What is the term for speciation due to a geographic barrier?

The term for speciation caused by a geographic barrier is allopatric speciation.

In this process, a population is divided by an external barrier such as a river, mountain range, canyon, changing climate, or newly formed ocean channel. Once gene flow is reduced or stopped, the separated populations evolve independently. Natural selection, genetic drift, and new mutations gradually make them genetically and reproductively different.

If the populations later meet again, they may no longer be able to produce viable, fertile offspring. At that point, reproductive isolation has developed and the two populations are treated as separate species. Vicariance, in which a barrier splits an existing range, is a major allopatric model; isolation of a small peripheral population is often called peripatric speciation.

Allopatric means “different fatherland,” from Ancient Greek. It contrasts with sympatric speciation, which occurs without geographic separation, and parapatric speciation, in which neighboring populations diverge despite some limited contact. The bonobo and common chimpanzee are often discussed as an example involving the Congo River.