Note: This post may be subject to some revision. I’ve hopefully managed to accurately summarise some scientific findings outside my area of proficiency. Also, there is a lot that could be said here but I’m be trying my best not to bog this down too much. Dialogue and suggestions are most welcome.
The bacterial flagellum (from latin “whip”) is one of the most complicated examples of biological “machinery” we know of today. The projecting filament of the flagellum is capable of rotating at thousands of revolutions a minute to propel bacteria through watery environments, in a similar way to an outboard motor. Explaining how such a system works and how it evolved has been a subject of interest for modern biologists.
In recent years, the interest in this curious piece of micro-machinery has increased dramatically. This has been as result of the intelligent design (ID) movement using it as a challenge to evolution and the flogging the subject took at the Dover trial a few years ago. Cdesign propensists, claim that the bacterial flagellum is evidence for ID as it exhibits an attribute called “irreducible complexity” (IC), which was coined by Michael Behe.
What he means by this is:
“An irreducibly complex system cannot be produced directly by numerous, successive, slight modifications of a precursor system, because any precursor to an irreducibly complex system that is missing a part is by definition nonfunctional.”
“Since natural selection can only choose systems that are already working, then if a biological system cannot be produced gradually it would have to arise as an integrated unit, in one fell swoop, for natural selection to have anything to act on.”
Essentially, an irreducibly complex system could be formed by the gradual processes of evolution as the non-functioning intermediates would not be preserved by natural selection. This harkens back to the old creationist arguments of “what good is half an eye” and William Paley’s argument for design (Perhaps these arguments and IC share a common ancestor?).
Despite the over-whelming rejection of ID by the scientific community and the gaping wound left by the rulings at Dover, ID proponents insist they have a case for design in irreducible complexity. Do they?
I don’t think so when I consider:
the bacterial flagellum isn’t irreducible
homology within the proteins involved
the many paths to bacterial mobility
complexity ≠ design
An article by Nick Matze mentions that some bacterial flagella can function without some of the proteins that make up the flagellum. According to the definition we should not be able to reduce such a system to a functioning simpler form. It should be by definition non-functional.
Another reason not to accept the notion of irreducible complexity is the existence of homologies. One popular example is that of the type 3 secretory system (T3SS) which pathogenic bacteria use to inject cells with toxins. Several of the T3SS proteins are found in the bacteria flagellum. It’s currently debated which came first or whether they evolved from a common ancestor, regardless, this example shows how a partial flagellum could have a use. If it has a use it can be preserved by natural selection and exaptation/co-option to act on it, which leaves no reason to claim evolution is incapable of producing it.
In reading some of the literature surrounding bacterial flagella I came across something I had previously overlooked, which is possibly my new favourite reason to reject ID’s notions. The fact is that there is no such thing as the bacterial flagellum. Instead, we find huge variation in how flagella function and are assembled. Of the 40-50 proteins used in making a flagellum only 20 are universal. I think it’s unnecessary to detail all the possibilities, but this raises some rather awkward questions for the ID camp such as:
Why did the intelligent designer reinvent the design so many times?
Did the intelligent designer create all of these varying different flagella in thousand of special creations?
Is the intelligent designer trying to trick us by making it look like these different systems evolved from a common ancestor?
The final question for me is how on earth would we recognise design? Complexity is not necessarily a good indication that something is designed. What about simplistic designs? Paperclips and bricks aren’t complicated but we know that they are designed (albeit by human designers which is another limitation for intelligent design). What we’re actually looking for in design is artificiality, which is difficult to find in a biological system.
Bacterial flagella are complicated and their evolutionary history is not well understood. However, I think that it is much better to leave science to explain, rather than invoking the arguments from psuedo-science which halt scientific enquiry. Pointing at gaps in scientific knowledge without providing a scientific and testable alternative is not good science. Furthermore, intelligent design implicitly limits God’s intelligence and power (most ID proponents think of the Judeo-Christian God). A God who has to repeatedly intervene to maintain his creation is less intelligent than one who could had a process in mind which could produce such complex micro-machines.
Dolittle, W.F. & Zhaxybayeva, O., (2007). Evolution: Reducible Complexity — The Case for Bacterial Flagella , Current Biology, 17(13) , 510-512
Pallen, M.J., & Matzke, N.J., (2006). From Origin of Species to the origin of bacterial flagella, Nature Reviews Microbiology, 4, 784-790
Wong, T., Amidi, A., Dodds, A., Siddiqi, S., Wang, J., Yep, T., Tamang, D.G., & Saier, M.H., (2007). The Evolution of Bacterial Flagellum, Microbe, 2(7), 335-340