By the way, the notion of "purpose" does not necessary imply "complexity." Something can be quite simple in structure and still designed; something else can be quite complex and still be reducible to chance. It isn't the complexity of the eye that suggests it is designed by an outside Designer; it's the irreducible nature of the complexity that does so. As Behe shows in his book, only the entire vision cascade has any value; 1/2 or 1/3 of the chemical steps, coming together over time through blind chance (as per Darwinism) has no value -- it doesn't lead to 1/2 or 1/3 the amount of vision. It leads to no vision. It's a biochemical dead end. Either we admit the notion of "design" in nature, or we admit the idea that Nature makes incredible jumps ("saltations"). The second notion is just as anathema to Darwinism as the first, as it smacks a little too much of "miracle" at least for orthodox materialists such as Richard Dawkins.
You obviously haven't read any of the refutations of Behe's book. Irreducible complexity can evolve, and has been shown to evolve.