SPATIAL AND TEMPORAL VARIATION IN GROUP RELATEDNESS: EVIDENCE FROM THE IMPORTED WILLOW LEAF BEETLE.

Abstract

The distribution of genetic variance within and among socially interacting groups, often quantified as the average relatedness or r̄, is an important determinant of the evolution of social behaviors. Models of social evolution often treat this average as a constant characteristic of a species. In this paper, we present data documenting the degree of temporal and spatial variation in the average relatedness of larval groups of imported willow leaf beetles, a species whose immatures display several primitive social behaviors. Collections of groups were made over three generations at three localities in Virginia and three localities in Illinois. Average relatedness was estimated from the distribution of electrophoretically determined genotypic frequencies within and among groups for each collection. Average relatedness ranged from 0.20 at one locality in Illinois to 0.65 at one locality in Virginia. Individual cases of pairwise differences between samples indicated that there were both temporal and locality effects. Further, statistical analysis showed the set of relatedness values to be heterogeneous with significant locality effects. Geographic genetic variance was also partitioned among trees sampled within localities in Illinois, among localities within each state, and between states. Notably, significant gene-frequency variation among groups of beetles occupying closely spaced trees was detected at several localities.