Leptodactylus fuscus (Schneider 1799) as currently understood has a broad geographic range, extending from Panama to Argentina east of the Andes and on the islands of Trinidad and Tobago. We obtained 16 samples throughout its distributional range for electrophoretic analysis to obtain estimates of genetic differentiation within the taxon. Twenty-four loci were scored for analysis. Analytical techniques were used that were appropriate for analyzing inter-population variation of open genetic systems and genetic systems with reduced or no gene flow among populations. The techniques used are: multidimensional scaling; correlation of geographic and electrophoretic distances; gene flow estimates; phylogenetic techniques. The results indicate that the series of samples from Trinidad, Tobago, French Guiana, and Roraima, Brazil have low genetic distances that correspond to an isolation-by-distance model of differentiation, thus comprising a system of populations linked by gene flow within a single species. However, the samples from Panama and those south of the Amazon River demonstrate genetic partitioning, such that there is insignificant gene flow among some sets of these samples as well as with samples north of the Amazon River. Leptodactylus fuscus is a “weedy” species, characteristic of open habitats and able to colonize and survive in human altered habitats. Such a “weedy” species would be expected to have relatively low levels of genetic diversity, contrasting strikingly with the levels of genetic differentiation discovered in our study. If these results are typical for other neotropical frogs, we are currently grossly underestimating the amount of diversity in tropical frogs, which has obvious conservation consequences. KEY WORDS: Leptodactylus fuscus, genetic differentiation, zoogeography, neotropics, conservation.
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