page_title: River barrier hypothesis
section_title: Critique
hierarchical_section_title: River barrier hypothesis / Critique
caption_attribution_description: Wild specimen of the arrow-poison frog, Dendrobates reticulatus; easternmost Peru, near Amazon River.
context_page_description: 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.
context_section_description: Not all studies have found support for the hypothesis. One study tested the riverine hypothesis by observing populations of four species of Amazonian frogs along the Juará River. The team expected to see gene flows of different volumes when comparing sites that were on the same bank to sites that were across the river. They found that this was not the case. Gene flow seemed to be in almost equal quantities between either set of sites.
  Another study took the hypothesis a step further. They postulated that since rivers are supposed to be barriers for gene flow for certain taxa, then it should be a barrier at a community level. Variation in species of frogs and small mammals along and across the riverbanks of the Juará River were evaluated. No obvious gradient of decreasing similarity in species of frogs and mammals from the headwaters to the mouth of the river was found. Another discovery was the fact that there was no greater similarity between species that lived on the same bank than those that were on opposite banks of the river.
  These results indirectly dispute the aspects of the hypothesis that insist on speciation caused by riverine barriers. The validity of the hypothesis was tested further by examining poison dart frogs. This study (Lougheed et al.)'s results were incongruent with the hypothesis that species on either side of a river would be monophyletic relatives. The Lougheed study aimed to show that the ridge hypothesis has more credibility than the river hypothesis. Eighty-one species of non-flying mammals were trapped at cross-river sites along the Juará River in another experiment. 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 underappreciated factor in diversification in Amazonia. This project further suggests that riverine barriers are not the only mechanism for speciation.
  All these critics argue that other factors influence speciation in Amazonia. Another shortcoming of the hypothesis is that it has been researched mostly in Amazonia, rather than in other river basins. Also, shifts of rivers may prevent the establishment of any patterns across rivers, further complicating means to test for the strength of the hypothesis.
caption: Red-backed poison frog (Dendrobates reticulatus)
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