TRANSITIONAL OBSERVATIONS. “The costs and benefits of cooperation, plus evolutionary susceptibility to defecting genotypes, were analysed to determine conformation to theory. Cooperation was costly to individuals, but beneficial to the group. Defecting genotypes evolved in populations founded by the cooperating type and were fitter in the presence of this type than in its absence.”
http://www.nature.com/nature/journal/v425/n6953/abs/nature01906.html