Allopatric speciation begins when a geographic barrier reduces gene flow between parts of a population.

The barrier might be a newly formed river or mountain range, fragmentation of habitat, or colonization of a distant island. Once migration between the separated populations becomes rare, they no longer share genetic variants freely.

After isolation, several evolutionary processes can make the populations diverge:

• mutation introduces new variants independently;
• genetic drift changes allele frequencies by chance;
• natural selection favors different variants if environments differ.

Geographic separation alone does not instantly create species. Speciation is complete only when enough reproductive isolation evolves that renewed contact no longer restores substantial gene flow.

Causal sequence

continuous population
        ↓
geographic separation
        ↓
reduced gene flow
        ↓
independent drift, mutation and selection
        ↓
genetic and phenotypic divergence
        ↓
reproductive isolation

For example, if a small population colonizes an island, founder effects can immediately shift allele frequencies. Different food, climate or predators can then impose different selection pressures. Over many generations, mating signals or genetic incompatibilities may diverge enough that migrants from the source population no longer reproduce effectively with island descendants.

Allopatric speciation therefore connects geography to evolutionary independence through reduced gene flow.