A hybrid zone exists where the ranges of two interbreeding species or diverged intraspecific lineages meet and cross-fertilize. Hybrid zones can form in situ due to the evolution of a new lineage[1][page needed] but generally they result from secondary contact of the parental forms after a period of geographic isolation, which allowed their differentiation. Hybrid zones are useful in studying the genetics of speciation as they can provide natural examples of differentiation and gene flow between populations that are at some point on the continuum between diverging populations and separate species with reproductive isolation.
- ^ Endler, J. (1977). Geographic Variation, Speciation and Clines. Monographs in Population Biology. Vol. 10. Princeton, NJ: Princeton University Press. pp. 1–246. ISBN 9780691081922. PMID 409931.