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. 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.

Hybrid zones are areas 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. The two (or more) genetically differentiated species or lineages contributing to formation of a hybrid zone are regarded as parental forms. 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. 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. under what has become known as the Biological Species Concept. Under this definition, both parental forms could be argued to be the same species if they produce fertile offspring at least some of the time. However, the two parental populations or species often remain identifiably distinct, conforming to an alternative, and presently preferred concept of species as "taxa that retain their identity despite gene flow". 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. bounded hybrid superiority model, and tension zone model. Different hybrid zones may fit different models and no single theory or model serves to explain all the hybrid zones found in nature.

Some early evolutionary biologists who preferred a biological species concept, such as Ernst Mayr and Theodosius Dobzhansky, believed that hybrid zones are generally rare and ephemeral, with an eventual fate of either merging of the hybridizing populations or reinforcement, which leads to a speciation event. The extinction of one of the hybridizing populations through introgression is sometimes termed “waves of advance”. (Although this term can also refer to the spreading of advantageous allele across a reproductive barrier) The ephemerality of hybrid zone has been countered by the discovery of many hybrid zones that has lasted for a long period of time,

The bounded hybrid superiority model predicts that hybrids have higher fitness in a habitat that is intermediate between those of their parental populations. Anderson also considered natural hybridization as a positive evolutionary stimulus that allows different populations and lineages to exchange adaptive genetic elements similar view that place a high evolutionary importance on hybrid zones is more prevalent among botanists, in contrast to zoologists who are more likely see hybrid zones as more of a “natural laboratory” of population genetics. some also point out that there has not been direct empirical evidence of higher hybrid fitness along an ecological gradient (i. e. ecotones). This term was later taken by Nicholas Barton and Godfrey Hewitt to denote a hybrid zone maintained by a balance between selection and dispersal. A tension zone is characterized by a dispersal dependent cline (in contrast to a dispersal independent cline such as a bounded hybrid superiority zone) maintained between the “homogenizing effect” of dispersal and some forces of “spatial heterogeneity”, such as differential selection and introgression. i. e. the comparative size of the dispersal distance and the average size of a patch. 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. In the centre of such a cline, hybrizymes are commonly found. These are alleles that are normally rare in both species but, probably due to genetic hitchhiking on genes for hybrid fitness, reach high frequencies in the areas where most hybrids are formed.

Hybrid zones involving a rare species and a more common one can be at risk for outbreeding depression that reduces the fitness of the rare species. Another risk that can arise is the assimilation of the rare species through loss of rare genotypes or phenotypes. This risk is especially high when an invasive species hybridizes with an endemic species on an island. However, hybridization can also serve to introduce genetic diversity into small, inbred populations, such as the case with the Florida panther. In this respect, conservation policies based on taxa instead of genetic structure can be disadvantageous to rare species experiencing inbreeding depression. Hybrid zones can be a good indicator in the study of climate change. Monitoring the range of hybrid zones through genetic methods such as geographical cline analysis of genotype distribution can tell us the populations’ response to historical as well as ongoing changing environments. Examining the exchange of adaptive alleles or genomic regions can also be useful in mapping the populations’ adaptation to climatic niches. 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 times, 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.