Effect of phenotypic variation on kin selection.

An expression for the equilibrium of the mean phenotypic value of a quantitative character is derived for a model in which the fitness of an individual depends on its own phenotype and the mean phenotypic value of a group of related individuals. When selection is weak the equilibrium mean is well predicted by Hamilton's k > 1/r rule (k is the ratio of mean fitness gained by recipient of altruistic behavior to mean fitness lost by donor; r is mean coefficient of relationship between donor and recipient). When selection is strong, however, the equilibrium mean depends on the heritability of the character. Low heritability can lead to substantially more "altruism" than predicted by the k > 1/r rule.