Richard Polt, a professor of philosophy at Xavier University, explains why theories of evolution cannot and do not provide a foundation or reasoning for ethical and moral truths:
I have no beef with entomology or evolution, but I refuse to admit that they teach me much about ethics. Consider the fact that human action ranges to the extremes. People can perform extraordinary acts of altruism, including kindness toward other species — or they can utterly fail to be altruistic, even toward their own children. So whatever tendencies we may have inherited leave ample room for variation; our choices will determine which end of the spectrum we approach. This is where ethical discourse comes in — not in explaining how we’re “built,” but in deliberating on our own future acts. Should I cheat on this test? Should I give this stranger a ride? Knowing how my selfish and altruistic feelings evolved doesn’t help me decide at all. Most, though not all, moral codes advise me to cultivate altruism. But since the human race has evolved to be capable of a wide range of both selfish and altruistic behavior, there is no reason to say that altruism is superior to selfishness in any biological sense.
In fact, the very idea of an “ought” is foreign to evolutionary theory. It makes no sense for a biologist to say that some particular animal should be more cooperative, much less to claim that an entire species ought to aim for some degree of altruism. If we decide that we should neither “dissolve society” through extreme selfishness, as Wilson puts it, nor become “angelic robots” like ants, we are making an ethical judgment, not a biological one. Likewise, from a biological perspective it has no significance to claim that Ishould be more generous than I usually am, or that a tyrant ought to be deposed and tried. In short, a purely evolutionary ethics makes ethical discourse meaningless.
Read the entire essay, "Anything But Human", on the New York Times' Opinionator blog.