Darwin on Group Selection
Another data point:
When two tribes of primeval man, living in the same country, came into competition, if (other things being equal) the one tribe included a great number of courageous, sympathetic and faithful members, who were always ready to warn each other of danger, to aid and defend each other, this tribe would succeed better and conquer the other (Darwin, Descent of Man, 1871, p. 113).
Darwin - no surprise - foresaw the objection:
It is extremely doubtful whether the offspring of the more sympathetic and benevolent parents, or of those who were the most faithful to their comrades, would be reared in greater numbers than the children of selfish and treacherous parents belonging to the same tribe. He who was ready to sacrifice his life, as many a savage has been, rather than betray his comrades, would often leave no offspring to inherit his noble nature (Darwin, p. 114).
It must not be forgotten that although a high standard of morality gives but a slight or no advantage to each individual man and his children over the other men of the same tribe, yet that an increase in the number of well-endowed men and an advancement in the standard of morality will certainly give an immense advantage to one tribe over another. A tribe including many members, who from possessing in a high degree the spirit of patriotism, fidelity, obedience, courage and sympathy, were always ready to aid one another, and to sacrifice themselves for the common good, would be victorious over most other tribes; and this would be natural selection (Darwin, 115-116).
For a very good discussion and commentary, follow the link. The author's basic point, if I understand it, is that it's possible to build models in which group selection, suitably defined, occurs and possible to build models with a slightly different definition in which it doesn't. This point is evaded by Pinker and like minded critics, but then the question becomes how useful are the competing definitions.