Hybrid zone

A hybrid zone is an area where the ranges of two interbreeding species, or of diverged lineages within one species, meet and cross-fertilize, producing a genetic gradient between the two populations. Most hybrid zones form through secondary contact, when populations that were once separated by geography evolve independently and then reconnect, though some arise in place as new lineages develop within an otherwise continuous population. Because they capture ongoing differentiation and gene flow, hybrid zones are valuable natural laboratories for studying how species develop reproductive isolation. Blue mussels illustrate the pattern: the species Mytilus edulis, M. trossulus and M. galloprovincialis hybridize across Atlantic and Pacific coastlines, and in a Newfoundland hybrid zone pure parental types make up more than 75 percent of individuals while first generation hybrids account for under 2.5 percent, showing that reproductive barriers persist even where fertile hybrids are produced.