Populations that have diverged can come back into contact before reproductive isolation is complete. A hybrid zone is a region where members of the diverged populations meet and produce hybrid offspring.

Secondary contact can have several outcomes.

Reinforcement

If hybrids have lower fitness than offspring produced within each parental population, selection can favor traits that reduce interbreeding. Prezygotic isolation then becomes stronger. This process is reinforcement.

Fusion

If hybrids have high fitness and reproductive barriers are weak, gene flow can erase much of the accumulated divergence. The populations may merge back into a single evolutionary lineage.

Stability

A hybrid zone can also persist. Hybrids may continue to be produced because dispersal brings parental individuals together, even if hybrid fitness is somewhat reduced.

These outcomes show that speciation is not always an instantaneous or irreversible boundary crossing. Divergence and gene flow can oppose one another over long periods.

The key comparison is:

low hybrid fitness + selection against interbreeding → reinforcement
high hybrid fitness + substantial gene flow          → fusion
continued contact with persistent differences        → stable hybrid zone

Hybrid zones therefore provide direct evidence about how strong reproductive isolation has become after populations diverge.