Stochastic group selection model for the evolution of altruism

We study numerically and analytically a stochastic group selection model in which a population of asexually reproducing individuals, each of which can be either altruist or non-altruist, is subdivided into M reproductively isolated groups (demes) of size N. The cost associated with being altruistic is modelled by assigning the fitness 1−τ, with τ [0,1], to the altruists and the fitness 1 to the non-altruists. In the case that the altruistic disadvantage τ is not too large, we show that the finite M fluctuations are small and practically do not alter the deterministic results obtained for M→∞. However, for large τ these fluctuations greatly increase the instability of the altruistic demes to mutations. These results may be relevant to the dynamics of parasite–host systems and, in particular, to explain the importance of mutation in the evolution of parasite virulence.