Forms
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 adapatation 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 existin 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 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.
Read more about this topic: Hybrid Zone, Definition
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