Figure 2.

Average relative fitness of cooperators with respect to defectors, measured by the quantity log₁₀ v (see §3a and the electronic supplementary material, S2), as a function of the initial fraction of cooperators and of the host population size. The frequency-dependent effect depicted in figure 1 breaks down at low densities. In this scenario, the virus does not evolve—but hosts may mutate to a resistant form. Parameter values are b₁ = 0, w = 0, μ_cd = μ_H = 10⁻⁴ h⁻¹, 1000 runs per parameter set; other parameters and initial conditions as in figure 1.