Bacterial flagella are irreducibly complex (Talk.Origins)
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Bacterial flagella and eukaryotic cilia are irreducibly complex, Since non-functional intermediates cannot be preserved by natural selection, these systems can only be explained by intelligent design.
Source: Behe, Michael J. 1996. Black Box, New York: The Free Press, pp. 59-73.
(Talk.Origins quotes in blue)
1. This is an example of argument from incredulity, because irreducible complexity can evolve naturally. Many of the proteins in the bacterial flagellum or eukaryotic cilium are similar to each other or to proteins for other functions. Their origins can easily be explained by a series of gene duplication events followed by modification and/or co-option, proceeding gradually through intermediate systems different from and simpler than the final flagellum.
This just shows that Talk Origins does not understand the problem. This is not about developing the proteins or even getting the exact flagellum used by bacteria but getting a minimal functional bacterial flagellum. To do that requires not only getting all the right proteins, but getting them all in the right configuration to work together as a flagellum.
One plausible path for the evolution of flagella goes through the following basic stages (keep in mind that this is a summary, and that each major co-option event would be followed by long periods of gradual optimization of function):
Creating a plausible sounding scenario for a totally impossible event is a mark of good science fiction not good science.
The most obvious flaw in Talk Origins' suggestion is the reference to an ion pump complex, since they do nothing to explain its origin; they just assume that it is there, already fully functional. The other obvious problem is the vagueness of their scenario. It conveniently lacks any details that could be used actually to test the theory.
2. The bacterial flagellum is not even irreducible. Some bacterial flagella function without the L- and P-rings. In experiments with various bacteria, some components (e.g. FliH, FliD (cap), and the muramidase domain of FlgJ) have been found helpful but not absolutely essential. One third of the 497 amino acids of flagellin have been cut out without harming its function. Furthermore, many bacteria have additional proteins that are required for their own flagella but that are not required in the "standard" well-studied flagellum found in E. coli. Different bacteria have different numbers of flagellar proteins (in Helicobacter pylori, for example, only thirty-three proteins are necessary to produce a working flagellum), so Behe's favorite example of irreducibility seems actually to exhibit quite a bit of variability in terms of numbers of required parts.
Behe's definition of irreducibly complex reads
By irreducibly complex I mean a single system composed of several well-matched, interacting parts that contribute to the basic function, wherein the removal of any one of the parts causes the system to effectively cease functioning.
When talking about the removal of "any one of the parts", Behe is talking specifically about the well-matched, interacting ones that contribute to the basic function. If X is an irreducibly complex process, adding Y to improve it does not make the result not irreducibly complex. It just means that Y is not part of the basic function.
- Being able to lose a part does not make something not irreducibly complex. See the above definition.
- If losing a part of an irreducibly complex assembly changes its function, it would still be classified as irreducibly complex.
- The existence of flagella that lack some of the proteins or other parts is irrelevant if the missing items are not part of what is irreducibly complex.
- When examining one particular flagellum, it is irrelevant that other flagella exist that lack some of the proteins or other parts that are part of what is considered irreducibly complex in the instant subject, if those other flagella operate with a different basic mechanism.
To contradict Behe's argument would require that the other flagella operate on the same principles as the one Behe describes as well as lacking some of the proteins that Behe describes as part of what is irreducibly complex.
3. Eubacterial flagella, archebacterial flagella, and cilia use entirely different designs for the same function. That is to be expected if they evolved separately, but it makes no sense if they were the work of the same designer.
While it is true that all three have the same basic function, that being to produce motion in water, the different designs provide different levels of manoeuvrability and speed; as a result their functions are not exactly the same.