POPULATION-STRUCTURE IN A SOUTH-EASTERN US FRESH-WATER FISH, HETERANDRIA-FORMOSA - II - GENE FLOW AND BIOGEOGRAPHY WITHIN THE ST.-JOHNS RIVER DRAINAGE

A previous study of gene flow in Heterandria formosa, the least killif ish, suggested that populations on the Florida peninsula may be close to equilibrium for migration and genetic drift, but the high average v alue of N(e)m calls that conclusion into question. In this study I exa mine patterns of gene flow at a fine scale within the St. Johns River drainage to test the predictions that (i) N(e)m within a drainage shou ld be equal or greater than that among drainages, and (ii) gene flow w ithin a drainage should approximate a one-dimensional stepping-stone s ystem at equilibrium. I used four isozyme systems (six loci) that were polymorphic in the previous study to characterize population structur e and gene flow. N(e)m within each river was equal to or greater than the average value reported in the previous study, which strengthens th e equilibrium interpretation of the species-wide results. However, the results showed no isolation by distance within two forks of the St. J ohns drainage (the upper St. Johns and the Ocklawaha rivers), but sign ificant differentiation between the two forks. This result has two imp lications: (i) that populations within each river are not at equilibri um; and (ii) that there was a historical barrier to migration between the two forks. There were substantial differences among loci, but an ( appropriate) application of Lewontin and Krakauer's test revealed no e vidence that locus-specific forces led to a violation of the assumptio n of neutrality.