Restricting gene flow
Separate species can arise only when the movement of alleles between groups becomes restricted. Reproductive isolation develops when differences between groups prevent successful breeding between their members.
With little or no gene flow, the gene pools are free to change independently. Allele frequencies in one group no longer have to follow the same pattern as those in the other.
From divergence to separate species
Over successive generations, the genetic differences between isolated groups can build up. Eventually, those differences may prevent individuals from the two groups from mating successfully and producing fertile offspring.
Once this reproductive separation is complete, the two groups represent different species.
Exam Tip: A complete speciation answer should finish with the biological consequence of divergence: members of the separated populations can no longer interbreed and produce fertile offspring.