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How do rivers create separate populations of related animal species?

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World map using color coding to show the distribution of major floral kingdoms, subkingdoms, and regions. Key floral kingdoms include Boreal (green), Neotropical (pink), Paleotropical (yellow), South African or Capensic (purple), Australian (orange), and Antarctic (blue). Boundaries for subkingdoms, regions, and divisions are marked with different colored lines, and major regions such as Indoafrican, North American, and Malesian are labeled across continents. — biogeographic region Floral kingdoms, subkingdoms, and major regions of the world.

Rivers act as physical barriers that contribute to the formation of separate populations through a process known as allopatric speciation. When a river divides a population, it creates geographic isolation, preventing members of the two groups from interbreeding.1

This separation leads to the following evolutionary outcomes:

Genetic Divergence: Once isolated, the two populations adapt to their own unique environments. Over time, they accumulate distinct genetic differences.1
Reproductive Isolation: As these genetic differences increase, the populations may become so distinct that they can no longer interbreed, even if the physical barrier were removed.1
Phenotypic Differentiation: The populations may develop different observable physical characteristics as they adapt to their specific surroundings.1

This process is a primary driver of speciation, the formation of new and distinct species. While rivers serve as barriers for many animals, the effectiveness of such a barrier depends on the mobility of the species involved; for instance, while birds might easily cross a river, it may remain an impassable boundary for terrestrial animals. This fragmentation of the environment is a key concept in vicariance biogeography.123