I agree with everything Matthew Silverstein says about the crucial difference between descriptive and prescriptive theories. I'd add the following. There is a large body of work in evolutionary theory which explains how altruistic behavior, both in ourselves and other animals, might have been selected for. For an overview, you might see Elliott Sober and David Sloan Wilson's Unto Others: The Evolution and Psychology of Unselfish Behavior. If evolutionary theorizing along such lines is correct, then "moral traits" such as our capacity to care about others, our outrage at "cheaters," our tendency to want to help those who have helped us, and so on, might have just as deep an evolutionary basis as our more selfish traits. In that sense, there is not the total opposition that you might think between the kind of creature evolutionary theory suggests we are (a descriptive claim) and the kind of creature morality says we should be (a prescriptive claim).
Why do the laws of morality and the laws of nature seem to be completely opposite one another? For example, most moral codes encourage monogamy while the theory of evolution states the strongest seed should be spread around.