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 - tau, with tau is an element of [0, 1], to the altruists and the fitness 1 to the non-altruists. In the case that the altruistic disadvantage tau is not too large, we show that the finite M fluctuations are small and practically do not alter the deterministic results obtained for M --> infinity. However, for large tau 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. (C) 1999 Elsevier Science B.V. All rights reserved.