John Allen Paulos in his consistently good Who’s Counting column at ABC News:
The theory of intelligent design, the purportedly more scientific descendant of creation science, rejects Darwin’s theory of evolution as being unable to explain the complexity of life. How, ask supporters of intelligent design, can biological phenomena like the clotting of blood have arisen just by chance?
A key supporter of intelligent design likens what he terms the “irreducible complexity” of such phenomena to the irreducible complexity of a mousetrap. If just one of the trap’s pieces is missing — whether it be the spring, the metal platform, or the board — the trap is useless. The implicit suggestion is that all the parts of a mousetrap would have had to come into being at once, an impossibility unless there were an intelligent designer.
Design proponents argue that what’s true for the mousetrap is all the more true for vastly more complex biological phenomena. If any of the 20 or so proteins involved in blood clotting is absent, clotting doesn’t occur, and so, the creationist argument goes, these proteins must have all been brought into being at once by a designer.
But the theory of evolution does explain the evolution of complex biological organisms and phenomena, and the above argument from design, which dates from the 18th century, has been decisively refuted. Rehashing the latter explanation and refutation is not my goal, however. Those who reject evolution are usually immune to such arguments anyway.