 Some other birds from the region, in contrast, have sister species that are "separated" by the major rivers, which thus apparently acted as natural barriers to gene flow.Whether a refugia or a barrier model describes superspecies distribution in the Amazonian basin most appropriately thus seems to be a direct consequence of the animals' ability to cross major waterways.But even in the "Selenidera" toucanets which, though largely sedentary, are technically able to disperse widely, the Amazon River forms a barrier that was simply too wide to cross in significant numbers as to inhibit speciation.