Irreducible Complexity

The Bacterial Flagellum Revisited: A Paradigm of Design

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Going back to my undergraduate days, I have long been struck by the engineering elegance and intrinsic beauty of that familiar icon of intelligent design, the bacterial flagellar nano-motor. In tribute to this masterpiece of design, I have just published a detailed (31 pages, inclusive of references) literature review in which I describe the processes underlying its self-assembly and operations.

My essay also attempts to evaluate the plausibility of such a system having evolved by natural selection. Here’s a short excerpt to whet your appetite.

The bacterial flagellum is a reversible, self-assembling, rotary nano-motor associated with the majority of swimming bacteria. There exists a number of different models of this rotary motor (Pallen and Matzke, 2006; Soutourina and Bertin, 2003). Flagella are produced by a very tightly regulated assembly pathway (Chevance and Hughes, 2008; Macnab, 2003; Aldridge and Hughes, 2002), and the archetypical system for understanding flagellar assembly belongs to Salmonella enterica serovar Typhimurium, a rod-shaped gram negative bacterium of the family Enterobacteriaceae.

Flagella receive feedback from the environment by virtue of an elegant signal transduction circuit and can adjust their course in response to external stimuli by a mechanism known as chemotaxis (Baker et al., 2006 Bourret and Stock, 2002; Bren and Eisenbach, 2000). The most extensively studied chemotaxis system belongs to Escherichia coli.

By itself, the rotor is able to turn at a speed between 6,000 and 17,000 rotations per minute (rpm) but normally only achieves a speed of 200 to 1000 rpm when the flagellar filament (that is, the propeller) is attached. Its forward and reverse gears allow the motor to reverse direction within a quarter turn.

The bacterial flagellum, which has been described as a “nanotechnological marvel” (Berg, 2003), has long been championed as an icon of the modern intelligent design movement and the flagship example of “irreducible complexity” (Behe, 1996). But even biologists outside of this community have been struck by the motor’s engineering elegance and intrinsic beauty. As one writer put it, “Since the flagellum is so well designed and beautifully constructed by an ordered assembly pathway, even I, who am not a creationist, get an awe-inspiring feeling from its “divine’ beauty” (Aizawa, 2009).

The mechanistic basis of flagellar assembly is so breathtakingly elegant and mesmerizing that the sheer engineering brilliance of the flagellar motor — and, indeed, the magnitude of the challenge it addresses to Darwinism — cannot be properly appreciated without, at minimum, a cursory knowledge of its underlying operations. The purpose of this essay is to review these intricate processes, and evaluate the plausibility of such a system evolving by natural selection.

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9 Replies to “The Bacterial Flagellum Revisited: A Paradigm of Design

  1. 1
    JoeCoder says:

    Its forward and reverse gears allow the motor to reverse direction within a quarter turn.

    Is this still amazing for such a microscopic scale or just a product of the square-cubed law? Honest, non-critical question.

  2. 2
    Joe says:


    The issue is it is under control- ie something is controlling the flagellum. And that means not only do you have to get all the parts in the proper configuration but you also need a command and control center to operate the thing.

  3. 3
    Genomicus says:

    Interesting essay. Some comments:

    1. With regards to the type III secretion system, the current phylogenetic evidence actually suggests that the TTSS and flagellar systems share a common ancestor, but this is based on the rather dubious arguments of Gophna et al., 2003. IMHO, the bulk of the evidence suggests that flagellum is ancestral to the TTSS, as you point out. Plus, we already have an example of an export system evolving from the flagellum – i.e., the Buchnera homologs of the flagellar apparatus.

    2. Regarding this statement:

    Moreover, the T3SS homologue of the flagellar proteins FliG and FliN exhibit very limited sequence similarity.

    It should be noted that FliG actually has no known homolog in the TTSS. The sequence similarity between FliG and any TTSS protein is so insignificant that no one can reasonably make a case for homology.

    3. With regards to:

    The motor itself exhibits irreducible complexity, and is dependent on the critical proteins FliG, MotA and MotB. Remove any one of those proteins and the motor will completely cease to function. And yet none of these proteins have non-flagellum-related homologues in other systems: In other words, these proteins are flagellum-specific.

    I have to disagree. FliG shares vague homology with MgtE, and MotA/MotB are thought to be homologous to ExbB/ExbD of the TonB system.

  4. 4
    gpuccio says:


    That seems really good (and absolutely needed). I will take the time to read everything carefully. Thank you.

  5. 5
    Jonathan M says:

    Hi Genomicus,

    Thanks very much for your comments. I have just uploaded a revision. You’re correct regarding FliG having no homologue in the T3SS but rather is somewhat homologous to MgtE, although Pallen and Matzke (2006) observe that “These similarities fail to achieve unequivocal significance using BLAST/PSI–BLAST under any of the above conditions, but are supported by other structural or functional considerations.” You are also correct about the MotA/MotB being homologous to ExbB/ExbD of the TonB system (surprised I missed that one!). I may expand upon that section at a later date as well to include other cases of irreducible complexity.


  6. 6
    Mung says:

    Hi Jonathan,

    In your search for irreducible complexity are you limiting yourself to ‘molecular machines’ such as the flagellar motor?


  7. 7
    JLAfan2001 says:

    Hasn’t the idea of irreducible complexity in the flagellum motor been refuted by Ken Miller? I thought he has shown that parts of the mouse trap can be used for different purposes. The idea is that one part of the motor may not have had that original function but was given that function when a new mutation took it’s place. The motor built and improved itself through progressive mutations.

  8. 8
    bornagain77 says:

    To refute the irreducible complexity of the flagellum motor should not one be required to actually produce a flagellum (or a single novel functional protein of the flagellum for that matter) by Darwinian processes instead of just conjuring up a some imagination of how it could have possibly been done??? But then again if you take away story telling from the Darwinian myth what have you got??? Nothing!

    Michael Behe on Falsifying Intelligent Design – video

    Orr maintains that the theory of intelligent design is not falsifiable. He’s wrong. To falsify design theory a scientist need only experimentally demonstrate that a bacterial flagellum, or any other comparably complex system, could arise by natural selection. If that happened I would conclude that neither flagella nor any system of similar or lesser complexity had to have been designed. In short, biochemical design would be neatly disproved.– Dr Behe in 1997

  9. 9
    Joe says:


    Hasn’t the idea of irreducible complexity in the flagellum motor been refuted by Ken Miller?


    I thought he has shown that parts of the mouse trap can be used for different purposes.

    That doesn’t matter:

    Irreducible Complexity is an Obstacle to Darwinism Even if Parts of a System have other Functions by Dr Behe himself

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