River barrier hypothesis

Organisms endemic to terra firme forest should be more affected than those that live in alluvial forests alongside the river, as they have a longer distance to cross before reaching appropriate habitat and lowland populations can rejoin relatively frequently when a river shifts or narrows in the early stages of oxbow lake formation.

Finally, if a river barrier is the cause of speciation, sister species should exist on opposing shores more frequently than expected by chance.

Rivers that present a barrier for some species in a region may not necessarily do so for all, leading to species-by-species and clade, or genetically distinct group, differences in degree of isolation and differentiation on opposing shores.

Additionally, rivers more effectively divide species that prefer terra firme forest as meanders and the process of oxbow formation in alluvial regions can narrow otherwise impassable streams.

The antbirds' diversification and distribution were examined throughout the Amazon, three monophyletic, genetically distinct, populations of the bird were found; two of them are currently valid subspecies.

Another study found that saddle-back tamarins follow the premise that variant gene flow occurs at different parts of a river.

For example, there is evidence that genetic variation in the Blue-crowned Manakin may have been influenced by river barriers, Andean uplift, and range expansions.

The river seemed to be a barrier for only a few taxa, with the majority either homogeneous throughout the research area or divided into monophyletic upriver and downriver clades.

Patton argues that the geographic location of these clades suggest that landform evolution is an under-appreciated factor in diversification in Amazonia.