 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.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.