Superb!
Can anyone put me straight on 'accelerations' or 'stagnations' in evolution?
In immunology, there is the 'Generator of Diversity' (GOD), a mechanism to in the thymus generate the (almost) infinite diversity of T-cell specificities towards antigens (bacteria, viruses...) It is a 'machine' generating minor changes in the gene of the hypervariable part of the antibody molecule. These clones then get selected by the environment.
Jan
At 17:28 21/01/2005, you wrote:
Having just
read all of Behe's examples I could most humorously relate to the
article below called "Irreducible Complexity Demystified."
by <mailto:pdunkelberg _at_>earthlink.net>Pete Dunkelberg where he does a most excellent job of totally
destroying the entire concept of "Irreducible Complexity."
It appears, in the end, to boil down to two major problems with Behe
(among many others.) The first is the way Behe "conceptualizes"
biological "parts" creating a non-existent state of biological existence.
The second is the result of truly sloppy scientific work on the part
of Behe.
To me, the essential argument against Behe's idea that "irreducible complexity" cannot evolve, is that Behe ignores or dismisses "indirect" evolution, i.e. evolution following some circuitous route towards a particular function or organization, by evolving a variety of other functions or organizations that at first sight don't seem to have anything to do with the function in focus, but that eventually produce that function as a kind of "side-effect".
Stephen Jay Gould summarized this idea using the term "exaptation", i.e. an evolved featured is not initially adapted to perform a particular function A, but a wholly independent function B. But then it turns out that the organization that allows B can also be used to do A, with some minor modifications. And finally, perhaps because the environment has changed, it turns out that B is no longer necessary, so the full selective pressure goes towards optimizing the feature for A.
But then an intelligent design theorist comes along, sees how beautifully the feature performs A, and observes that a more primitive version of this feature would not have been able to perform A. Therefore, he concludes, the feature could not have evolved, and must have been designed. But he ignores the possibility that the primitive version might have performed the wholly different function B.
A classic example of exaptation are the lungs of land-living vertebrates, whose function is to extract oxygen from the air (A). They evolved from the swimbladder used by fish to keep their vertical position in the water (B). But once the first fishes started to live on the land, they no longer had a need for a swimbladder, and so function B was lost. However, since the swimbladder contained air, it could be easily adapted to the function of extracting oxygen from that air. The gills, on the other hand, which fish use to extract oxygen from water, could not be adapted to air, and so were lost.
Another simple example mentioned by Pete Dunkelberg is the tail used by cows merely to swat flies, where it is obvious that tails initially evolved for very different purposes.
The essential point is that evolution does not distinguish between "direct" and "circuitous" routes towards evolving some feature: it merely moves from the present state to a state that is locally more fit, but what is fit depends on a multitude of factors and a constantly changing context. Therefore it is difficult for us as observers to recognize a concrete direction in the process. When we try to imagine how the process might have taken place, we are biased to look in a particular direction, namely the one where the function we are interested in can be reached via the shortest, most direct route. But then we often find that there is an insurmountable obstacle when we try to follow that route. Yet, evolution doesn't even notice the obstacle, as it cannot look ahead, and has no bias towards short, direct routes: it will just take any route going anywhere, with the only condition that none of the states along that route reduce fitness.
--
Francis Heylighen Evolution, Complexity and Cognition group Free University of Brussels http://pespmc1.vub.ac.be/HEYL.html
