Technically, exaptation is not a gradualistic mechanism--at least the way
Mike is defining gradualism, but it has been comfortably drawn into the
neo-Darwinian explanatory apparatus. Del says of Mike's position
> - it is
>whether or not such could be generated by *Darwinian* means (defined,
>again, in terms of selectionist, gradualist processes).
>
and
> And the support for *that* claim is not a simple "I don't see
>how it could" - it is an argument generated out of some of the
>implications of the selectionist and gradualist constraints he is
>focusing on, in conjunction with his definition of irreeucible
>complexity.
>
>
The exaptation scenario is this. Components of the irreducibly complex
system all exist in other systems. This is true. Virtually no one in this
discussion denies this. This is the sequence comparison/gene duplication
argument. Now how did the original irreducibly complex system arise in the
first place. Well, even though neo-Darwinists use this argument, it was
not by means of selection and gradualism!!!!! The original system that
produced the useful function did not arise by Darwinian selection and
gradualism, rather it arose by the novel combination of the parts that
functioned for other purposes in other systems.
But here comes the power of natural selection. Once that novel function is
useful, no matter how poorly formed initially, it is selected for and via
selection fine-tuned and perfected. But notice that the primordial event
that resulted in the formation of the novel complex system DID NOT OCCUR BY
DARWINIAN MEANS unless, of course, you re-define Darwinism to include
exaptation (which everyone, except for Behe has done). Interestingly,
Daniel Dennett seems willing to call Stuart Kauffman's self-organizational
proposal Darwinian. He seems to be suggesting that all evolutionary
mechanism should be called Darwinian. Basically, this is semantics, but it
is an important semantic shift.
The question which now arises is "are these "molecular machines" flexible
enough to reassemble into novel, potentially useful, structures. Well, I
happen to think that the answer is yes. Behe seems to think no. One of
the spectacular shortcomings of the machine analogy that Mike uses is that
there are forces at work at the molecular level that lead to self-assembly
that simply have no analogy at our macro machine level. Dennett brought
this out quite nicely in the debate at Notre Dame. Commenting on Mike's
analogy of cans of tuna that have ridges on one end and rounded edges at
the other end so that fit together to be compared with tubulin the protein
subunit of microtubules, Dennett asked "have you even seen a stack of tuna
cans stack up by themselves?" But, you see, this is what happens in the
cell due to the intermolecular forces between molecules! We are only
scratching the surface of the range of protein-protein and protein-DNA
interactions that occur in the concentrated molecular environment of the
cell. Most biochemistry, has been conducted in dilute aqueous solution,
that only remotely resembles the conditions in the cell.
Here's one concrete example that shows the flexibility of proteins. We all
know the sickle-cell anemia story. SSA is the result of a single amino
acid substitution in the beta subunit of hemoglobin. The result of this
single substitution is that deoxy tetramers form a novel macro structure--a
large fiber that is a helical arrangement of deoxy hemoglobin tetramers.
This structure is a novel structure that results from new interactions
caused by the glutamic acid --> valine substitution. The valine introduces
a oily spot on the surface that gloms together with another pre-existing
oily spot. This fiber is what produces the sickling characteristic of the
disease. We also know that this mutation appears to be beneficial against
malaria, at least in the heterozygotic state. How does that work? Well,
I'm not sure that the whole story is in--but it appears that the sickling
caused by the fibers interferes with the life span of the plasmodium
possibly by a simple mechanical disruption of the red blood cell.
So here is a case of one moleculear machine, hemoglobin, being converted
into another molecular machine, a red blood cell-lysing molecular spear, by
a single amino acid substitution. Wow! That is amazing to me! But did it
arise by evolutionary means--no doubt. Did it arise by Darwinian means?
Well, not exactly. The glutamic acid --> valine mutation occurred by
ordinary mutagenic processes, probably random and from our perspective
undirected. SSA fiber formation is a simple biochemical structural
consequence of that mutation. So the new machine arose not by selection or
gradualistically, but by principles of self-organization and suddenly. NOW
DARWINISM KICKS IN. This new machine has a function and so this mutation
gets fixed into a population where malaria is prevalent.
Obviously, this is a simple case, because the new machine is simply a
rearranged version of an old machine. No gene duplication. But it
doesn't seem to me to be such a far fetched first step in the evolution of
some new structure and function.
I will use Dennett's language to summarize. The bottom line is that there
are easily imaginable evolutionary CRANES that produce irreducibly complex
systems. (I strongly disagree with David Tyler's admission that the
critics' task is to show that systems are not irreducibly complex. Behe's
critics must show that irreducible complexity can arise via normal
evolutionary processes.) The sequence comparison data and the gene
duplication inference drawn from it are some of the tell-tale signs that
such an evolutionary crane did exist.
Terry G.
_____________________________________________________________
Terry M. Gray, Ph.D. Department of Chemistry and Biochemistry
Calvin College 3201 Burton SE Grand Rapids, MI 49546
Office: (616) 957-7187 FAX: (616) 957-6501
Email: grayt@calvin.edu http://www.calvin.edu/~grayt
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