So it looks like there are a number of people who have published on this, and all the papers that I've so far looked at basically agree that the following are not all true:I don't understand all the arguments that lead up to this conclusion well enough to know whether to agree with it or not, but it's rather a fascinating conclusion if true.
1. Evolution occurred pretty much as science describes it.
2. Naturalism is true.
3. Moral realism is true.
4. We have moral knowledge.
Alexander, being a theist and a moral realist, naturally takes (3) and (4) to be true, and so concludes that either (1) or (2) is false - he then opts for naturalism being false.
I don't know enough about the whole realism/anti-realism debate to have a firm position, but I'm leaning anti-realist at the moment, so the argument doesn't present a real problem for me. But it seems to be a real problem for someone like Sam Harris, who is a naturalist and a moral realist. I don't think he would want to deny (4), either. From what I understand, he thinks that science can discover moral truths. So I wonder how Sam would respond to this argument.
But then, Sam thinks that discussions of metaethics and deontology only add to the amount of boredom in the world, so probably he wouldn't bother.