In studying evolutionary biology, "group-selection" has a specific meaning, an individual sacrifices its own fitness in order to improve the group fitness.
I think it's quite limiting to think strictly in terms of genetics, because there is more than one level of description going on when it comes to selection pressure.
It is interesting to take that step back and view the culture as an individual. The human super-organism (e.g., a tribe, or more generally, a culture) competes with others for resources. It consumes, metabolizes, and excretes, which is to say that it lowers entropy locally and raises it globally. With others, it fights, defends, cooperates, and merges/assimilates. It gets sick, fights the antigens, heals itself, or dies. New super-organisms are spun off of or otherwise born from parents. We may look at the "evolution of language" through the lens of the human super-organism. Language is the DNA of culture, to make a rough analogy.
The dynamics that propagate the super-organism are not reducible to genetics. It's a different level of description, because culture emerges from the interaction of large numbers of individuals. And you can't deny that if one culture has guns and confronts another that doesn't, that dynamic is going to place a harsh selective pressure indeed on the culture without the firepower. So genetics is not the whole story, and that's what I mean by group selection.
Quick comment: Terren Suydam's version of "evolutionary psychology" is not the academically accepted one. Conventional academic evolutionary explanations of morality rely on neither group selection nor selection on cultures.
Be that as it may, I would have to say that an explanation of morality made strictly in terms of the academically accepted version of evolutionary psychology is not possible. I'm not trying to redefine the term - just saying what else would be necessary to make an explanation of morality on that basis possible.
Great dialog, which I think can be summarized in Nietzsche's aphorism: "Morality is the herd-instinct in the individual."
Actually, I think the dialog could have been a lot shorter if it became clear earlier on that preference (as in morality-as-preference) referred not to individual preference but the "preference of the collective". Which is to say, morality is determined by evolutionary psychology. There are however two assumptions built into the evolutionary psychological explanation of morality which ought to be made explicit.
The first is that one has to adopt the group-selection stance. As in, groups in which morals evolved had higher stability (its members were more likely to survive and procreate). If we focus only on the selfish individual, then it's obvious that morals make no sense.
The second assumption is that evolution, in humans, bifurcated into both physical and cultural domains. This is because morality is almost certainly not determined by genetics. At best, genetics predisposes us psychologically to accept morality (whatever that means), but the content of morality is too well specified to be so determined. Thus we have to assume a selection process that selects groups on the basis of culture as well as genetics. This is actually a common sense notion, that for example groups that develop better weapons will dominate or eliminate other groups.
There may be a selection effect in that cultures with guns are more likely to persist, but that's different from saying that selection pressures play a really important role in designing the particular features of a culture.
That's what I'm saying - selection pressures are important in determining cultural features, because those features in turn determine a culture's viability. The global-level organization of a culture - including its moral code, political organization, and other important social structures - are key considerations in what makes a culture healthy or stable, and thus competitive in an arena of limited resources. Keep in mind, I think these ideas lend themselves more easily to ancient history, in which the boundaries between cultures were so much clearer, rather than the globalized cultures of our modern world.
A lot of these ideas come from Jared Diamond's fascinating Guns, Germs, and Steel, which talks in depth about cultural evolution throughout human history.