Population structure and interdemic selection in the cooperative spider Anelosimus eximius

The degree of relatedness among interacting individuals helps determine the fitness consequences of particular behaviors, whereas the partitioning (and amount) of genetic variation among and within groups controls the level at which selection will act most effectively. Three criteria are considered necessary for selection to act at the group or interdemic level: high rate of group initiation/extinction; differential survival and reproduction among groups; and highly subdivided population structure. The first two criteria have been demonstrated by earlier studies of Anelosimus eximius colonies. This study employs hierarchical analysis of allozyme polymorphisms to demonstrate the third criterion, subdivided population structure. Anelosimus eximius were collected from Suriname, Panama, Ecuador, Peru and Trinidad. Seven of 40 scorable enzyme loci revealed variation; 4 of these were polymorphic within colonies or regions. Expected heterozygosities were low, ranging from 0 (Ecuador, Peru) to ∼0.03 (Suriname). For each polymorphic locus, hierarchical F ‐statistics were used to partition overall genetic variation into among‐region (or among‐population; F rt ), among‐colony ( F sr ), and within‐colony ( F is ) components. Samples from Suriname (43 colonies, 4 local populations) were the most informative; lack of scorable variation limited the inferences that could be drawn from other regions. A. eximius colonies are highly inbred: negative estimates of F is imply very small effective colony sizes (∼6.5 for Suriname samples). By contrast, estimates of F sr were very high: the mean for Suriname samples was 0.890, indicating neglibible gene flow among established colonies. Inbreeding within colonies, and genetic differentiation among colonies are consistent with demographic and behavioral observations of A. eximius . We suggest that interdemic selection is probable in this species and other cooperative spiders with this type of social system, and that mutual tolerance and absence of nest‐mate recognition, as well as female‐biased sex ratios, may have arisen by interdemic selection.