Evolution of the Bacterial Flagellum

IAN MUSGRAVE

The bacterial flagellum is an organelle that looks strikingly similar to a machine constructed by humans (Namba et al. 2003). This similarity has led to claims that it is a construct rather than a product of evolution. Indeed, the bacterial flagellum has become the mascot of the intelligent-design movement: it is one of only two examples of alleged design considered in any depth, graces the cover of William Dembski's No Free Lunch (2002b), and features in a recent video promoting intelligent design. Yet "the" bacterial flagellum does not exist.

The image that graces Dembski's book is a representative flagellum of eubacteria, one of the two fundamental subdivisions of prokaryotes (bacteria in general). Archaebacteria, the other fundamental prokaryote group, have flagella that are superficially similar but do not look like a human-constructed machine. Given the importance that "the" eubacterial flagellum has assumed in debates over intelligent design, it is worthwhile looking at "the" flagellum in more detail.

In this chapter, I outline the construction and function of eubacterial and archaebacterial flagella and their relationship to other systems. I discuss some of Dembski's objections to current accounts of flagellar evolution and end with a possible scenario for the evolution of eubacterial flagellum, based on structures and functions in known, related bacteria.

Michael Behe (1996) has listed the eubacterial flagellum as one of the systems that he believes is irreducibly complex and unable (or unlikely) to have been produced by evolution. Building on Behe's claims, Dembski (2002b, 289) has made the eubacterial flagellum a central point of his key chapter, "The Emergence of Irreducibly Complex Systems," and has produced an analysis of the flagellum by assuming that all elements of the flagellum arose randomly. He claims that his analysis supports the intelligent design of the flagellum.

Kenneth Miller (2003) and David Ussery (chapter 4 in this book) have addressed key aspects of our understanding of the flagellum and its evolution. Dembski (2003) has not found such accounts convincing. First, he does not seem to understand that the eubacterial flagellum is only one of a range of motility systems in bacteria—systems, moreover, that revolve around a common thread—and that motility is just one function of the flagellum. Further, he artificially categorizes the eubacterial flagellum as a machine. By viewing the eubacterial flagellum as an isolated outboard motor rather than a multifunctional organelle with no explicit, human-constructed analog, Dembski makes the problem of flagellar evolution artificially and misleadingly difficult.

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