In addition to merely nitpicking evolution and calling God 'Designer', the ID movement has two distinguishing features among creationists: irreducible complexity and another, fuzzier concept variously referred to as 'functional information' or 'complex specified information'.
Irreducible complexity generally falls under the rubric of nitpicking evolution, but since the concept was much touted back in the day, it's worth looking at in depth. The basic idea as put forward by Behe is that certain biological systems are composed of several interdependent parts and don't work without all of the other parts. The
most famous example is the bacterial flagella, which more or less couldn't work without all their proteins. Behe then goes on to argue that such a system could not have evolved naturally—there are no functional intermediate steps leading up to the whole structure. “An irreducibly complex system cannot be produced directly by numerous, successive, slight modifications of a precursor system, because any precursor to an irreducibly complex system that is missing a part is by definition nonfunctional.” What good is half a flagella?
The problems with this are concurrent development, removal of parts, and exaption. Concurrent development means, roughly stated, that the parts evolve together into interdependence. Suppose that you have a protein that does something. Now imagine that a mutation produces a second protein which greatly enhances the function of the original but depends on it to work. Then, imagine that another mutation changes the first protein in such a way as to be more efficient still, but to depend on the presence of the second protein. Congratulations, you have just imagined the evolution of an irreducibly complex system in just two beneficial mutations.
The second problem is removal of parts. One can argue that an arch is irreducibly complex: take away any given piece, and the whole structure collapses. However, suppose that a wooden support was first placed under the arch. Then, the pieces are added one by one, and when the last piece was added the wooden support was removed. Likewise, in biological systems, one 'support' part that allows the other parts of the system to evolve might eventually be removed at a certain point in evolution when the other parts function perfectly well without it.
The final point is that biochemical parts are often co-opted by mutations to do something other than their original function. If you take apart a flagellum, then it won't work anymore—as a flagellum. But the individual parts and structures work perfectly well as something else: the base, for instance, has been co-opted from a secretory system common in other related bacteria. A mousetrap is irreducibly complex: all the parts depend on each other. Take away the release mechanism and it won't catch any mice. It will function just fine as a tie-bar or paperclip, though—having parts which existed anyway but then were brought together for other functions is another way in which an irreducibly complex system can evolve. In our analogy, the tie-bar is a transitional stage, just as the secretory systems were transitional stages for the flagellum.
What good is half a flagella? asked Behe. As it turns out, it's an excellent secretory system.
So, back to Behe's statement: “any precursor to an irreducibly complex system that is missing a part is by definition nonfunctional.” Well, no. No, they are not—and this holds true for every single system yet put forward as irreducibly complex. Credit where it's due: Behe has distinguished himself from all others in the the movement by making a definite, testable prediction. Unfortunately, it's wrong.