Setting aside the possibility that inclusive fitness theory has been more fruitful simply because group selection was ruled out as a matter of orthodoxy for many years, Sober and Wilson explain in their book that these aren’t really rival theories:
“Social interactions among genetic relatives correspond to the nonrandom formation of groups. The significance of relatedness for the evolution of altruism is that it increases genetic variation among groups, thereby increasing the importance of group selection” (77).
They go on to quote W.D. Hamilton: “it obviously makes no difference if altruists settle with altruists because they are related… or because they recognize fellow altruists as such, or settle together because of some pleiotropic effect of the gene on habitat preference” (77). (For the uninitiated, pleiotropy is when a trait that isn't directly selected for comes as a package deal with a trait that is, usually because both are products of the same genes.)
And they conclude that Hamilton “left no doubt that the difference between inclusive fitness theory and group selection theory is a matter of perspective, not process” (77).
The idea is that groups must vary genetically if they are to serve as a unit of selection. The corollary is that there is some genetic similarity within each of the groups that contrasts with the differences between groups. What accounts for the similarity within groups isn’t important to the process of differential reproduction between groups. The group to which I belong can be made up of family members or people who share a set of important traits with me but who aren’t related. Either way the stage is set for group selection.
Still, how powerful and how important selection at the group level is has yet to be determined. And I’m just dipping into the issue here. (Sober and Wilson’s book is 12 years old, and I spent more time on Saul Bellow’s Collected Stories this summer than on evolutionary biology.) I will argue though that there are few reliable markers of kinship that even a species as cognitively sophisticated as humans can use to apportion altruistic favors. True, we have language to describe and label all sorts of relationships, but those labels are frequently applied to nonkin. At least in humans, group (tribal) membership seems a much more salient signal for deciding who deserves unselfish treatment.
But here is what Dr. Larry Kuznar, an anthropologist at my alma mater specializing in evolutionary models, said in response to my inquiry about how Sober and Wilson's ideas are faring:
"Wilson and Sober are at the vanguard of reintroducing group selection to biology. In anthropology, Richard Boyd and Peter Richerson are advancing group selection under the guise of "cultural group selection" (see their book, Not be Genes Alone). How is this holding up in the field? Good question. Wilson and Sober are still a minority view in biology, and I think for good reason.
Boyd and Richerson have had more success in anthropology, but mostly through clever argumentation and by attacking individual selectionist work. I have not seen good robust tests of their ideas. There is a lot of experimental work, even done in different cultures (see Joseph Henrich), but I think that the experiments are rather contrived. Nonetheless, cultural group selection theories are very fashionable among some anthropologists (certainly not all, probably not the majority), behavioral economists, some political scientists, and some evolutionary psychologists. One hears a lot about altruism these days. It has even emerged as one of the competing explanations for suicide bombing."