Gene flow

There is also the factor of gene flow. Selection might reinforce any isolation between the populations but, until isolation is complete, gene flow will be acting to equalize their gene frequencies. Once the two populations become genetically the same, there can no longer be selection to decrease breeding between them. Selection for reinforcement is likely to be strongest immediately after the two populations meet. If the necessary genetic variation in mating preferences is present, and selection is strong enough, the two species may completely split; but a gradual slide back into a single species is possible.

Allopatric speciation does not need a theory of reinforcement

A theory of speciation can avoid theoretical difficulties if it does not depend on reinforcement. The allopatric theory has this virtue. In the allopatric theory, it could be that reproductive isolation only ever evolves in allopatry (and when it does not the two populations simply collapse back into one when they meet); alternatively, it could be that partial reproductive isolation sometimes evolves allopatrically and is then reinforced on secondary contact. Either possibility fits in with the general theory of allopatric speciation.

The two alternatives - parapatric and sympatric speciation - require the theory of reinforcement: if reinforcement does not operate, neither do they.

John Maynard Smith discusses the question of whether geographic isolation is necessary for speciation.