We examine seven geographically separate subpopulations of Heliconius charithonia, a butterfly with well-documented home range behaviour, in Miami-Dade County, Florida, for genetic differentiation using cellulose acetate electrophoresis. These subpopulations exhibit little genetic variation (percent polymorphic loci=27, average heterozygosity=0.103) especially in comparison to populations of the same and related species from mainland South America. Allele frequencies do not differ among the subpopulations in south Florida and estimates of Wright's fixation index (FST) support that there is no detectable genetic differentiation among them. This result supports an earlier finding that the dispersal ability of Heliconius butterflies may be underestimated. However, it is unlikely that increased dispersal ability alone could account for the lack of genetic differentiation observed among subpopulations separated by almost 80 km. Given the likely effective population size of these subpopulations (Ne=205) and the average generation time of this species in the subtropics (in the range of 30-90 days), this lack of genetic differentiation is best explained by current or very recent gene flow following a stepping-stone model. Furthermore, this result provides evidence that the current extensive degree of habitat fragmentation surrounding the city of Miami does not limit gene flow among urban subpopulations of Heliconius charithonia.
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