Selection for prezygotic barriers to reproduction by the existence of postzygotic barriers.

Reinforcement is a description particularly of the process that drives speciation events once two populations have diverged genetically. Note, though, that postzygotic barriers must be tested – hybridization has to have some potential to occur – for reinforcement to operate.

Reinforcement thus can only occur if two populations are sympatric, which means sharing ranges such that gene flow, particularly in the course of sexual reproduction, has the potential to occur.