Here's why:

* Allopatric speciation specifically describes the evolution of new species due to *geographic separation*. This separation prevents individuals from different populations from interbreeding, leading to independent evolutionary paths.

* Gene flow refers to the exchange of genetic material between populations. Geographic isolation acts as a physical barrier, blocking this exchange and preventing interbreeding.

Examples of geographic isolation:

* Mountain ranges: A mountain range can separate populations of a species, preventing them from mixing.

* Oceans: A body of water can separate populations on different continents or islands.

* Glaciers: Glacial movements can split populations, isolating them in different areas.

* Rivers: A large river can act as a barrier to gene flow, especially for species that cannot cross it.

Key takeaway: Geographic isolation is the *fundamental* initial barrier that sets the stage for allopatric speciation. It prevents gene flow and allows populations to diverge independently, ultimately leading to the formation of new species.