Is the Flagellum Evolvable

Dembski's calculation method for deciding whether or not systems are designed first requires elimination of systems assembled by natural laws such as natural selection. Given our finite state of knowledge, there is always the possibility that if we currently do not have an explanation due to natural laws, we may find one in the future (Wilkins and Elsberry 2001). To avoid this problem, Dembski (2002b) attempts to provide a proscriptive generalization that will eliminate any explanation based on natural law and then also allow him to eliminate chance hypotheses. He gives as an example of a proscriptive generalization the second law of thermodynamics, which proscribes the possibility of a perpetual motion machine (274).

To provide a proscriptive generalization with regard to the flagellum, Dembski accepts Behe's (1996) description of the flagellum as irreducibly complex (IC). He claims that the flagellum, considered as a system of motor, shaft, and propeller, cannot be built sequentially. He thus claims that describing the flagellum as IC eliminates natural selection as a possible mechanism and proceeds to his calculations to eliminate chance. There are, however, two problems with using the alleged IC nature of the flagellum that are not covered in his book.

First, while allegedly eliminating directly evolved systems (see, however, chapters 4 and 5 in this book), Behe (1996) himself points out that these systems may evolve indirectly: "Even if a system is irreducibly complex (and thus cannot have been produced directly) . . . one can not definitely 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" (40). IC by itself does not provide the proscriptive generalization that Dembski requires.

Second, the specification of an outboard motor, which provided the IC system description of motor, shaft, and propeller, is a flawed human analogy for the actual flagellar system. Thinking in terms of human design has misled Dembski. Indeed, in terms of Dembski's (2002b) modification of Behe's original definition, "A system performing a given basic function is irreducibly complex if it includes a set of well-matched, mutually interacting, non-arbitrarily individuated parts such that each part in the set is indispensable to maintaining the system's basic, and therefore original, function. The set of these indispensable parts is known as the irreducible core of the system" (285, emphasis added).

The flagellum is probably not IC at all because the original function of the eubacterial flagellum, which can survive massive pruning of its components, is almost certainly secretion, not motility (Hueck 1998, Berry and Armitage 1999, Aizawa 2001). To explain this claim, I will examine the variety of motility systems found in bacteria and show that the eubacterial flagel-lum functions is far more than just a motility system.

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