Frequency-dependent selection in a periodic environment

We examine the action of natural selection in a periodically changing environment where two competing strains are specialists, respectively, for each environmental state. When the relative fitness of the strains is subject to a very general class of frequency-dependent selection, we show that coexistence rather than extinction is the likely outcome. This coexistence may be a stable periodic equilibrium, stable limit cycles of varying lengths, or be deterministically chaotic. Our model is applicable to the population dynamics commonly found in many types of viruses. (c) 2007 Elsevier B.V. All rights reserved.