A hybrid zone exists where the ranges of two interbreeding species or diverged intraspecific lineages meet and hybridization takes place. Hybrid zones can form in situ, but generally they result from secondary contact of the parental forms after a period of geographic isolation, which allowed their differentiation (speciation). Hybrid zones are useful in the study of the processes of speciation as they provide natural examples of 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 races) 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 hundreds of metres to hundreds of kilometres. The shape or the zones (clines) can be gradual or stepped.
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 as they can produce fertile offspring at least some of the time. Despite this, the two populations remain identifiably different, conforming to an alternative definition 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.
Hybrid zones can be either primary or secondary. Primary hybrid zones occur where divergence is taking place between adjacent populations of a previously homogeneous species, possibly leading to parapatric speciation. As a population spreads across a contiguous area it may spread into an abruptly different environment. Through adaptation to the new environment, the adjacent populations begin parapatric divergence. The point of contact between the older population and the newer population will ideally be a stepped cline, but due to dispersal across the line, hybridization will take place and a hybrid zone will arise. Secondary hybrid zones in turn arise from secondary contact between two populations that were previously allopatric.In practice it can be quite difficult to distinguish between primary and secondary contact by observing an existing hybrid zone. Most of the prominent, recognized hybrid zones are thought to be secondary.
One form of hybrid zone results where one species has undergone allopatric speciation and the two new populations regain contact after a period of geographic isolation. The two populations then mate within an area of contact, producing 'hybrids' which contain a mixture of the alleles distinctive for each population. Thus novel genes flow from either side into the hybrid zone. Genes can also flow back into the distinct populations through interbreeding between hybrids and parental (non-hybrid) individuals (introgression). These processes lead to the formation of a cline between the two pure forms within the hybrid zone.
Hybrid zones and gene flow do not inevitably lead to merging of the two populations involved, but the zones can instead be retained for thousands of years. The prevailing explanation for the stability or retention of hybrid zones is that they represent a 'tension zone', where the conflicting effects of dispersal of parental forms and selection against hybrids balance each other. Dispersal of individual parents leads to the creation of more hybrids within the hybrid zone. This results in gene flow between the two populations because of introgression. However, in many cases hybrids are less fit than parental forms because they lack the complete gene complexes of the parentals that make them well adapted to the environments either side of the hybrid zone. The more frequent death and sterility of hybrids forms a barrier to gene flow by making a 'hybrid sink' into which genes from parentals flow but rarely continue into the other population. Statistical models suggest that neutral alleles flow across this barrier very slowly while positively selected alleles will move across quite rapidly. An interesting outcome of this model is that hybrid zones are almost environment independent and can therefore move. Hybrids may not always be unfit in the very first generation, which can show hybrid vigour.
Several other models exist to explain hybrid zone stability, although the tension zone model is used in most cases. The dispersal-independent cline model does not consider dispersal at all, with the frequency of alleles finding different equilibria depending on the precise environmental conditions in a particular area. In each location, selection maintains a stable equilibria for each allele, resulting in a smooth cline. The hybrids must therefore be fitter at some point along the cline. The wave of advance model sees multiple clines for individual alleles forming due to the progression of advantageous alleles from one population the other.
Certain factors contribute to stability and steepness of hybrid zones within these models by reducing the frequency of inter-population mating and introgression. These include positive assortative mating within populations, habitat selection of different populations and hybrid unfitness. Additionally, it is suggested that individuals in a populations near a tension zone (in which hybrids are less fit), will evolve methods of only mating with their own population to reduce the prevalence of unfit hybrids. This is dubbed reinforcement, and controversy remains as to its importance.
Marine hybrid zone case study 
Hybrid zones are thought to be less common in marine than terrestrial environments. However, blue mussel populations show extensive hybridisation worldwide and are a well studied example of a marine hybrid zone. There are multiple sites of hybridisation between the closely related species Mytilus edulis, M. trossulus and M. galloprovincialis across the North Atlantic and Pacific coasts. These hybrid zones vary considerably. Some hybrid zones, such as the one in Newfoundland in Canada show remarkably few hybrids, while in the Baltic Sea most individuals are hybrids.
Based on the fossil record and genetic marker studies the following chronology is used to explain the Canadian mussel hybrid zone:
- The genus Mytilus is at one point restricted to the North Pacific but spreads to the Atlantic through the Bering Strait around .
- M. trossulus evolves in the North Pacific and M. edulis in the Atlantic in near allopatry as migration across the Bering Strait is very low.
- Recently, in post-glacial time M. trossulus from the Pacific enters the Atlantic and colonises shores on both sides, and meets with the local M. edulis.
The Canadian mussel hybrid zone is unusual because both species are found along the entire shore (a mosaic pattern) instead of the typical cline found in most hybrid zones. Studies of mtDNA and allozymes in adult populations show that the distribution of genotypes between the two species is bimodal; pure parental types are most common (representing above 75% of individuals) while backcrosses close to parental forms are the next most prevalent. F1 hybrid crosses represent less than 2.5% of individuals.
The low frequency of F1 hybrids coupled with some introgression allows us to infer that although fertile hybrids can be produced, significant reproductive barriers exist and the two species are sufficiently deviated that they are now able to avoid recombinational collapse despite habitat sharing. One reason that could account for keeping taxa separate through prezygotic isolation is that in this region M. edulis spawns over a narrow 2–3 week period in July, while M. trossulus spawned over a more extensive period between late spring to early autumn. No infertility or developmental retardation was found in the hybrid individuals, allowing them to introgress with pure species.
See also 
- Endler, J. (1977). Geographic Variation, Speciation and Clines. Monographs in Population Biology (Volume 10). Princeton, NJ: Princeton University Press. ISBN 9780691081922.
- N. H. Barton & G. M. Hewitt (1985). "Analysis of hybrid zones". Annual Review of Ecology and Systematics 16: 113–148. doi:10.1146/annurev.es.16.110185.000553.
- Murray, 1985
- Ernst Mayr (1942). Systematics and the Origin of Species. New York: Columbia University Press.
- N. H. Barton (1983). "Multilocus clines". Evolution 37 (3): 454–471. JSTOR 2408260.
- J. A. Endler (1982). "Problems in distinguishing historical from ecological factors in biogeography". American Zoologist 22 (2): 441–452. doi:10.1093/icb/22.2.441. JSTOR 3882673.
- Mark Ridley (2003). Evolution (3rd ed.). Blackwell Publishers. ISBN 978-1-4051-0345-9.
- M. J. D. White, R. E. Blackith, R. M. Blackith & J. Cheney (1967). "Cytogenetics of the viatica group of morabine grasshoppers. I. The "coastal" species". Australian Journal of Zoology 15 (2): 263–302. doi:10.1071/ZO9670263.
- A. D. Bazykin (1969). "Hypothetical mechanism of speciation". Evolution 23 (4): 685–687. JSTOR 2406862.
- N. H. Barton (1979). "The dynamics of hybrid zones". Heredity 43: 341–359. doi:10.1038/hdy.1979.87.
- William S. Moore (1977). "An evaluation of narrow hybrid zones in vertebrates". Quarterly Review of Biology 52 (3): 263–278. JSTOR 2824163.
- Jaroslav Pialek & Nick H. Barton (1997). "The spread of an advantageous allele across a barrier: the effects of random drift and selection against heterozygotes". Genetics 145 (2): 493–504. PMC 1207813. PMID 9071602.
- D. J. Howard (1993). "Reinforcement: origin, dynamics, and fate of an evolutionary hypothesis". In R. G. Harrison. Hybrid Zones and the Evolutionary Process. New York: Oxford University Press. pp. 46–69.
- G .J. Vermeij (1991). "Anatomy of an invasion: the trans-Arctic interchange". Paleobiology 17 (3): 281–307. JSTOR 2400870.
- C. Riginos & C. W. Cunningham (2005). "Local adaptation and species segregation in two mussel (Mytilus edulis × Mytilus trossulus) hybrid zones" (PDF). Molecular Ecology 14 (2): 381–400. doi:10.1111/j.1365-294X.2004.02379.x. PMID 15660932.
- Carlos Saavedra, Donald T. Stewart, Rebecca R. Stanwood & Eleftherios Zouros (1996). "Species-specific segregation of gender-associated mitochondrial DNA types in an area where two mussel species (Mytilus edulis and M. trossulus) hybridize" (PDF). Genetics 143 (3): 1359–1367.
- J. E. Toro, R. J. Thompson & D. J. Innes (2002). "Reproductive isolation and reproductive output in two sympatric mussel species (Mytilus edulis, M. trossulus) and their hybrids from Newfoundland" (PDF). Marine Biology 141 (5): 897–909. doi:10.1007/s00227-002-0897-3.