The evolutionary determinist charge
There may be several ways of dealing with this objection, but the clearest, it seems to me, is a version of the genetic fallacy argument. This reply grants that creatures with a moral faculty would be selected for in worlds like ours, but denies that the content of morality--how we ought to act--is thereby determined. Evolution, one might think, is a condition for moral experience, but one need not also think that we are therefore constrained to think moral what some or all of our ancestors thought moral.1 So it's wrong to think that the origin of moral experience tells us anything about its content, thus the similarity to genetic fallacy arguments.
That strategy does a good job, I think, of taking seriously the idea that morality must have some relation to our best scientific theory about our development, while making vivid the need for moral theory.
How convincing is that argument, from an interlocutory point of view? The evolutionary determinist's underlying intuition is, I think, that there's something suspect about the whole idea of morality. Moral experience feels like it compels us or strongly urges to do or think certain things. But it only feels like that because it's a powerful, biologically mandated, adaptive feature of beings like us. In investigating moral experience, we therefore ought not to take moral experience as data. We ought to be suspicious of our experience and instead start from our best scientific theories to explain why we have the moral experience we have.
My response has been to deny that evolution need be thought to determine the content of moral experience. It's at least as plausible to think that evolution selects for the conditions of moral experience--reasoning or empathy or whatever--so that moral philosophy still has an important job to do. If in response the evolutionary determinist insists that the normative force--the "ought"--of moral experience is illusory, so that moral philosophy is on a hiding to nothing, then he, like the relativist, shades into moral nihilism, not taking seriously the phenomenon he set out to explain.
A final point that I should have noted in the original conversation is that relativism, at least in its individual-level form,2 is in some tension with evolutionary determinism. If what's right is right at my whim, as individual-level relativism claims, then what's right is not determined by my evolutionary history. (The reply that what I can choose to think of as right is evolutionarily determined reduces individual-level relativism to evolutionary determinism.)
1. De Marco 77 suggests something similar, that different moral standards are consistent with the same value for survival that evolution selects for. I'm drawing the distinction in a different way, though, as a sort of form/content distinction. And my argument doesn't, as de Marco's does, leave me vulnerable to the relativist reply that any of the extant systems of morality are equally acceptable.↩
2. Probably also in its cultural form, if less clearly.↩