Frank, S. A. 1995. The origin of synergistic symbiosis. Journal of Theoretical Biology 176:403-410.

A dominant theme in the history of life has been the evolutionary innovations of cooperative symbioses: the first genomes near the origin of life, integrated prokaryotic cells, the complex symbiotic communities that evolved into modern eukaryotic cells, lichens, mycorrhizae, and so on. I analyze a model of cooperative symbiosis that shows a threshold condition for the evolution of cooperation. The threshold is not easily passed, but cooperative evolution proceeds rapidly once a symbiosis overcomes the threshold. I assume in my model that each species has genetic variability for a symbiotic trait. The trait imposes a reproductive cost on its bearer but enhances the reproduction of its partner species. For example, in the origin of genetic systems, the trait may cause biochemical synergism for the rate of replication of primitive RNA strands as in Eigen and Schuster's hypercycle model. I contrast models of growth synergism, which are most appropriate for the evolution of genetic systems and for mutualisms such as lichens, with the strategic and psychological applications of the Prisoner's Dilemma model.