Allopatric speciation

Allopatric speciation is the formation of new species after populations become geographically isolated and evolve separately. Reduced gene flow allows genetic differences to accumulate until reproductive isolation develops.

Geographic isolation: The separation of populations by physical distance or barriers that restrict movement and interbreeding. It interrupts gene flow and begins the divergence that can produce separate species.
Charles Darwin: The English naturalist who developed the theory of evolution by natural selection. Darwin connected geographic separation and divergent conditions with the origin of species.
Kaibab squirrel: A squirrel population on the North Rim of the Grand Canyon, often compared with populations south of the canyon. The canyon separates it from closely related squirrel populations and illustrates geographic divergence.
Sympatric speciation: The formation of new species without geographic separation between diverging populations. It contrasts with allopatric speciation because divergence begins within the same geographic area.
Speciation continuum: The range of stages from initial population divergence to complete reproductive isolation. It captures why geographic separation does not always produce fully distinct species.
Gene flow: The transfer of genetic variants between populations through migration and reproduction. Isolation reduces gene flow, allowing populations to diverge independently.
Alfred Russel Wallace: The British naturalist who independently developed the theory of evolution by natural selection. Wallace’s biogeographic work emphasized how barriers divide related populations.
Grand Canyon: A deep canyon system in Arizona that separates habitats and populations across the Colorado Plateau. Its barrier has limited movement between some populations on opposite rims.
Peripatric speciation: Speciation in a small population founded beyond the main range of a larger population. It is a geographically isolated form in which a small peripheral population often drives divergence.
Secondary contact: The renewed geographic overlap of populations that diverged while separated. It tests whether accumulated differences persist when isolated populations meet again.

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