Frank, S. A. 1994. Kin selection and virulence in the evolution of protocells and parasites. Proceedings of the Royal Society of London B 258:153-161.

The evolution of parasite virulence and the origin of cooperative genomes in primitive cells are both problems that balance cooperative and competitive interactions among symbionts. I analyze the tradeoff among three correlated traits: competitiveness against other genotypes for resources within hosts (protocells), damage to the host (virulence), and rate of horizontal transmission from one host to another. All three life history components are strongly influenced by kin selection. For example, when genetic relatedness within hosts is high, then each genotype is competing for resources with closely related genotypes. This competition among relatives favors increased horizontal transmission in order to colonize new hosts and compete against nonrelatives. My analysis shows that many aspects of parasite and protocell evolution must be studied with the theoretical tools of social evolution. I discuss extensions that are required for a general theory of symbiosis.