Hybrid zone
Encyclopedia
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 to distinguish between primary and secondary contact (Endler, 1982).
populations show extensive hybridisation worldwide and are a well studied example of a marine hybrid zone. There are multiple sites of hybridisation between the species Mytilus edulis, M. trossulus and M. galloprovincialis across the Atlantic, Scandinavian and the Mediterranean Sea
s. These hybrid zones vary considerably. Some hybrid zones, such as the one in Newfoundland
in Canada
show remarkably few hybrids, while in Scandinavia
most individuals are hybrids to some degree. Morphological and genetic differences are clear between these populations and it is believed that they are close to full speciation. They are probably maintained through a combination of hybrid unfitness, positive assortative mating and habitat segregation. In this summary focus will be on the Canadian hybrid zone in the North Atlantic, particularly that near Newfoundland which has been studied by researchers at Newfoundland Memorial University.
Based on the fossil record and genetic marker studies the following chronology is used to explain the Mussel hybrid zone:
The Canadian 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 (Bates and Innes, 1995). Studies of mtDNA and allozyme
s 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 (Saavedra et al., 1996).
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. Jiggins and Mallet (2000) have found that hybrid zones with low levels of F1 hybrids (bimodal distribution of hybrids) are highly stable and usually the result of assortative mating or fertilization, and not related to gross levels of genetic divergence or intrinsic genomic incompatibility. They hypothesized that such zones occurred between two populations that were close to full speciation and with some reinforcement. Toro et al. (2002) investigated whether different reproductive patterns and behaviour were the cause of this prezygotic isolation and discovered that M. edulis spawned over a 2-3 week period in July, while M. trossulus spawned over a more extensive period between late spring to early autumn. It was also found that hybrids were not infertile and exhibited normal reproductive development, allowing them to introgress with pure species. It was concluded that “differences in reproductive traits may partially explain the maintenance of the mussel hybrid zone in Newfoundland.”
The other likely candidate for hybrid zones stability is species segregation by habitat which has been investigated but not conclusively. Several studies have suggested that M. edulis are found in areas of lower salinity
and less wave exposure at the heads of bays more than M. trosullus. M.trosullus appears to be favoured in habitats with higher wave exposure (Bates and Innes, 1995). The one subtidal (low wave action) site sampled by Bates and Innes had just 8% M. trossulus individuals. A similar segregation has been found in the Mediterranean hybrid zone with M. edulis also favouring more sheltered habitats compared to M. galloprovincialis (Bierne et al. 2003). If this is the case, this would provide partial habitat separation and reduce the probability of gametes of two species encountering one another and cross-fertilising. This would increase genetic distinctiveness despite the populations living in sympatry. However, conflicting results have been identified to this trend of habitat segregation and so these results are not conclusive (Riginos and Cunningham, 2005). It is suggested that differences in habitat are what has led to the very different type of hybrid zones in Scandinavia and Canada. Hybrid mussel fitness has not been properly investigated, so it is not possible to judge its effects on postzygotic isolation and whether it could cause reinforcement (Riginos and Cunningham, 2005).
Species
In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...
meet. For a hybrid zone to be stable, the offspring produced by the cross (the hybrids) have to be less fit
Fitness (biology)
Fitness is a central idea in evolutionary theory. It can be defined either with respect to a genotype or to a phenotype in a given environment...
than members of the parent species, although this condition does not need to be met in the very first hybrid generation (F1 hybrid
F1 hybrid
F1 hybrid is a term used in genetics and selective breeding. F1 stands for Filial 1, the first filial generation seeds/plants or animal offspring resulting from a cross mating of distinctly different parental types....
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
Taxonomy
Taxonomy is the science of identifying and naming species, and arranging them into a classification. The field of taxonomy, sometimes referred to as "biological taxonomy", revolves around the description and use of taxonomic units, known as taxa...
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
Hybrid speciation
Hybrid speciation is the process wherein hybridization between two different closely related species leads to a distinct phenotype. This phenotype in very rare cases can also be fitter than the parental lineage and as such natural selection may then favor these individuals. Eventually, if...
), as they are believed to be in transition to reproductive isolation
Reproductive isolation
The mechanisms of reproductive isolation or hybridization barriers are a collection of mechanisms, behaviors and physiological processes that prevent the members of two different species that cross or mate from producing offspring, or which ensure that any offspring that may be produced is not...
.
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
Cline (population genetics)
In biology, an ecocline or simply cline describes an ecotone in which a series of biocommunities display continuous gradient...
(Barton & Hewitt, 1985). Precise definitions of hybrid zones vary, some insist on increased variability of fitness
Fitness (biology)
Fitness is a central idea in evolutionary theory. It can be defined either with respect to a genotype or to a phenotype in a given environment...
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
Paradox
Similar to Circular reasoning, A paradox is a seemingly true statement or group of statements that lead to a contradiction or a situation which seems to defy logic or intuition...
for the biological definition of a species
Species
In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...
, 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
Speciation
Speciation is the evolutionary process by which new biological species arise. The biologist Orator F. Cook seems to have been the first to coin the term 'speciation' for the splitting of lineages or 'cladogenesis,' as opposed to 'anagenesis' or 'phyletic evolution' occurring within lineages...
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
Reproductive isolation
The mechanisms of reproductive isolation or hybridization barriers are a collection of mechanisms, behaviors and physiological processes that prevent the members of two different species that cross or mate from producing offspring, or which ensure that any offspring that may be produced is not...
.
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
Allopatric speciation
Allopatric speciation or geographic speciation is speciation that occurs when biological populations of the same species become isolated due to geographical changes such as mountain building or social changes such as emigration...
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
Introgression
Introgression, also known as introgressive hybridization, in genetics is the movement of a gene from one species into the gene pool of another by the repeated backcrossing of an interspecific hybrid with one of its parent species...
) (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
Infertility
Infertility primarily refers to the biological inability of a person to contribute to conception. Infertility may also refer to the state of a woman who is unable to carry a pregnancy to full term...
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
Parapatric speciation
Parapatry is a term from biogeography, referring to organisms whose ranges do not significantly overlap but are immediately adjacent to each other; they only occur together in the narrow contact zone, if at all. This geographical distribution is opposed to sympatry & allopatry or peripatry...
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 to distinguish between primary and secondary contact (Endler, 1982).
Hybrid Zone Case Study
MusselMussel
The common name mussel is used for members of several families of clams or bivalvia mollusca, from saltwater and freshwater habitats. These groups have in common a shell whose outline is elongated and asymmetrical compared with other edible clams, which are often more or less rounded or oval.The...
populations show extensive hybridisation worldwide and are a well studied example of a marine hybrid zone. There are multiple sites of hybridisation between the species Mytilus edulis, M. trossulus and M. galloprovincialis across the Atlantic, Scandinavian and the Mediterranean Sea
Mediterranean Sea
The Mediterranean Sea is a sea connected to the Atlantic Ocean surrounded by the Mediterranean region and almost completely enclosed by land: on the north by Anatolia and Europe, on the south by North Africa, and on the east by the Levant...
s. These hybrid zones vary considerably. Some hybrid zones, such as the one in Newfoundland
Newfoundland and Labrador
Newfoundland and Labrador is the easternmost province of Canada. Situated in the country's Atlantic region, it incorporates the island of Newfoundland and mainland Labrador with a combined area of . As of April 2011, the province's estimated population is 508,400...
in Canada
Canada
Canada is a North American country consisting of ten provinces and three territories. Located in the northern part of the continent, it extends from the Atlantic Ocean in the east to the Pacific Ocean in the west, and northward into the Arctic Ocean...
show remarkably few hybrids, while in Scandinavia
Scandinavia
Scandinavia is a cultural, historical and ethno-linguistic region in northern Europe that includes the three kingdoms of Denmark, Norway and Sweden, characterized by their common ethno-cultural heritage and language. Modern Norway and Sweden proper are situated on the Scandinavian Peninsula,...
most individuals are hybrids to some degree. Morphological and genetic differences are clear between these populations and it is believed that they are close to full speciation. They are probably maintained through a combination of hybrid unfitness, positive assortative mating and habitat segregation. In this summary focus will be on the Canadian hybrid zone in the North Atlantic, particularly that near Newfoundland which has been studied by researchers at Newfoundland Memorial University.
Based on the fossil record and genetic marker studies the following chronology is used to explain the Mussel hybrid zone:
- The genus Mytilus is at one point restricted to the North Pacific but spreads to the Atlantic through the Bering Straight around 3.5 million years ago (Vermeij 1991).
- M. trossulus evolves in the North Pacific and M. edulis in the Atlantic in near allopatry as migration across the Bering Straight is very low.
- Subsequently M. galloprovincialis undergoes cladogenesis from M. edulis in the Mediterranean Sea after it is temporarily isolated from the Atlantic.
- Recently M. trossulus from the Pacific enters the Atlantic and colonises shores on both sides. It spreads and forms secondary contact hybrid zones with M. edulis populations on coasts across Scandinavia and the eastern Atlantic.
- Riginos and Cunningham (2005) includes a suggested pattern of migration of M. trossulus across the ocean based on a review of genetic marker studies.
The Canadian hybrid zone is unusual because both species are found along the entire shore (a mosaic
Mosaic
Mosaic is the art of creating images with an assemblage of small pieces of colored glass, stone, or other materials. It may be a technique of decorative art, an aspect of interior decoration, or of cultural and spiritual significance as in a cathedral...
pattern) instead of the typical cline found in most hybrid zones (Bates and Innes, 1995). Studies of mtDNA and allozyme
Allozyme
Variant forms of an enzyme that are coded by different alleles at the same locus are called allozymes. These are opposed to isozymes, which are enzymes that perform the same function, but which are coded by genes located at different loci....
s 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 (Saavedra et al., 1996).
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. Jiggins and Mallet (2000) have found that hybrid zones with low levels of F1 hybrids (bimodal distribution of hybrids) are highly stable and usually the result of assortative mating or fertilization, and not related to gross levels of genetic divergence or intrinsic genomic incompatibility. They hypothesized that such zones occurred between two populations that were close to full speciation and with some reinforcement. Toro et al. (2002) investigated whether different reproductive patterns and behaviour were the cause of this prezygotic isolation and discovered that M. edulis spawned over a 2-3 week period in July, while M. trossulus spawned over a more extensive period between late spring to early autumn. It was also found that hybrids were not infertile and exhibited normal reproductive development, allowing them to introgress with pure species. It was concluded that “differences in reproductive traits may partially explain the maintenance of the mussel hybrid zone in Newfoundland.”
The other likely candidate for hybrid zones stability is species segregation by habitat which has been investigated but not conclusively. Several studies have suggested that M. edulis are found in areas of lower salinity
Salinity
Salinity is the saltiness or dissolved salt content of a body of water. It is a general term used to describe the levels of different salts such as sodium chloride, magnesium and calcium sulfates, and bicarbonates...
and less wave exposure at the heads of bays more than M. trosullus. M.trosullus appears to be favoured in habitats with higher wave exposure (Bates and Innes, 1995). The one subtidal (low wave action) site sampled by Bates and Innes had just 8% M. trossulus individuals. A similar segregation has been found in the Mediterranean hybrid zone with M. edulis also favouring more sheltered habitats compared to M. galloprovincialis (Bierne et al. 2003). If this is the case, this would provide partial habitat separation and reduce the probability of gametes of two species encountering one another and cross-fertilising. This would increase genetic distinctiveness despite the populations living in sympatry. However, conflicting results have been identified to this trend of habitat segregation and so these results are not conclusive (Riginos and Cunningham, 2005). It is suggested that differences in habitat are what has led to the very different type of hybrid zones in Scandinavia and Canada. Hybrid mussel fitness has not been properly investigated, so it is not possible to judge its effects on postzygotic isolation and whether it could cause reinforcement (Riginos and Cunningham, 2005).