In the opening chapter of The Blind Watchmaker, Richard Dawkins penned the following words:
An atheist before Darwin could have said, following Hume: "I have no explanation for complex biological design. All I know is that God isn't a good explanation, so we must wait and hope that somebody comes up with a better one." I can't help feeling that such a position, though logically sound, would have left one feeling pretty unsatisfied, and that although atheism might have been logically tenable before Darwin, Darwin made it possible to be an intellectually fulfilled atheist.
Creationists have quote-mined this statement ad nauseam in a bid to prove - admitted by the archbishop of atheism, Richard Dawkins himself! - that accepting evolution requires one to give up belief in God. But if they use this statement for that purpose, they haven't read it very carefully. What Dawkins is actually saying is something different: not that Darwin made it necessary to be an atheist, but possible. Before Darwin, there was no compelling reply to the argument that only a supernatural creator could have brought about the well-adapted diversity of life. Now there's an alternate explanation, one that does not require any godly intervention. A believer could say that God used evolution as his means of creation, but an atheist can just as well say that evolution happened in God's absence.
Of course, if it's a vital tenet of your religion that the Earth is 6,000 years old and was created in seven days, then there's no help for it: evolution contradicts your religious beliefs. But that doesn't mean evolution was dreamed up by a bunch of nefarious atheists seeking to undermine faith. It just means you foolishly placed your religious beliefs out where science had something to say about them. If it's a vital tenet of your religion that lightning and thunder are the strike of Thor's hammer, then modern meteorology will likewise seem to you to be an atheistic conspiracy invented to turn people away from belief in your god. That's your fault, though, not the meteorologists'.
As I've written before, I despise the idea of hiding my true beliefs for the sake of political expediency. If I really did believe that evolution necessarily and inevitably implied atheism, I would say so. But I don't believe that.
As I said, evolution does disprove clumsy, literalist interpretations of theism, although no more so than any other branch of science. Just as Richard Dawkins said, it also robs the force of the argument from design, providing a powerful means of producing complex life that does not require the intervention of a deity.
It does not, however, prove that no deity exists. No branch of science, evolution included, can ever fully rule out the idea of a god who works behind the scenes to control the course of events in a way indistinguishable from natural law and coincidence. Certainly, it's possible that God has been subtly guiding the evolution of life over the eons, nudging molecules at the right time to create just the right mutations that would shape the future to his will. There's no way to disprove that, although I also don't see any reason to believe it.
That said, I don't think this gets theism completely off the hook. One could postulate a god like the kind I described - but then one is also faced with the unavoidable problem that a god which controlled evolution would be a god who deliberately brought about enormous amounts of suffering, terror, death and extinction to achieve his goals. Evolution proceeds by the death of the losers, and those organisms that are weak or cannot defend themselves are ruthlessly exploited. Infectious disease, parasitism, and predation are the rule at every level and in every niche of life. It's quite amazing that this process of ceaseless struggle and violence has brought forth living things as beautiful, intricate and diverse as the ones as we see. We can appreciate that beauty, but we shouldn't forget what lies behind it. This is just what Darwin meant when he wrote about the parasitic wasps who implant their hungry larvae in a caterpillar's living body, and how he could not persuade himself that a benevolent god would have deliberately designed such a thing.
In this respect, I think evolution does pose a problem to believers in God. Of course, Darwin didn't invent all the pain and ugliness in the world. The ruthless amorality of nature is an observed fact, not subject to debate. Even if evolution turned out to be false or had never been proposed, theists would still face the challenge of explaining it.
With that in mind, my conclusion is this: Evolution does not disprove the existence of God. It does add force to the atheist's argument from evil, but it's just one point in a larger picture, and the problem as a whole would remain even if evolution were to fall. A person who accepts evolution and still believes in God can do so consistently. But evolution does cast into sharper relief the problem of evil, a recurring problem for theism of all varieties. It doesn't create a problem for theism where none existed previously, but it does further drive the point home.