Intelligent Design advocates rely heavily on an argument called irreducible complexity.
The basic idea behind Intelligent Design is that Darwinistic evolution is insufficient to explain the development of life as we know it, and thus the world shows evidence of intelligent purposeful design.
The most basic and best evidence that IDers have for their theory is irreducible complexity. There are structures in nature, they say, that could not have evolved in gradual steps because they are irreducibly complex.
Irreducibly complex means that if any component of the structure is removed, then it will cease to function.
Charles Darwin agreed that finding irreducibly complex structures would destroy his theory:
If it could be demonstrated that any complex organ existed, which could not possibly have been formed by numerous, successive, slight modifications, my theory would absolutely break down. (Origin of Species, ch. 6)
But exactly how are you going to prove irreducible complexity?
The poster child of the irreducible complexity movement was the flagellum on bacteria that they use for movement. The claim of Behe and others was that if you remove any part of the flagellum it would become nonfunctional.
Because the bacterial flagellum is necessarily composed of at least three parts—a paddle,a rotor, and a motor—it is irreducibly complex. Gradual evolution of the flagellum, like the cilium, therefore faces mammoth hurdles. (Michael Behe, Darwin's Black Box, p. 72; cited from www.veritas-ucsb.org)
A white raven argument is one that attempts to prove a negative. In irreducible complexity the argument is that there are no precursors to certain structures in nature (such as the bacterial flagellum discussed in the text).
The problem with white raven argument is they're impossible to prove. How do you know you won't turn up a white raven somewhere?
It's the same with any supposedly irreducibly complex system. Just because you can't think of what a partial bacterial flagellum might be used for doesn't mean that there is no such use. Most people could not suggest a use for a partial eye or partial lung, but that's because they don't know about light spots and swim bladders like scientists do.
Scientists are human, too. They cannot say that a system or organ is irreducibly complex because there might be a use for a partial system or organ that is currently unknown.
That sounded convincing when originally proposed. However, when given time …
The most powerful rebuttals to the flagellum story … have emerged from the steady progress of scientific work on the genes and proteins associated with the flagellum and other cellular structures. … As the evidence has shown, nature is filled with examples of "precursors" to the flagellum that are indeed "missing a part," and yet are fully-functional. Functional enough, in some cases, to pose a serious threat to human life. (Kenneth Miller, "The Flagellem Unspun")
That problem will always be there. Any suggested irreducibly complex organ or structure may prove to be found in nature in successive steps. In this case, it turns out that some bacteria have what is known as a Type III Secretory System, which they use to inject toxins into cells, including our cells.
You guessed it. That system is made from the same proteins that make up the bacterial flagellum. We now have one step on the way to this supposedly irreducibly complex flagellum. How much do you care to wager that science won't soon find more?
The original irreducibly complex organ advanced against evolution was the eye. However, the evolution of the eye is easily enough explained that Darwin was able to do it in 1859. In fact, the many versions of the eye found in nature are thought to have evolved separately so that an eye of some sort has probably evolved at least 40 times!
Since I wrote this I got to hear Dr. Behe try to explain away these findings.
Simply put, no one is ever going to be able to prove irreducible complexity. There will always be the possibility of finding a "white raven" (see sidebar). So far, science has been remarkably successful at turning up the reductions that destroy irreducible complexity.