Nearly 150 years ago, the famed naturalist Alfred Russell Wallace noted that range boundaries for a number of animal species in the Amazonian rain forest seemed to coincide with the region's many rivers. That observation marked the origin of one of the leading hypotheses for why the Amazon harbors such extraordinary biodiversity for its size. In its modern form, this "riverine barrier hypothesis" posits that the Amazon's major rivers functioned as natural barriers to gene flow between populations. As a result, the populations ultimately diverged. This model has received a certain amount of support from molecular studies in recent years. But findings published today in the Proceedings of the National Academy of Sciences point to a different explanation.

When Claude Gascon of Conservation International in Washington, D.C., and his colleagues examined data from frog and small mammal communities along the Jurua River, a major tributary of the Amazon River, they found that it did not fit with predictions based on riverbank affiliation. Rather the composition of these communities was best predicted by geographic distance and habitat type. What is more, the team reports that distributions of small mammals terminated perpendicular to the river and parallel to the Andes Mountains, which suggests that the topography of the Amazonian lowlands may generate the biodiversity. Thus far the team has studied only a single river. They believe, however, that they can extend their results to all large meandering rivers in the region as a working hypothesis, concluding that the "combined results of this and other studies significantly weaken the postulated role of rivers as major drivers of Amazonian diversification."