Riverine barriers and the geographic distribution of Amazonian species

Rivers have been suggested to have played an important role in shaping pres ent-day patterns of ecological and genetic variation among Amazonian specie s and communities. Recent molecular studies have provided mixed support for the hypothesis that large lowland Amazonian rivers have functioned as sign ificant impediments to gene flow among populations of neotropical species. To date, no study has systematically evaluated the impact that riverine bar riers might have on structuring whole Amazonian communities. Our analyses o f the phylogeography of frogs and small mammals indicate that a putative ri verine barrier (the Jurua River) does not relate to present-day patterns of community similarity and species richness. Rather. our results imply a sig nificant impact of the Andean orogenic axis and associated thrust-and-fold lowland dynamics in shaping patterns of biotic diversity along the Jurua. C ombined results of this and other studies significantly weaken the postulat ed role of rivers as major drivers of Amazonian diversification.