In this installment, I complete the series on Draper's critique of Behe's design argument from irreducible complexity.
I. Review and Setup
To review, recall that the article focuses on stage one of Behe's two-stage design argument, which argues that certain biochemical structures couldn't have arisen via gradualistic Darwinian processes. The argument of this stage crucially relies on his notion of irreducible complexity, where this is defined as a system "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". With this notion in hand, Behe argues that there are irreducibly very complex biochemical systems (e.g., the bacterial flagellum), and that these systems can't plausibly be explained in terms of gradualistic evolutionary processes. And the reason is that evolution can only create systems via direct and indirect evolutionary pathways. But evolution can create no irreducibly complex system via adirect evolutionary pathway. And while evolution can create simple irreducibly complex systems via indirect pathways, and reducibly complex systems via direct and indirect evolutionary pathways, the odds are overwhelmingly against creating irreducibly very complex systems via indirect pathways.
Given Behe's argument, there are at least three ways to criticize his argument directly:
(1) Undercut or rebut the claim that his example systems are irreducibly complex (or at least irreducibly very complex)
(2) Undercut or rebut the claim that irreducibly (very) complex systems can't be created via indirect evolutionary pathways
(3) Undercut or rebut the claim that irreducibly complex systems can't be created via direct evolutionary pathways
In previous installments, we saw that Draper has offered criticisms of type (1) and type (2). Each of these criticisms is sufficient by itself to defeat Behe's argument. However, Draper doesn't stop there. In this installment, we'll look at one of Draper's criticisms of type (3).
II. Behe's Argument Against Direct Evolutionary Pathways to Irreducible Complexity
Now as mentioned above, Behe argues that direct routes to irreducibly complex structures are impossible. But why think that? His argument can be stated as follows: consider some irreducibly complex system S composed of parts A and B, which together perform function F. Now since S is irreducibly complex, neither A nor B can perform F by itself. Therefore, if we assume that neither A nor B served some other useful function in the interim, then if either A or B came into existence before the other, it would've been eliminated before the second part came into being to interact with the first part to perform F. Therefore, direct routes to irreducibly complex systems are impossible.
III. Draper's Reply
What to make of this argument? To set up Draper's critique, recall that Behe distinguishes between 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. But as Draper points out, this account of evolutionary pathways is too coarse-grained, as it fails to distinguish different kinds of direct and indirect evolutionary pathways. And it turns out that these further distinctions have a bearing on whether Behe's argument succeeds.
Thus, to redress this shortcoming, Draper distinguishes between two sorts of direct routes to irreducible complexity: simple and complicated. A simple direct route amounts to adding parts to a system without changing the function or the mechanism. By contrast, a complicated direct route can involve both adding and subtracting parts (again, without changing the mechanism or the function).
Thus, Behe fails to distinguish between simple and complicated direct evolutionary pathways. But the problem is that Behe wrongly assumes that all direct pathways are of the simple variety. For as it turns out, it's possible for complicated direct routes to generate irreducibly complex systems. Draper states his criticism as follows:
"The possibility of an irreducibly complex system's being produced by a complicated direct path is fairly obvious. For example, an irreducibly complex two-part system AB that performs function F could evolve directly as follows. Originally, Z performs F, though perhaps not very well. (This is possible because, from the fact that AB cannot perform F without A or B, it doesn't follow that Z cannot perform F by itself.) Then A is added to Z, because it improves the function, though it is not necessary. B is also added for this reason. One now has a reducibly complex system composed of three parts, Z, A and B. Then Z drops out, leaving only A and B. And without Z, both A and B are required for the system to function."
Thus, Draper shows the logical possibility of a complicated direct path to an irreducibly complex system in four stages:
Stage 1: A system S is composed of Z, which performs function F. (simple system)
Stage 2: Part A is added to Z in S, leading to an improvement in F. (reducibly complex system)
Stage 3: Part B is added to Z and A in S, leading to an improvement in F. (reducibly complex system)
Stage 4: Part Z drops out of S, and without Z, BOTH A and B are required for S to continue performing F. (irreducibly complex system)
Draper's criticism surfaces an illicit assumption implicit in Behe's reasoning about what follows from his definition of irreducible complexity: that the irreducible complexity of a system is insensitive to the parts that make it up. But as Draper's counterexample shows, this isn't so: whether a system is irreducibly complex is relative to the constituents of which it's composed. Thus, while a system S may be irreducibly complex if composed of A and B, it may well be reducibly complex if it's composed of C and D. In short, irreducible complexity is parts-relative. And this leaves open the possibility of creating an irreducibly complex system directly by (e.g.) first starting with a reducibly complex system composed of CD, then successively adding parts A and B to improve its function, and finally losing CD, resulting in an irreducibly complex system composed of AB. And given this possibility, Behe's argument against the possibility of direct routes to irreducible complexity is defeated.
As I mentioned earlier, Draper raises a number of other criticisms of Behe's design argument from irreducible complexity, but I think enough of his criticisms have been discussed to indicate that Behe's argument is a failure. For his key clams -- that some biochemical structures are irreducibly complex; that irreducibly very complex systems can't be produced by indirect evolutionary pathways; and that irreducibly complex systems can't be produced by direct evolutionary pathways -- are defeated.
 Behe, Michael J. Darwin's Black Box: The Biochemical Challenge to Evolution (New York: The Free Press, 1996), p. 39.
 Recall that 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. 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.
 "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.
 As many -- including Draper -- have noted (e.g., Kenneth Miller), this assumption is fatal to the argument. For from the fact that neither A nor B can perform a particular function F without other parts, it doesn't follow that neither part can perform some other funtion(s). And in fact, there are plausible evolutionary pathways where Behe's paradigm case of an irreducibly complex system -- the bacterial flagellum -- has evolutionary precursors that performed different functions. (See the following YouTube clip of a talk by Kenneth Miller for a helpful example of such a criticism.). However, that sort of criticism is one demonstrating the possibility of indirect pathways to irreducible complexity. I therefore relegate the criticism to a footnote, not because it isn't important, but because it falls outside the scope of the current topic, which focuses on direct evolutionary pathways to irreducible complexity.
 This statement of Behe's argument is a paraphrase of Draper's. See Draper. Irreducible Complexity and Darwinian Gradualism: A Reply to Michael J. Behe", Faith and Philosophy 19:1 (2002), p. 15.
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