Behe defined an irreducibly complex biological system as a system which is "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".
Creationists, following Behe's lead, often claim that irreducibly complex structures cannot evolve (to be more precise, cannot evolve by the processes detailed in the theory of evolution).
The argument is that an irreducibly complex system of n parts can't have evolved because natural selection would not have favored the evolution of the first n-1 parts of the system, because the first n-1 parts of the system would have conferred no selective advantage to the organism until the nth part was added to the system.
(I believe that I have stated the creationist position fairly: it is no desire of mine to argue against a straw man, and if any creationist would like to refine this argument by quoting from Behe, I should welcome the intervention.)
* * *
These claims about irreducible complexity have, in my opinion, often been misapplied, in that some creationists have held up as examples of irreducibly complex systems things which are, by Behe's definition, not actually irreducibly complex.
Let us leave this aside. An argument is not, after all, falsified by its misapplications. Let us instead take a look at some examples of things that actually are irreducibly complex.
* * *
My favorite example is the bones of the mammalian middle ear. There are three of them, the malleus, the incus, and the stapes, and vibrations are transmitted from on to the other, rather like the toy known as "Newton's Cradle". Take one away, and the whole assembly wouldn't work --- in fact, the whole darn ear wouldn't work. Since the removal of one part would remove the function, this is an irreducibly complex system.
And yet we can see in the fossil record how this evolved. (Or, if you are a die-hard creationist, at least admit that we can see in the fossil record how this could have evolved.) We have the intermediate forms (or, if you are a creationists, the things that look exactly like intermediate forms) showing how the incus and malleus took on their present form and position and role by gradual adaptations of form and position and function from the quadrate and articular bones of the reptilian jaw.
Just to be clear, if you're a creationist, you don't have to concede that this is how the mammalian middle ear evolved. It is, but let's save that argument for a rainy day.
No, my point is that the evolutionary explanation for the middle ear involves gradual shifts of form and position and function. Behe's argument fails because we don't need to imagine an evolutionary process in which (for example) the malleus and the stapes came first, and then the incus poofed into existence to fill the gap between them.
The sequence in the fossil record shows that we don't have to envisage a scenario where two of the three parts acquired their modern form and position first and then the third was tacked on. And even if you are a die-hard creationist and don't think that the fossil record is evidence of anything, nonetheless you must concede that we can see how the irreducibly complex system might have been produced by ordinary evolutionary modifications, and without two of the parts evolving and then the third being added.
* * *
Let's take another example. Mammals have two jaws to grasp and chew their food with: an upper and a lower jaw. Remove one of these parts, and all function is lost --- one jaw is about as much use for chewing as one millstone is for grinding. This structure is therefore irreducibly complex.
So, what can we conclude? Clearly, this: that the evolution of this system can't have occurred by one jaw developing first and then the second one being added to make the complete system.
And guess what? No biologist anywhere ever has claimed that this happened. Instead, they trace the evolution of jaws from pharyngeal arches ("gill arches") which already had a lower and upper half before they functioned as jaws. Fossils of interest include the early chordates Yunnanozoon and Haikouella, which have no jaws but which possess pharyngeal teeth.
(Some modern fish also have pharyngeal teeth on the pharyngeal arches behind their jaws. Here's a picture of pharyngeal teeth in modern carp.
In cichlids, the lower left and right first pharyngeal arches have fused together to form a lower "pharyngeal jaw" behind the true jaw.)
Now, whether or not you accept the evolutionary explanation for this irreducibly complex structure, it's plain that the fact that it is irreducibly complex does nothing to argue against the evolutionary explanation, since the explanation does not involve the system evolving one jaw at a time, nor yet two fully-formed jaws simultaneously appearing at a single stroke, but rather the gradual processes of adaptation of form and function that one associates with the theory of evolution --- that is, with the actual theory of evolution, the one in biology textbooks, rather than the imaginary theory of evolution that must have been rattling around in Behe's head when he came up with this stuff about irreducible complexity.
* * *
As a final example, divide yourself (conceptually) into two parts --- your head and everything else. A series of experiments conducted by the French researcher Dr Joseph-Ignace Guillotin have proved conclusively that one is useless without the other. You yourself are an irreducibly complex system.
And this would be a strong objection to any evolutionary scenario involving a bunch of human heads rolling about without bodies for a few millenia; or, alternatively, a bunch of headless people strolling around and bumping into things.
But this is not the scenario that biologists have put forward. Instead, they suggest that the process involved gradual encephalization starting with a segmented ancestor whose frontmost segments played no special role.
Again, you can believe that or not; but it is no objection to this theory to point out that today the human body that represents one of the end points of that process forms an irreducibly complex system. It is only an objection to a hypothesis in which heads magically appeared on human bodies which previously had lacked them, or vice versa --- a hypothesis to which no-one at all subscribes.
* * *
The fact that a system is irreducibly complex in no way suggests that it could not have evolved. It does, however, suggest that such a system cannot have evolved by a process in which the last step was the sudden wholesale addition of a part to a system which was otherwise identical to the system in question.
Which is why no biologist has ever attempted to explain any such system in any such way.
Now if Behe wants to argue against evolution, he should try arguing against the actual processes that are supposed to underlie evolution: the gradual modifications of form and function which biologists claim have occurred. Instead, he has an excellent argument against parts suddenly poofing into existence out of nothing to fulfill a role in a system which wouldn't work without them.
He is right to suggest that this does not happen. I suppose that pretty much every evolutionist from Darwin on would heartily concur. Hurrah, Behe has discovered one of the more obvious consequences of the theory of evolution! But he also has mistaken it for an argument against that same theory; and it is hard to see why he has made such an atrocious blunder except under the hypothesis that he's some kind of bloody fool.
* * *
To the moderators: slevesque indicated that he would be interested in having a one-on-one debate on this subject with some evolutionist in the Great Debate forum.
I immediately PM'd him and took him up on this.
For some reason he didn't answer. One can only speculate as to why. My own favored hypothesis is that he realized that I am such a delicate and sensitive soul that debating with me would be like trampling on a fragile little flower.
There might be another reason.
Anyway, my suggestion is that you should put this in whatever forum you see fit and open it up to general comments; and if slevesque decides that he wants to make it a Great Debate topic between the two of us, then I'd be happy to continue it there.
I would be happy to take on this discussion with you in the Great Debate section.
I'm sorry I didn't answer your PM, in retrospect I should have maybe given my intentions, which were simply to wait to see if someone who had participated actively in the discussion of IC in the other thread would want to do this, as I would have given them priority over you who made little contribution. (Don't take this personnally, it seemed like simply the right thing to do. Be assured I am very happy that you proposed yourself).
And so between preparing an opening post and studying for my quiz of today, I told myself I would just wait a day or two to see if either you would make an OP, or if I would receive another PM.
So in the meantime I won't promote this topic myself, and instead leave that to another admin because in any case even if I did do it now I wouldn't get a written answer posted before tonight.
I'll end by saying that hopefully the peanut gallery gets going, but I won't be answering directly there so if you see something worthwhile mentioned over there you'll simply have to be the one to bring it into the discussion here. Idem for me if any other points by creationists I think merits attention.