The effect of stochastic variation on kin selection in a budding-viscous population.

Recent theoretical work suggests that kin selection cannot operate in a viscous, group-structured population. This view is based on the separation of kin selection dynamics into two components: a within-group component that must always select against an altruist allele and an among-groups component that may favor the allele if the benefit/cost ratio is sufficiently high. The argument is that in a viscous population, the within-group component will dominate, and an altruist allele cannot succeed by kin selection. However, stochastic variation in group composition maintains the among-groups component of kin selection and allows an altruist allele to succeed if it meets the requirements of Hamilton's rule. Computer simulations confirm that in populations with a viscous, permanent group structure, an altruist allele always fails in the absence of stochastic variation but follows Hamilton's rule if realistic levels of random variation are maintained.