Population genetic structure of the Northern Idaho ground squirrel (Spermophilus brunneus brunneus)

Spermophilus brunneus is one of the rarest species of North American ground squirrels. it occurs in just five counties in western Idaho. We sampled bl ood from 14-64 individuals in II populations of the northern subspecies S. b. brunneus. Analyses of allozymes at five polymorphic loci revealed signif icant population structure (F-ST = 0.167). Four populations located within a 2-km radius in Bear Meadow (Adams Co.) were indistinguishable with respec t to allelic frequencies (Nei's D = 0.002), but the remaining seven, more d istant populations (also in Adams Co.) were differentiated from this group and from each other. F-ST values ranged from 0.034 to 0.124 when the four p opulations in Bear Meadow were pooled and compared with each of the seven i solated populations. Regression of Slatkin's (M) over cap against geographi c distance among the 55 pairwise comparisons of populations suggested a str ong effect of isolation-by-distance, consistent with a one-dimensional step ping-stone model of gene flow. Significant genetic structure in S, b. brunn eus populations apparently is due to genetic drift in populations with smal l N-e (i.e., 20-50), reinforced by lack of gene flow following recent habit at fragmentation. Fragmentation results from the shrinking of meadows due t o invasion by conifers, itself the consequence of fire suppression in the p ast 100 years.