From: Peter Ruest (pruest@pop.mysunrise.ch)
Date: Fri Nov 08 2002 - 00:53:46 EST
David Campbell wrote:
> >[PR:] Some time ago, on this list, Behe's and Dembski's use of the
> >bacterial flagellum as a system of irreducible complexity was
> >dismissed with the claim that it probably evolved out of the type
> >III protein secretion system. I tried to check the evidence
> >available to support this claim.<
> [useful detailed comparison deleted]
> >In any case, the similarities between flagellum and type III export
> >are very restricted, making any derivation claim highly speculative.
> >Whether the sequence similarities which do exist derive from
> >divergence from common ancestral genes or from functional
> >convergences remains equally uncertain.<
> >Does this situation "explain" the flagellum and demonstrate that it
> >is not "irreducibly complex"? I doubt it. The claim remains to be
> >substantiated.<
>
> The similarities between parts of the flagellum and type III systems
> certainly does not provide a full explanation of the origin of the
> flagellum. However, they do contradict a basic assumption of
> identification of irreducible complexity, namely the assertion that
> the parts are only useful as components of the whole system and could
> not be gradually assembled piecemeal. Instead, we have a subunit
> that can be used as a component of multiple systems. Thus, all the
> parts of a flagellum did not have to be assembled all at once.
>
> Thus, ID is not disproven, but it is not supported, either.
The review by Hueck about the type III protein secretion systems (MMBR
62 (1998), 379-433) showed similarities between 10 of the type III
system proteins and 10 of the flagellum system proteins, representing at
most 25-30% of the components of each system. The similarities
represented small to appreciable amino acid identities in regions
presumably important for analogous functions of the two export systems
(the flagellar system is composed of an export system for flagellar
components, in which all the similarities occur, and the operative
flagellum system proper). In our present state of knowledge, it isn't
even possible to exclude convergent evolution, at least in some of these
cases. This implies that incorporation of the same ancestral proteins
into the two systems cannot be inferred with any certainty. There is no
proof that any of the flagellum proteins is evolutionarily derived from
a type III protein. In fact, Hueck speculated about the inverse
relationship. We have no experimental evidence that any type III protein
(or any few-mutations-derivative) could functionally replace the
supposedly homologous flagellum protein.
Thus, the comparison does not (yet) contradict the assumption of
irreducible complexity. Irreducible complexity is given, as well, if
there is at least one component whose evolution would require passing
through a series of several non-selectable intermediates to finally
achieve (at least minimal) functionality of the complete flagellum
system. More precisely, for proving a possible evolutionary path, we
would need evidence that the product of the probabilities of occurrence
of the evolutionary random-walks of consecutive non-selected mutational
steps for all flagellar system components is sufficiently large. This is
no problem only if, for all components, these random-walks through
non-selected path segments were few and very short.
By the way, there is no need to claim that all had to be assembled at
once. The components just had to be assembled at some time into a
minimally functional flagellum, after having gone their own paths. In
any case, the probabilities for any non-selected-intermediates segments
cumulate multiplicatively.
This implies that, from what is presently known about it, the type III
system does not disprove irreducible complexity of the flagellum system.
Of course, this, in itself, does not prove it, either.
Peter
-- Dr. Peter Ruest, CH-3148 Lanzenhaeusern, Switzerland <pruest@dplanet.ch> - Biochemistry - Creation and evolution "..the work which God created to evolve it" (Genesis 2:3)
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