Selection Within and between Kin Groups of the Imported Willow Leaf Beetle

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Abstract
Two opposing levels of selection determine the evolution of cannibalism in the imported willow leaf beetle, Plagiodera versicolora. Within kin groups, cannibals exhibit a developmental and survival advantage over non-cannibals. Between groups, individuals in groups with low rates of cannibalism enjoy a survival advantage owing to an increase in survival with larval group size. This effect of the cannibal on group survivorship is not a simple function of group size; rather, the effect of an individual''s phenotype on the fitness of the other members of its kin group depends on the array of phenotypes in the group. Two factors determining the evolution of cannibalism vary in space and time: the relative strength of selection within and between groups; and the average degree of relatedness within groups, which determines the evolutionary outcome of selection. This sets up the possibility that the balance between selection for cannibalism within and between groups may vary in a complicated manner across the range of this species.