The bacterial flagellum could not have evolved.
The contention that the bacterial flagellum could not have evolved rests upon the further contention that the bacterial flagellum is irreducibly complex. However, as shown in 4.2, the irreducible complexity of a structure or mechanism does not preclude its evolution.
The bacterial flagellum is not even irreducibly complex. A number of scientists have pointed out that parts of the flagella of various types of bacteria appear to correspond to other structures that have different functions: the archaebacterial flagellum supposedly is closely related to class IV pilins, and part of the eubacterial flagellum is homologous to the type III secretory system, each of which have different functions than flagella. None of this appears to be in dispute.
Nor is the subsection of the flagellum that corresponds to the type III secretory system the only part of the flagellum that is homologous to something that has another function. Molecular pharmacologist Ian Musgrave, who offers a step-by-step example of how the eubacterial flagellum might have evolved (in Musgrave 2004), points out that "between 80 and 88 percent of the eubacterial flagellar proteins have homologs with other systems" (Musgrave 2004: 81).
An area of contention is whether or not class IV pilins and type III secretory systems are the actual precursors to the flagella. Scholarly articles have argued for such derivation, but others have argued for derivation in the other direction, and others have argued that both are equally derivative from some other common precursor, which might itself be an ancestral version of the modern type III secretory system.
One should note that when creationists respond in this way, they are backing away from their original assertion and reframing the debate. The original issue was whether or not the flagellum is irreducibly complex. If a type III secretory system can be derived from the eubacterial flagellum by eliminating some parts, then this is enough to show that the flagellum is not irreducibly complex and that the creationist argument that it could not have evolved is false. It makes absolutely no difference whether flagella really are derived from type III secretory systems. Scientists can now even plead complete ignorance of how the flagellum evolved, and it will hardly do any damage to evolutionary theory since the onus of opposite proof is on the creationists, and they have failed to make one.
Nick Matzke has proposed a model for the origin of the flagellum. There is a YouTube video going over the main points of Matzke's model. According to Matzke, the video is not free of error, but gets the basic idea across very well.
Edis T and Young M (eds). 2004. Why Intelligent Design Fails. New Brunswick, NJ: Rutgers University Press.
Musgrave I. 2004. Evolution of the bacterial flagellum. pp. 72-84 in Edis and Young 2004.
Last updated 13 Jan 2009
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