SPATIAL-ANALYSIS OF MICROGEOGRAPHIC GENETIC-STRUCTURE IN RICHARDSONS GROUND-SQUIRRELS

Local genetic structure has a sound theoretical basis, yet empirical d emonstration in animal species has proved elusive, even in apparently ideal candidate species. Techniques based on the distribution of indiv idual genotypes may offer a more complete picture of population struct ure than traditional measures focusing on isolation by distance and di spersal behavior. We used spatial autocorrelation and contiguous clust ering to identify structure in a population of Richardson's ground squ irrel (Spermophilus richardsonii) for which deviation from Hardy-Weinb erg expectations indicated population subdivision. Nonrandom aggregate s of genotypes were detected at five of six enzyme loci examined and s election at one locus. Genetic structuring was highly sex-dependent, b eing prominent only among females. Isolation by distance cannot accoun t for the patterns of gene diversity observed, but Mantel matrix proce dures of inter-individual distance based on demographic-behavioral cha racteristics and geographic distance were significantly associated. So cial and breeding systems of S. richardsonii lead to significant local substructure. While philopatry alone may not account for fine-scale g enetic structure in small mammals with sex-biased dispersal, nonrandom aggregates will be detected if appropriate social models and rigorous sampling criteria are adopted. Identification of such localized struc ture remains fundamental to understanding evolutionary models of popul ation genetic structure and sociality.