Random amplified of polymorphic DNA (RAPD) analysis was used to assess genetic distance and the genetic structure of populations of Nectomys squamipes, a semiaquatic rodent species distributed along watercourses. DNA samples of five populations were analyzed using three primers, producing 45 scorable bands, 31 of which were polymorphic. There was a significant differentiation among populations [F ST = 0.17; phiST = 0.14 (P < 0.004)] but gene flow (Nm = 1.25) was sufficient to overcome genetic drift effects. No fixed specific markers were found for any population. The Mantel's test and UPGMA cluster analysis showed a lack of relationship between genetic and geographic distances. The apparent homogeneity indicated by RAPD markers coincided with morphometric data, despite the wide geographic range of N. squamipes. Alternative hypotheses for explaining our results include recurrent processes of local extinction and recolonization or a recent and sudden increase in the geographic distribution of this species.
