MEASURES OF GENE FLOW IN THE COLUMBIAN GROUND-SQUIRREL

From analyses of published data and a review of the literature, I stud ied indirect and direct measures of gene flow among populations of Col umbian ground squirrels, Spermophilus columbianus. New analyses were u sed to examine an allozyme data set (seven polymorphic loci) that had been collected by Zammuto and Millar (1985a) from six populations of g round squirrels that were spread over 183 km. G-tests indicated signif icant variation in allele frequencies among populations, but F-statist ics revealed relatively little population differentiation (average F(S T)=0.026). F(ST) values were used to estimate rates of gene flow indir ectly and indicated fairly high rates of gene flow (average N(e)m=13.5 ). Recorded dispersal distances of individual ground squirrels were fa irly short (most < 4 km, maximum recorded distance was 8.5 km), and th e minimum distance between populations used to create the allozyme dat a set was about 25 km. Thus, direct dispersal among the populations in the allozyme data set was highly unlikely. Small genetically effectiv e populations may have experienced high rates of migration over short distances (about 43% of adults in local populations were immigrants), however, resulting in homogeneous allele frequencies over the geograph ic range. This explanation provides an alternative to invoking gene fl ow in the recent past to explain discrepancies between dispersal dista nces in the field and homogenization of allele frequencies over large ranges. Mammalian species that have virtually complete dispersal of su badult males from the natal area might be expected to exhibit relative ly high rates of gene flow, regardless of actual dispersal distances. Genetically effective populations may be much smaller than more extens ive ecological populations and experience higher rates of gene flow.