As discussed in Key Concept 22.2, once a barrier to gene flow is established, reproductive isolation will begin to develop through genetic divergence. Over many generations, differences accumulate in the isolated lineages, reducing the probability that individuals from each lineage will mate successfully with individuals in the other when they come back into contact. In this way, reproductive isolation can evolve as a by-product of the genetic changes in the two diverging lineages.

If reproductive isolation is incomplete when incipient species come back into contact, some hybridization is likely to occur. If hybrid individuals are less fit than non-hybrids, selection will favor parents that do not produce hybrid offspring. Under these conditions, selection will result in the strengthening, or reinforcement, of mechanisms that prevent hybridization.

Mechanisms that prevent hybridization from occurring are called prezygotic isolating mechanisms. Mechanisms that reduce the fitness of hybrid offspring are called postzygotic isolating mechanisms. Postzygotic isolating mechanisms result in selection against hybridization, which in turn leads to the reinforcement of prezygotic isolating mechanisms.