Hybrid zone

A hybrid zone exists where the ranges of two interbreeding species or diverged intraspecific lineages meet and cross-fertilize.

[2] The two (or more) genetically differentiated species or lineages contributing to formation of a hybrid zone are regarded as parental forms.

They can be wide (gradual) or narrow (steep) depending on the ratio of hybrid survival to recombination of genes.

The four major models operate mostly under a general framework of either a balance between natural selection and dispersal or interaction between genotypes and environment.

Some early evolutionary biologists who preferred a biological species concept, such as Ernst Mayr[11] and Theodosius Dobzhansky,[12] 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.

[13] (Although this term can also refer to the spreading of advantageous allele across a reproductive barrier[14]) The ephemerality of hybrid zone has been countered by the discovery of many hybrid zones that has lasted for a long period of time,[9] up to 100,000 years found between the iguanid lizards, Sceloporus woodi and S. undulatus undulatus.

In fact, botanist Edgar Anderson suggested that hybrid populations are more likely to inhabit ecologically disturbed areas, which often occur under human’s modification of landscapes or geological events that create novel habitat conditions.

[19] This term was later taken by Nicholas Barton and Godfrey Hewitt to denote a hybrid zone maintained by a balance between selection and dispersal.

If a transect is taken on a mosaic zone, the distribution of a genotypic trait may present itself as a wave with multiple peaks or plateaus, which may be interpreted as clines depending on the size of the geographical study.

Primary hybrid zones occur where divergence is taking place between adjacent populations of a previously homogeneous species, possibly leading to parapatric speciation.

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.

Genes can also flow back into the distinct populations through interbreeding between hybrids and parental (non-hybrid) individuals (introgression).

[26] However, hybridization can also serve to introduce genetic diversity into small, inbred populations, such as the case with the Florida panther.

[27] In this respect, conservation policies based on taxa instead of genetic structure can be disadvantageous to rare species experiencing inbreeding depression.

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.

However, blue mussel populations show extensive hybridisation worldwide and are a well studied example of a marine hybrid zone.