River barrier hypothesis

The river barrier hypothesis is a hypothesis seeking to partially explain the high species diversity in the Amazon Basin, first presented by Alfred Russel Wallace in his 1852 paper On Monkeys of the Amazon. It argues that the formation and movement of the Amazon and some of its tributaries presented a significant enough barrier to movement for wildlife populations to precipitate allopatric speciation. Facing different selection pressures and genetic drift, the divided populations diverged into separate species.

There are several observable qualities that should be present if speciation has resulted from a river barrier. Divergence of species on either side of the river should increase with the size of the river, expressing weakly or not at all in the headwaters and more strongly in the wider, deeper channels further downriver. 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 random chance.

River barrier speciation occurs when a river is of sufficient size to provide a vicariance for allopatric speciation, or when the river is large enough to prevent or interfere with a genetic exchange between populations. Population division is initiated either when a river shifts into or forms within the range of a species that cannot cross it, effectively splitting the population in half, or when a small founder group is transported across an existing river through random chance. Usually a river's strength as a barrier is viewed as proportional to its width; wider rivers present a longer crossing distance and thus a greater obstacle to movement. Barrier strength varies within a given river; narrow headwaters are easier to cross than wide downstream channels. 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. Large mammals and birds have little trouble crossing most streams, whereas small birds unaccustomed to long distance flight can have particular difficulties and thus may be more subject to population division. 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.

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