As everyone is tired now of hearing, Intelligent Design booster Michael Behe has argued there is an "irreducible complexity" in some biological processes that means they cannot have evolved. The basic logic is pretty simple - if a system needs all its parts, then the lack of any part means it would be nonfunctional. This means, he says, that for it to have evolved, those other parts would be twiddling their thumbs evolutionarily, until all the parts are in place.
The incredibly smart, handsome and active Ian Musgrave has a piece on Panda's Thumb on Behe's key example, the clotting cascade, showing that all the homologues of the mammalian clotting cascade can be found doing other things in other organisms, and that it could very well have evolved by hijacking prior functions to a new task.
It's cool to see some real science being done to rebut these hoary old claims. But being who I am, I have to look at these arguments historically, and guess what? It's all been done before, at least in argument form. At the beginning of the 19th century, Cuvier dismissed transformism of species because all the parts of animals were so tightly correlated that any change would make them nonfunctional. Cuvier famously argued that from any part, such as a tooth, he could reconstruct the lifestyle and basic form of an animal, because of this tight integration of parts.
Darwin knew Cuvier's work well, and knew too that he had to address this question of functional integration, and did so in a passage that is often misquoted by creationists - the evolution of the complexity of the eye. Darwin in effect took the most complex organ he could, and showed how it could indeed work in intermediate functional forms. He wrote:
Organs of extreme perfection and complication.—To suppose that the eye, with all its inimitable contrivances for adjusting the focus to different distances, for admitting different amounts of light, and for the correction of spherical and chromatic aberration, could have been formed by natural selection, seems, I freely confess, absurd in the highest possible degree. Yet reason tells me, that if numerous gradations from a perfect and complex eye to one very imperfect and simple, each grade being useful to its possessor, can be shown to exist; if further, the eye does vary ever so slightly, and the variations be inherited, which is certainly the case; and if any variation or modification in the organ be ever useful to an animal under changing conditions of life, then the difficulty of believing that a perfect and complex eye could be formed by natural selection, though insuperable by our imagination, can hardly be considered real. How a nerve comes to be sensitive to light, hardly concerns us more than how life itself first originated; but I may remark that several facts make me suspect that any sensitive nerve may be rendered sensitive to light, and likewise to those coarser vibrations of the air which produce sound. In looking for the gradations by which an organ in any species has been perfected, we ought to look exclusively to its lineal ancestors; but this is scarcely ever possible, and we are forced in each case to look to species of the same group, that is to the collateral descendants from the same original parent-form, in order to see what gradations are possible, and for the chance of some gradations having been transmitted from the earlier stages of descent, in an unaltered or little altered condition. Amongst existing Vertebrata, we find but a small amount of gradation in the structure of the eye, and from fossil species we can learn nothing on this head. In this great class we should probably have to descend far beneath the lowest known fossiliferous stratum to discover the earlier stages, by which the eye has been perfected. [Origin, first edition, p186ff]
and then proceeds to look at different groups with eyes of varying complexity, all of which work well in the organism's environment. He noted, and this is something the IDevotees should pay close attention to:
We should be extremely cautious in concluding that an organ could not have been formed by transitional gradations of some kind. Numerous cases could be given amongst the lower animals of the same organ performing at the same time wholly distinct functions. [p190]
The logic used by Cuvier is revivified in the ID arguments, but the solution was given in general terms by Darwin. What Ian has done here is effectively a Darwin number on Behe, who is very far from the sophistication of a Cuvier, even if Ian is being a grand Darwin to him. Also, go check out
Thornhill, R. H., and D. W. Ussery (2000), "A classification of possible routes of Darwinian evolution", J Theor Biol 203 (2):111-116.
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I'm far from the first to point this out, but as remarkably complex as the eye is, it is far from the perfection one would expect of a supernatural engineer. Aside from all of the compensatory neurological activity required to deal with the blind spot in each eye and habituation of photoreceptors, we are essentially blind for anywhere from 8-13 hours per day. Human designers, as well as nature, have produced visual systems that can see in the dark, so why did the intelligent designer deprive us of this capacity, leaving us with such a major shortcoming in this complex system?
Well, then there are wisdom teeth...
Even the fans of intelligent design have said that a design might be less than optimum (although perhaps it was with the same breath that they used to identify the Designer as a space alien). I suppose they could classify some anatomical features as "intelligently designed" or "half-assed designed." The sub-optimal design is not a show-stopping argument; showing that naturalistic process can achieve the same result renders design unnecessary.
Although "irreducible complexity" fails as an argument against evolution, I'm wondering if it can be (or has been) turned around and used as an argument against another creationist canard - that the evolution of increasing complexity is unlikely. It seems to me that the evolution of an existing "IC" system would be restricted to adding on or modifying components rather than removing them, since removing them would render the system non-functional. This would favor the evolution of additional complexity. One could argue that the system could evolve backward in the same way that went forward, but at this point it becomes a probability argument.
I was reading about jellyfish the other day in the Big Book o' Invertebrates and was fascinated to learn that they use the same chemical system, namely potassium and chlorine ions, to send nervous impulses throughout their flesh and stimulate motion; but those ions are not found in and around nerves the way they are in animals with more complex nervous systems. I believe that some of them have a "neural net" but no ganglia nor brain.
I'm still trying to figure out how they detect light without even eyespots.
Some of them also move to avoid levels of carbon dioxide that are too high, which keeps them in water with adequate oxygen. Oddly enough, it's rising carbon dioxide that makes us breathe more quickly--we can't detect low oxygen levels, vital though that sense would be.
So we have at least one example of reducible complexity (nervous impulses without nerves) and two of common mechanism when we compare ourselves with jellyfish. Common biochemical pathways descent from a common ancestor to me.