A hybrid zone exists where the ranges of two interbreeding species meet. For a hybrid zone to be stable, the offspring produced by the cross (the hybrids) have to be less fit than members of the parent species, although this condition does not need to be met in the very first hybrid generation (F1 hybrid which can exhibit hybrid vigour). Some hybrid zones move, typically at a rate of 0.1-10 metres per year.
Hybrid zones are relatively rare, although a surprising number are now known to science. They present a problem to the taxonomy of the species involved, and the definition of species more generally. They are also important study systems for understanding how new species form (Hybrid speciation), as they are believed to be in transition to reproductive isolation.
These zones are often mapped including the current range of both species, with the overlap ranges highlighted.
Hybrid zones are locations where the hybrid offspring of two divergent populations (sometimes defined as subspecies or races) are prevalent and form a cline (Barton & Hewitt, 1985). 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 (Murray, 1985). They occur at the area of contact between two closely related but genetically different populations, each regarded as parental forms. Reviews of hybrid zones show varying widths between hundreds of metres and hundreds of kilometres and the presence of both gradual clines and stepped clines (Barton & Hewitt, 1985). They present a paradox for the biological definition of a species, usually “a population of actually or potentially interbreeding individuals that produce fertile offspring” (Mayr, 1942). Both parental forms by this definition are one 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” (Barton and Hewitt, 1989). They 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.
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. They almost always 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 (Barton, 1983).
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) (Ridley 2003). 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 the recombination of the two populations involved, but can instead continue for thousands of years (White, et al., 1967.). The predominant explanation for this is that the hybrid zone represents a 'tension zone' between the conflicting effects of dispersal of parental forms and selection against hybrids (Bazykin, 1969.). 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 (Barton & Hewitt, 1985 p.135). An interesting outcome of this model is that hybrid zones are almost environment independent and can therefore move (Barton, 1979). 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. (Moore, 1977) 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 (Pialek and Barton, 1997).
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 (examples of both these found in question 1 part B and question 2) 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 (Howard, 1993).
Hybrid zones can also occur across regions of primary contact in which parapatric speciation is taking place. As a population spreads across a contiguous area it may spread into an abruptly different environment. The populations will deviate and begin parapatric speciation – those in the new environment adapting to the different conditions. The point of contact between the older population and the newer population will be a stepped cline and a hybridisation zone can form. Despite this, the two populations will never have been fully isolated from one another, unlike in cases of secondary contact. This distinction may not be a very useful one as in practice it can be quite difficult
Hybrid zones are relatively rare, although a surprising number are now known to science. They present a problem to the taxonomy of the species involved, and the definition of species more generally. They are also important study systems for understanding how new species form (Hybrid speciation), as they are believed to be in transition to reproductive isolation.
These zones are often mapped including the current range of both species, with the overlap ranges highlighted.
Hybrid zones are locations where the hybrid offspring of two divergent populations (sometimes defined as subspecies or races) are prevalent and form a cline (Barton & Hewitt, 1985). 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 (Murray, 1985). They occur at the area of contact between two closely related but genetically different populations, each regarded as parental forms. Reviews of hybrid zones show varying widths between hundreds of metres and hundreds of kilometres and the presence of both gradual clines and stepped clines (Barton & Hewitt, 1985). They present a paradox for the biological definition of a species, usually “a population of actually or potentially interbreeding individuals that produce fertile offspring” (Mayr, 1942). Both parental forms by this definition are one 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” (Barton and Hewitt, 1989). They 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.
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. They almost always 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 (Barton, 1983).
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) (Ridley 2003). 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 the recombination of the two populations involved, but can instead continue for thousands of years (White, et al., 1967.). The predominant explanation for this is that the hybrid zone represents a 'tension zone' between the conflicting effects of dispersal of parental forms and selection against hybrids (Bazykin, 1969.). 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 (Barton & Hewitt, 1985 p.135). An interesting outcome of this model is that hybrid zones are almost environment independent and can therefore move (Barton, 1979). 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. (Moore, 1977) 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 (Pialek and Barton, 1997).
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 (examples of both these found in question 1 part B and question 2) 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 (Howard, 1993).
Hybrid zones can also occur across regions of primary contact in which parapatric speciation is taking place. As a population spreads across a contiguous area it may spread into an abruptly different environment. The populations will deviate and begin parapatric speciation – those in the new environment adapting to the different conditions. The point of contact between the older population and the newer population will be a stepped cline and a hybridisation zone can form. Despite this, the two populations will never have been fully isolated from one another, unlike in cases of secondary contact. This distinction may not be a very useful one as in practice it can be quite difficult