David Sloane Wilson in This View of Life:
Major controversies in science have a way of appearing obvious in retrospect. We find it hard to understand why smart people took so long to agree that the earth revolves around the sun, that glaciers once covered the northern latitudes, that the continents drift, and that species are derived from other species.
So it is with group selection, a theory that was declared dead in the 1960’s, only to come to life as an essential tool for understanding animal and human societies. Group selection theory employs the following assumptions.
1) Natural selection is based on relative fitness.
2) Selection among individuals within groups tends to favor traits that are called selfish in human terms; that is, traits that benefit individuals at the expense of other members of the group and the group as a whole.
3) The evolution of group-advantageous traits typically requires a process of selection among groups in a multi-group population.
4) Groups are defined as the individuals who influence each other’s fitness with respect to the evolving trait.
Or, as Edward O. Wilson and I put it in a 2007 article[i], “Selfishness beats altruism within groups. Altruistic groups beat selfish groups. Everything else is commentary.”