Saturday, June 28, 2008

Notes on Draper's Article on Behe's Design Argument, Part 4: Behe's Revision

In the last installment, we saw that there are two serious problems for Behe's key claim that some biochemical systems are irreducibly very complex: (i) Behe fails to demonstrate that his example systems in Darwin's Black Box are irreducibly very complex, and (ii) a number of scientists (e.g., cell biologist Kenneth Miller and biochemist David W. Ussery) have given excellent evidence to show that his examples aren't irreducibly complex. However, Draper points out that Behe has responded to this criticism by (in effect) revising his account of irreducibly complex systems. To see how, we’ll need to give a slightly more precise formulation of Behe’s original account of that notion. Recall that his original account defined irreducible complexity 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". Thus, Behe's original account of irreducible complexity (henceforth 'IC1') can be expressed in terms of three clauses:

(IC1) A system S is irreducibly complex if and only if:

(i) S is composed of several interacting parts
(ii) S's parts are well-matched
(iii) removal of one or more of S's parts would cause S to cease functioning

Now in response to Ussery's criticisms of Behe's examples of irreducibly complex systems (e.g., Ussery's point that bacterial flagella can perform their function with less than the 40 parts in its system that Behe claimed were essential to its functioning), Behe replies:

"some systems may have parts that are necessary for a function, plus other parts that, while useful, are not absolutely required. Although one can remove the radio from a car and the car will still work, one can't remove the battery or some other parts and have a working car."

(As Draper points out, Behe's analogy isn't quite apt, since a radio isn't a part of a car's primary function. However, Draper helps Behe out by replacing his example of a radio with that of a set of tires. Since tires aren't required for a car to drive, but do enhance its function, we have an apt illustration of Behe's point.)

Two initial remarks concerning Behe's reply to Ussery are in order. First, Behe's response tacitly concedes that Ussery is right, and thus that the bacterial flagellum fails to satisfy Behe's original definition of irreducible complexity (i.e., IC1). Second, and more to the main point in our discussion here, it tacitly replaces IC1 with a new definition of irreducible complexity -- call it 'IC2':

(IC2) A system S is irreducibly complex if and only if:

(i) S is composed of several interacting parts
(ii) S's parts are well-matched
(iii') A subset x of S's parts are such that removal of one or more of x's parts would cause S to cease functioning

The most important thing to notice about IC2 is that clause (iii) of IC1 has dropped out, and with it, its crucial implication that an irreducibly complex system requires all of its parts to function. In its place is a new clause -- clause (iii') -- which only entails the weaker claim that an irreducibly complex system requires a subset of its parts to function. Thus, unlike IC1, IC2 allows a system to count as irreducibly complex even if it has parts that aren't essential to its function.

Thus, with his revised account of irreducible complexity (IC2) in hand, we can put Behe's reply to Ussery as follows: granted, the point about the bacterial flagellum not being irreducibly complex, while strictly speaking correct, doesn't defeat the fundamental point that it contains a subset of parts, each of which must be present from the get-go for it to function at all. But systems like that -- systems that have at least a portion that is irreducibly complex -- can't evolve.

What to make of Behe's new definition of irreducible complexity? If you've been following the earlier posts, you might already see the problem with Behe's reply. For as we have seen in earlier installments, Behe allows that evolution can create simple irreducibly complex systems via indirect evolutionary pathways, and it that it can create reducibly complex systems via direct evolutionary pathways. But if so, then Behe has left open the very real possibility that his example systems have evolved via a two-staged combination of evolutionary pathways: an indirect pathway to create a simple yet irreducibly complex system in the first stage, and then a direct pathway to make that system very complex in the second stage. But such systems satisfy his revised account of irreducibly complex systems (i.e., IC2): systems containing both reducible complexity and an irreducibly complex (in the IC1 sense of 'irreducibly complex') core.

Now one might reply on behalf of Behe that although this sort of two-stage evolutionary process could produce irreducibly complex systems (in the IC2 sense) where the core set of interdependent parts is very simple, it can't account for Behe's example systems (e.g., the bacterial flagellum). For the core set of interdependent parts in those systems are very complex, and thus couldn't have evolved via indirect evolutionary pathways. But this reply won't work. For recall our discussion from the last installment. There, we saw Draper's point that Behe failed to show that his example systems are irreducibly very complex. For when Behe gets around to that task in Part II of his book, he only establishes examples of systems that are irreducibly complex and examples that are very complex. But showing those things is of course crucially different from showing what he needs to show here, viz, that at least some of his examples are irreducibly very complex.

But the problems with Behe's argument don't end here. For as Draper goes on to argue at the end of his article, Behe's arguments against simple direct and indirect evolutionary pathways to irreducibly very complex systems have very large holes in them. We'll wrap up our discussion of Draper's article by discussing these points in the next post or two.

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