Hybrid zones

A hybrid zone is an area of contact between two noticably different forms of a species at which hybridization takes place. A hybrid zone is a necessary stage for parapatric speciation to take place.

Hybrid zones are examples of stepped clines. A cline is a geographic gradient in the frequency of a gene and stepped clines have more dramatic gradients.

A hybrid zone can develop into a species barrier by the process of reinforcement.

The forms on either side of the zone may be different enough to have been classified as separate species: the classic example of this is the hooded and carrion crows (opposite) which interbreed and produce hybrids along a line in central Europe.