1. Department of Biology, University of Maryland, College Park, Maryland 20742, USA
2. Present addresses: Instituto Cavanilles de Biodiversidad y Biologìa Evolutiva and Departamento de Genètica, Universidad de València, 46100 València, Spain (P.E.T.) and Department of Biology, University of California San Diego, La Jolla, California 92093–0116, USA (L.C.).

Correspondence to: Paul E. Turner^1,2 Correspondence and requests for materials should be addressed to P.E.T. (e-mail: Email: paul.e.turner@uv.es).

The evolution of competitive interactions among viruses¹ was studied in the RNA phage 6 at high and low multiplicities of infection (that is, at high and low ratios of infecting phage to host cells). At high multiplicities, many phage infect and reproduce in the same host cell, whereas at low multiplicities the viruses reproduce mainly as clones. An unexpected result of this study¹ was that phage grown at high rates of co-infection increased in fitness initially, but then evolved lowered fitness. Here we show that the fitness of the high-multiplicity phage relative to their ancestors generates a pay-off matrix conforming to the prisoner's dilemma strategy of game theory^{2, 3}. In this strategy, defection (selfishness) evolves, despite the greater fitness pay-off that would result if all players were to cooperate. Viral cooperation and defection can be defined as, respectively, the manufacturing and sequestering of diffusible (shared) intracellular products. Because the low-multiplicity phage did not evolve lowered fitness, we attribute the evolution of selfishness to the lack of clonal structure and the mixing of unrelated genotypes at high multiplicity^{4, 5, 6}.