Talk:Flagellum

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Could we make this about Flagellum?

This article does not give any information about the flagellum function or structure but instead seems to be a soapbox about intelligent design and counters to it. Very unproductive if you ask me.--TimS 14:31, 4 May 2007 (EDT)

The flagellum and the argument for Intelligent Design

The flagellum has frequently been presented as a counter-example to evolution. [1] [2] Anti-evolutionists argue that it could not possibly have been produced by an evolutionary pathway.

The argument from irreducible complexity

The flagellum is said to possess "irreducible complexity," meaning it could not have been produced by evolution. This argues for an outside intelligent designer operating beyond the laws of nature. Behe defines an irreducibly complex structure as ". . . 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." [3]

Behe asserts that the flagellum is a structure "in which the removal of an element would cause the whole system to cease functioning" [4] and that its individual parts must have been specifically designed to work as a unified assembly.

The argument is that because a minimum number of protein components are needed for a working biological function, evolution could not have selected to produce those components a few at a time, since they do not have functions that natural selection would favor. The logic of irreducible complexity dictates that these individual components should have no function until all 30 are put into place, at which point the function appears. Behe wrote: " . . . natural selection can only choose among systems that are already working" [5]. An irreducibly complex system does not work unless all of its parts are in place. The flagellum is irreducibly complex, and therefore it must have been designed.

Counter arguments to the argument from irreducible complexity

The argument for intelligent design has received many detailed rebuttals have appeared in scientific literature [6][7][8][9] including counter-examples to the assertion that evolution cannot explain biochemical complexity. [10] Researchers have also investigated the means by which evolutionary mechanisms allow biological systems to increase in information content [11] [12]

Studies of the genes and proteins associated with the flagellum and other cellular structures have now established that there are precursors to the flagellum that are fully functional despite "missing a part."

Disease-causing bacteria threaten the organisms they infect in many ways, one of which is to inject poisons into a host cell's cytoplasm. Once inside, these toxins break down and destroy the host cells, producing illness, tissue damage, and sometimes even death. They do this by employing a Type III secretory system (TTSS)[13] The proteins transferred this way are how these bacteria make people sick.

Molecular studies of proteins in the TTSS show that they have the same relative structure as proteins in the basal portion of the bacterial flagellum. A detailed comparison of the proteins associated with both systems [14] notes that the two systems "consist of homologous component proteins with common physico-chemical properties". This demonstrates that a smaller subset of the full complement of proteins in the flagellum makes up the functional transmembrane portion of the TTSS.

It is normal for evolutionary processes to mix and match proteins opportunistically to produce new functions. According to the doctrine of irreducible complexity, however, this should not be possible. If the flagellum is irreducibly complex, then removing just one part, let alone 10 or 15, should render what remains nonfunctional. Yet the TTSS is fully-functional, even though it is missing most of the parts of the flagellum. Since the function of the TTSS is favoured by natural selection, it is obviously incorrect to argue that the flagellum must be fully assembled before any of its parts can be useful.

It is self-contradictory to argue that one irreducibly-complex system might contain another. The flagellum cannot be irreducibly complex if it contains a smaller functional set of components like the TTSS.

The Combinatorial argument

William Dembski [15] calculated the probability of assembling an object like the flagellum by working out the probabilities that each of its components might have originated, been localized to the same region of the cell, and assembled in the right order by chance.

Dembski states that, given enough time, any event with a probability larger than 10-150 might well take place. He puts the odds of evolving the 30 proteins of the bacterial flagellum at 20-30 ("around 10-39") and explains that since the flagellum requires 30 such proteins, they "will all need to be multiplied to form the origination probability", which would give an origination probability for the flagellum of 10 -1170. This is well below the universal probability bound, demonstrating that the flagellum could not have evolved.

Counter arguments to the Combinatorial argument

Dembski's calculation considers only the probability of spontaneous, random assembly for each of the proteins of the flagellum. His conclusion that the flagellum is unevolvable fits with his view that "The Darwinian mechanism is powerless to produce irreducibly complex systems...".

However, by treating the flagellum as a "discrete combinatorial object" he has only shown that it is unlikely that the parts of the flagellum could assemble spontaneously. Unfortunately, no scientist has ever proposed that the flagellum or any other complex object evolved that way. This is a straw man argument, and the calculations are irrelevant.

He assumed that none of the 30 proteins of the flagellum could have biological activity. We can see from the work on the TTSS that this is wrong. Nearly a third of those proteins are closely related to components of the TTSS, which does indeed have biological activity. This seems to invalidate the underlying assumption of his calculations.

References

  1. Dembski, W. 2002a, No Free Lunch: Why specified complexity cannot be purchased without intelligence. Lanham, Maryland: Rowman & Littlefield.
  2. Behe, M. 1996a. Darwin's Black Box. New York: The Free Press.
  3. Behe, M. 1996a. Darwin's Black Box. New York: The Free Press.
  4. Behe, M., 2002. The challenge of irreducible complexity, Natural History 111 (April): 74.
  5. Behe, M., 2002. The challenge of irreducible complexity, Natural History 111 (April): 74.
  6. Coyne, J. A., 1996. God in the details, Nature 383: 227-228.
  7. Miller, K. R. 1999. Finding Darwin's God. New York: Harper Collins
  8. Depew, D. J. (1998), Intelligent design and irreducible complexity: A rejoinder. Rhetoric and Public Affairs 1: 571-578.
  9. Thornhill, R. H., and D. W. and Ussery, 2000. A classification of possible routes of Darwinian evolution, The Journal of Theoretical Biology 203: 111-116
  10. Miller, K. R. 1999. Finding Darwin's God. New York: Harper Collins.
  11. Schneider, T.D. (2000), Evolution of biological information. Nucleic Acids Research 28: 2794-2799.
  12. Adami, C., C. Ofria, and T. C. Collier, 2000. Evolution of biological complexity, Proceedings of the National Academy of Sciences 97: 4463—4468
  13. Heuck, C. J., 1998. Type III protein secretion systems in bacterial pathogens of animals and plants, Microbiol. Mol. Biol. Rev. 62: 379-433.
  14. Aizawa, S.-I., 2001. Bacterial flagella and type III secretion systems, FEMS Microbiology Letters 202: 157-164
  15. Dembski, W. 2002a, No Free Lunch: Why specified complexity cannot be purchased without intelligence. Lanham, Maryland: Rowman & Littlefield.

Blanking

I'm just guessing, but I suspect the blanking was due to it being a repeat of the actual article, probably better to blank it again as it is confusing having the same thing on both 'article' and 'discussion' pages.

That's right and I wrote a message on Dpbsmith's page to tell him so. I'd blank it again myself but it seems a bit pointless if somebody's just going to revert it. Britinme 19.40 30 March 2007 (EDT)

This one seems better

The talk page Flagellum article seem to give a more balanced picture of the debate, rather than the one-sided article. Like many other articles here, both sides of the debate should be represented.--Phillipps 16:57, 27 February 2008 (EST)

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