Hybrid zone

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 but generally they result from secondary contact of the parental forms after a period of geographic isolation, which allowed their differentiation (or speciation). Hybrid zones are useful in the study of the genetics of speciation as they can provide natural examples of differentiation and (sometimes) gene flow between populations that are at some point between representing a single species and representing multiple species in reproductive isolation.

Hybrid zones are locations where the hybrid offspring of two divergent taxa (species, subspecies or genetic "forms") are prevalent and there is a cline in the genetic composition of populations from one taxon to the other. Precise definitions of hybrid zones vary; some insist on increased variability of fitness within the zone, others that hybrids be identifiably different from parental forms and others that they represent secondary contact alone. They occur at the area of contact between two closely related but genetically different populations, regarded as parental forms. The widths of such zones can vary from tens of metres to hundreds of kilometres. The shape of the zones (clines) can be gradual or stepped.

Some hybrid zones can be seen as presenting a paradox for the biological definition of a species, usually given as "a population of actually or potentially interbreeding individuals that produce fertile offspring". Under this definition, both parental forms could be argued to be same species if they can produce fertile offspring at least some of the time. Despite this, the two populations remain identifiably different, conforming to an alternative, and presently preferred concept of species as "taxa that retain their identity despite gene flow".

The clines of hybrid zones can be observed by recording the frequency of certain diagnostic alleles or phenotypic characteristics for either population along a transect between the two populations. Often the clines take the form of a sigmoid curve. They can be wide (gradual) or narrow (steep) depending on the ratio of hybrid survival to recombination of genes. Hybrid zones which show no regular transition from one taxon to the other, but rather a patchy distribution of parental forms and subpopulations with hybrid background, are termed mosaic hybrid zones.

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