbable to be a plausible explanation. Note (as Draper does in his article) that Behe's argument leaves open the possibility of relatively simple irreducibly complex systems (say, systems with two or three parts) being produced gradually via indirect evolutionary pathways. This will be important to keep in mind for later posts.
 Behe, Michael J. Darwin's Black Box: The Biochemical Challenge to Evolution (New York: The Free Press, 1996), p. 39.
 Draper, Paul. "Irreducible Complexity and Darwinian Gradualism: A Reply to Michael J. Behe", Faith and Philosophy 19:1 (2002), p. 5
 Irreducibly complex systems "cannot be produced directly, because any precursor to an irreducibly complex system that is missing even a single part is by definition nonfunctional." Darwin's Black Box, P. 39. The quote appears in Draper, ibid.
 "Even if a system is irreducibly complex (and thus cannot have been produced directly), however, one can not 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." Behe, Darwin's Black Box, P. 40.
 This summary is closely based on Draper's. See ibid, p. 5.
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