How meander loop cutoff can facilitate gene flow across a riverine barrier.

<p><b>A</b>, Isolated populations on opposite sides of a river may be genetically distinct (represented by red and brown lizards). <b>B</b>, Increasingly large meander loops may form due to erosion and re-deposition of channel sediments. <b>C</b>, Meander loops may expand to the point of being cut off at the neck by the forging of a more direct channel. Individuals living within the meander loop are passively transferred across the river. <b>D</b>, Ancient meander loops recede from the main river channel, forming oxbow lakes. Alleles transferred across the river are released into a novel population, resulting in unidirectional gene flow.