Advocates of so-called Intelligent Design claim that some biological systems are too complex to have evolved, so they must have been designed by some external intelligence (about which they, oddly, refuse to hold any testable theory...). What they really mean by this, of course, is "I don't understand how this could have evolved, so it must have happened by magic." There are so many things wrong with Intelligent Design it's hard to know where to start -- it's not just bad science, it's bad religion (but that's a topic for another, much longer post).
Personally, I'm an advocate of Idiotic Design. It's clear to me that some biological systems are too complex to have been designed, so they must have evolved without any external intelligence. Interestingly, most of the best examples for Idiotic Design are the same ones invoked by Intelligent Design advocates (at least, up until science proves them wrong, and then they start casting around for another example). For instance, consider the human eye: a horrendously complex sack of components with a blind spot in the middle of an upside-down projection of the world that's channeled to the opposite hemisphere of the brain, which then has to sort the whole mess out. And look around you at the number of people who are near-sighted, long-sighted, astigmatic, have glaucoma or cataracts or any number of other eye diseases. Who could possibly have designed such a bad solution to a straightforward problem?
The evidence, then, admits of only two possible conclusions: Either the eye emerged slowly through small random steps combined with selection pressure (i.e. evolution). Or it was Designed by an Idiot.