PATTERNS OF GENETIC POPULATION DIFFERENTIATION IN 4 SPECIES OF AMAZONIAN FROGS - A TEST OF THE RIVERINE BARRIER HYPOTHESIS

Patterns and levels of allozyme variation among populations of Amazoni an frogs were used to test the riverine barrier hypothesis of species differentiation. Two frog species were sampled from each of the two ma in forest habitats on both banks of the Jurua River in the southwester n Brazilian Amazon Basin at various points along its course to contras t different barrier strengths. Scarthyla ostinodactyla and Scinax rubr a were sampled from flooded forest (varzea), and Physalaemus petersi a nd Epipedobates femoralis from non-flooded forest (terra firme). All s pecies showed high levels of within-population genetic variation. Aver age Nei's (1978) and Rogers' (1972) genetic distances between sampled sites for all species were high indicating substantial among-populatio n differentiation. The observation of low gene flow between sampled si res within species was further substantiated with Slatkin's (1993) M a nalyses. Randomization tests suggested that there was some population structure at a few assayed polymorphic loci that was consistent with t he riverine barrier hypothesis. However, it was apparent from the raw allozyme frequency data that these results were largely driven by subs tantial differentiation al one or a few collecting localities rather t han by basin-wide patterns of riverine differentiation. Phenograms usi ng genetic distance matrices supported this interpretation. Patterns o f geographic variation are probably more consistent with the idea of t his region being a zone of secondary contact.