Hybrid Zone

Hybrid zones normally are situated at the geographical interfaces (or areas of secondary overlap) between pairs of closely related species.

From: Sketches of Nature , 2016

Plant Hybrids

Robert S. Fritz , in Encyclopedia of Biodiversity, 2004

II.A. Types of Hybrid Zones

Hybrid zones are locations where hybrids between species, subspecies, or races are found. Typically, hybrid zones are described as clines, spatial gradients in traits or alleles across a geographic transect where two taxa meet. The balance between selection on hybrids and dispersal of genes determines cline width. Clines are expected to be narrow if there is strong selection against hybrids, if gene flow is limited, or if there are steep environmental gradients. Clines will be wider if selection is weak, gene flow is more extensive, or environmental gradients are gradual. Cline shape is predicted to be a smooth, sigmoid curve if selection acts on single genes or on quantitative traits, but linkage disequilibrium combined with dispersal of genes can distort the smooth shape of the cline, creating a stepped cline. In stepped clines most of the change in allele frequency or trait expression occurs in a narrow range in the middle of a cline. Allele frequency, linkage disequilibrium with other alleles or traits, and gene flow can be used to infer the form by which selection acts in these clines. A clinal hybrid zone is one type of spatial pattern of plant genetic diversity that can influence biodiversity and spatial distribution of animal and pathogen species.

Narrow hybrid zones 10   m wide are found between the sedges Carex canescens and C. mackenziei at the edge of water along the Bothnian coast in northern Sweden. Oak hybrid zones approximately 30   m wide between the oaks Quercus depressipes and Q. rugosa are found on steep slopes in northern Mexico. A much wider hybrid zone extending about 20   km occurs in the same area between Q. coccolobifolia and Q. rugosa.

In contrast to the traditional clinal model of hybrid zones, mosaic hybrid zones occur when habitats of the hybridizing taxa are patchily distributed. Hybridization may occur where the different habitat patches abut, leading to the patchy distribution of hybrids. In contrast to the more or less discrete location of hybrids geographically in the clinal hybrid zones, mosaic hybrid zones can be as widely distributed as the distribution of habitats and parental species. Therefore, the impact of mosaic hybrid zones on the distribution of plant genetic variation and its effects on diversity of communities of herbivores and pathogens are geographically more widespread.

Louisiana irises Iris fulva, I. brevicaulis, and I. hexagona fit the mosaic model since species and hybrid genotypes are associated with different, interspersed habitats. For example, I. fulva is associated with maple-dominated forest habitats, I. brevicaulis is associated with black oak forests, and I. hexagona is found at the edge of freshwater marshes. Hybrid genotypes are either not strongly associated with specific habitats or occupy intermediate or novel habitats. More dispersed mosaic hybrid zones occur where broadly sympatric species hybridize, either occasionally or extensively, where their habitats mix. The sunflowers Helianthus annuus and H. petiolaris exemplify this type of hybrid zone. These sunflower species have overlapping ranges and form local hybrid swarms in the western United States. Hybrids between Salix sericea and S. eriocephala also fit the mosaic model, with hybrids being found throughout the sympatric range of these species in eastern North America.

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Hybrid Zone, Mouse

M. Macholán , in Brenner's Encyclopedia of Genetics (Second Edition), 2013

Abstract

Hybrid zones are areas where genetically distinct populations meet, mate, and leave offspring of mixed ancestry. The majority of hybrid zones are probably maintained by a balance between dispersal and selection against admixture. One of the best-studied hybrid zones is the zone of secondary contact between two house mouse subspecies ( Mus musculus musculus and Mus musculus domesticus) in Europe. Genetic analyses suggest that in general stronger selection is affecting X chromosome loci than autosomal loci. The number of loci that contribute to postzygotic isolation between the taxa is moderate to high, in agreement with the Dobzhansky–Muller model. This model proposes that accumulation of different mutations at several loci in allopatry will cause sterility/inviability of hybrids upon secondary contact, however, recent studies suggest some loci may benefit from contact with a 'naive' foreign genome and escape the barrier.

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Neotropical African Bees

Orley R. Taylor , in Encyclopedia of Insects (Second Edition), 2009

Hybrid Zones

A hybrid zone formed at the southern limit of AHBs in northern Argentina in the late 1960s and early 1970s. Although many anticipated that a similar zone would form in the United States, this has not happened. Coincident with the arrival of AHBs in the United States, the mite, Varroa destructor, an introduced brood parasite from Asia that kills EHB colonies, spread rapidly throughout the country eliminating feral EHB colonies. At present, feral EHBs, which usually are escaped swarms from managed apiaries, are transitory and persist only for a short time; this precludes the formation of a persistent hybrid zone. In Argentina, it is likely that Varroa, which arrived after the formation of the hybrid zone, has changed the dynamic of the interaction of both types of bee as well.

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Other Marine Fishes

John C. Avise , in Sketches of Nature, 2016

The evolutionary genetic status of Icelandic eels

Avise, J.C., W.S. Nelson, J. Arnold, R.K. Koehn, G.C. Williams, and V. Thorsteinsson. 1990. The evolutionary genetic status of Icelandic eels. Evolution 44:1254–1262.

Anecdote or Backdrop

Hybrid zones normally are situated at the geographical interfaces (or areas of secondary overlap) between pairs of closely related species. This next paper helped to confirm a puzzling exception to this pattern. It involves a hybrid population, in Iceland, between American eels and European eels, both of which were thought to spawn in the Sargasso Sea, thousands of kilometers away. So how in the world do the interspecific hybrids end up in Iceland?

Abstract

The Iceland population of Anguilla eels contains an elevated frequency of fish with vertebral numbers lower than those typical of European localities. Several distinct hypotheses have been advanced to account for these morphologically atypical fish: for example, they could represent (i) genetically "pure" American expatriates, (ii) genetically "pure" European types with ontogenetic abnormalities, or (iii) hybrids between American and European forms. Here we critically test these and other possibilities by examining the joint distributions of allozyme markers, mitochondrial DNA markers, and vertebral numbers in Icelandic eels. The particular patterns of association among the genetic and morphological traits demonstrate that the Icelandic population includes, in low frequency, the products of hybridization between American and European eels. Approximately 2–4% of the gene pool in the Iceland eel population is derived from American eel ancestry. This hybrid zone is highly unusual in the biological world, because the mating events in catadromous eels presumably take place thousands of kilometers from where the hybrids are observed as maturing juveniles. The molecular data, in conjunction with geographic distributions, strongly suggest that the differences in migrational behavior and morphology between American and European eels include an important additive genetic component. Evolutionary hypotheses are advanced to account for the original separation of North Atlantic eels into American and European populations, and for the presence of hybrids in Iceland.

Addendum

Subsequent genetic surveys of Icelandic eels (by other laboratories) have further confirmed that hybrids reside on that island and also that the hybrids include generations beyond the F 1 .

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Populations, Species, and Conservation Genetics

David S. Woodruff , in Encyclopedia of Biodiversity, 2001

III.E. Hybridization

Outbreeding depression occurs in nature in some hybrid zones between semispecies and species of plants and animals. Hybrids are interesting because they show that the evolution of many groups of plant and animal species involves both lineage splitting and lineage anastamosis. Fertile interspecific hybrids permit gene flow between species (introgression). Hybrids call into question species definitions that overemphasize reproductive isolation. The notion that "species" are somehow "purebred" and always reproductively isolated from their close relatives is not borne out by observations of some animal groups and many plants, in which low rates of hybridization between congeners often occur in nature.

In the past, it was argued that hybrid populations did not qualify for legal protection under the U.S. Endangered Species Act. However, hybrids are very much a normal part of nature. Rare or very rare in some groups, and more common in others, they present conservationists with a dilemma because their occurrence appears to diminish the value of a taxon. Should one save Florida panthers if they are known to harbor genes of introduced South American panthers, a different subspecies? Do Texas red wolves (Canis rufus) merit conservation if they are gray wolf–coyote hybrids? Should one save the remaining Przewalski's horses if it is shown that historical mismanagement resulted in a large fraction of the surviving animals being tainted by the genes of domestic horses, a karyologically distinct species? Whether hybrids should be afforded the same priority as nonintrogressed populations or species will remain controversial; in the previous cases, the answer was yes and geneticists contributed to pedigree management.

Habitat disturbance can result in increased opportunities for hybridization between species that would not normally interbreed. Fragmentation of the recently continuous Pacific Northwest old-growth forest has led to hybridization between the declining northern spotted owl (Strix occidentalis) and the barred owl (S. varia), which favors disturbed sites.

Hybridization is more common in plants than in animals; therefore, not surprisingly it is in plants that there are numerous examples of rare species being hybridized into extinction (genetic assimilation) by hybridization with a more common sympatric congener (Soltis and Gitzendanner, 1999). This is the case for the Catalina Island mahogany, in which 5 of the remaining 11 adult trees are actually hybrids with the more common mountain mahogany (Rieseberg and Swensen in Avise and Hamrick, 1996, p. 305–334). Other cases involve plants (Asteracea: Argyranthemum) in the Canary Islands. The Simien jackal (Canis simensis) of Ethiopia is at risk of being introgressed into extinction by hybridization with domestic dogs. It is now recognized that restocking rivers with genetically uniform hatchery-bred salmon has contributed to the collapse of the Pacific Northwest salmon runs. Hatchery fish show reduced fitness in the wild (they are not locally adapted) and compete and hybridize harmfully with the remaining wild salmon (Lande, 1999).

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Amphibians

John C. Avise , in Sketches of Nature, 2016

Abstract

We examine the influence of nonrandom mating and immigration on the evolutionary dynamics of cytonuclear associations in hybrid zones. Recursion equations for allelic and genotypic cytonuclear disequilibria were generated under models of: (i) migration alone, assuming hybrid zone matings are random with respect to cytonuclear genotype and (ii) migration in conjunction with refined epistatic mating, in which females of the pure parental species preferentially mate with conspecific males. Major results are as follows: (i) even the slightest migration removes the dependency of the final outcome on initial conditions, producing a unique equilibrium in which both pure parental genotypes are maintained in the hybrid zone; (ii) in contrast to nuclear genes, the dynamics of cytoplasmic allele frequencies appear robust to changes in the assumed mating system, yet are particularly sensitive to gene flow; (iii) continued immigration can generate permanent cytonuclear disequilibria, whether mating is random or assortative; and (iv) the order of population censusing (before vs. after reproduction by immigrants) can have a dramatic effect on the magnitude but not the pattern of cytonuclear disequilibria. Using the maximum likelihood method, the parameter space of migration rates and assortative mating rates was examined for best fit to observed cytonuclear disequilibria data in a hybrid population of Hyla tree frogs. An epistatic mating model with a total immigration rate of about 32% per generation produces equilibrium gene frequencies and cytonuclear disequilibria consistent with the empirical observations.

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Species and Speciation

C.L. Boggs , in International Encyclopedia of the Social & Behavioral Sciences, 2001

4 Space: Patterns of Biodiversity Resulting From Speciation

The different modes of speciation outlined above each result in a different spatial pattern of biodiversity. Hybrid zones between newly formed species are the predicted outcome of allopatric speciation, if the geographically isolated populations have not developed full reproductive isolation prior to removal of the geographic barrier. Further, the establishment of a geographic barrier is expected to divide ancestral populations of many species within the flora and fauna of a region. Removal of that barrier and recontact of newly divergent flora and fauna can result in multiple individual hybrid zones that overlap, or suture zones. Such suture zones mark boundaries between regional floral and faunal assemblages. Remington ( 1968) outlined six major suture zones and a number of minor suture zones in the Nearctic region, including a compendium of known hybridizing pairs of species within each suture zone and an analysis of the likely geographic barriers whose removal resulted in the now-observable suture zone pattern.

Ring species also result from the interaction of speciation with a geographic barrier, but with a twist. As populations of a species expand into new areas, moving around a geographic barrier, they can form a ring around that barrier. While the immediately adjacent populations at each point around the ring interbreed, the terminal populations are reproductively isolated and cannot interbreed. Ring species thus can be thought of as a result of geographically extended parapatric speciation. Irwin et al. (2001) present an outstanding example of a ring species in the case of the greenish warbler (Phylloscopus trochiloides), whose six sub-species circle the Tibetan Plateau. The sub-species that are sympatric at the ends of the ring in central Siberia do not interbreed. The structure of songs involved in mate choice have diverged due to selection pressures favoring increased song complexity as the species expanded from the southwest around on the eastern and western sides of the plateau, eventually contacting again north of the plateau. Interestingly, there is now a gap in the ring in central China, which is probably due to habitat destruction by humans. The recentness of this gap has not allowed time for song divergence across the gap, but one could predict speciation across the gap in the future.

Sympatric speciation can generate a different pattern of biodiversity, hot spots of endemic species. Endemic species have very localized distributions, being found in one (usually small) area. Such species are not necessarily rare; the localized populations may be quite large. The cichlid fish species assemblages found in the crater lakes of Cameroon and cited above, along with the much richer cichlid diversity in the rift lakes of east Africa, are classic examples of hot spots of endemic species. In Lake Victoria, for example, over 300 species are found, of which very few are found anywhere else.

Overall biogeographic patterns of diversity within a region should therefore reflect the relative dominance of allopatric, parapatric, and sympatric speciation in shaping the flora and fauna. However, these biogeographic patterns are all too easily disturbed by human activities, which include habitat destruction resulting in species extinction and species transport resulting in new mixing of floras and faunas. This disturbance lends urgency to attempts to understand the causes of biogeographic patterns at the beginning of the twenty-first century, before the evidence is muddied by human activity.

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Species

E. Mayr , in Encyclopedia of Genetics, 2001

Incomplete isolation

Two incipient species, after a period of isolation, may spread and come secondarily in contact with each other. If speciation was not yet completed, they will form a parapatric hybrid zone. If there is a complete breakdown of the isolating mechanisms, the two incipient species must be ranked as subspecies. However, if only an occasional hybrid is produced, they are best considered full species. Occasional hybridization between related sympatric species is much more common in plants than in animals, but in spite of its frequency may not lead to a complete breakdown of the barrier between the two species.

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Species, Concepts of

James Mallet , ... Yuttapong Thawornwattana , in Reference Module in Life Sciences, 2022

Genotypic Cluster Criterion and Assignment Tests

For morphological or genetic gaps to exist between species, gene flow (if any) between species must be balanced by an opposing force of disruptive selection. In his own work, the first author had studied hybrid zones between geographic forms of butterflies, and attempted to show that a practical statistical definition of species versus geographic races could be constructed using morphological and genetic gaps alone, rather than employing the phylogenetic or evolutionary processes that caused the gaps to exist.

However, to define species by means of the gaps between them requires consideration of the nature of the gaps to avoid falling into the trap of defining polymorphic forms as separate species, or of lumping sibling species. Rather than merely using external morphology, in difficult cases we could consider genetics as well. DNA has a digital, rather than analog code, so there are genetic gaps between virtually any pair of individuals. Clearly, then, we cannot use just any discreteness at the genetic level to define species. Separate sexes and polymorphic female forms of mimetic Papilio butterflies also have gaps between them in exactly this way. A genetic element, which may be a single base pair, an allele at a gene, the entire mitochondrial genome, a chromosomal rearrangement, or perhaps a sex chromosome, may determine the genetic or morphological differences between such polymorphic forms.

To be considered part of a single local population, and therefore part of the same local species, we expect that polymorphic genetic elements like mimicry genes and sex chromosomes will be approximately randomly combined with polymorphisms at genetic elements found on other chromosomes or extrachromosomal DNA. Each individual may be a distinct multilocus genotype, but we recognize a single grouping of genotypes because polymorphisms at one genetic element are independent of polymorphisms at others. Conversely, if alleles at one locus are strongly associated with alleles at other, unlinked elements (i.e., linkage disequilibrium or gametic disequilibrium), we have evidence for more than one separate population; if these two populations overlap spatially, the groupings are probably also separate species.

Many of us therefore proposed a "genotypic cluster criterion" for species (Mallet, 1995; Feder, 1998). Avise and Ball (1990) pioneered the basic idea using the frequencies of allozyme loci and gene sequences of mitochondrial DNA, but called the method "genealogical concordance". The term "genomic cluster" would perhaps be a more appropriate synonym in today׳s postgenomic age. Species are recognized by morphological and genetic gaps between populations in a local area rather than by means of the phylogeny, cohesion, or reproductive isolation that are responsible for these gaps (Mallet, 1995). In a local area, separate species are recognized if there are several clusters separated by multilocus phenotypic or genotypic gaps. A single species (the null hypothesis) is recognized if there is only a single cluster in the frequency distribution of multilocus phenotypes and genotypes. The genotypic gaps may be entirely vacant, or they may contain low frequencies of intermediate genotypes, or hybrids (Fig. 1). The definition is useful because one avoids tautological thinking: hypotheses about speciation or phylogeny of taxa become independent of assumptions about the nature of reproductive isolation or phylogeny underlying the taxa studied.

Fig. 1

Fig. 1. Heuristic for species delimitation: genotypic clusters identifiable in sympatry. A sample of individuals is made at a single place and time. Numbers of individuals are represented by the contours in multidimensional genotypic space. Peaks in the abundance are represented by "+   ". Two species are detected if there are two peaks in the genotypic distribution (right, bimodal distribution; also see Jiggins and Mallet (2000)). Otherwise the null hypothesis of a single species is not rejected (left, unimodal distribution). This method is agnostic to species concept: it depends on the pattern of genotypes only. Note: the axes represent multidimensional morphological/genotypic space, not geographic space.

 Reproduced from Jiggins, C. D., Mallet, J., 2000. Bimodal hybrid zones and speciation. Trends in Ecology and Evolution 15, 250–255.

Statistical procedures, "assignment tests" based on multilocus allelic data are today regularly used to estimate the numbers of genotypic clusters from individual multilocus genotype data (Pritchard et al., 2000; Falush et al., 2003; Huelsenbeck and Andolfatto, 2007): if such genetic clusters are statistically distinguishable in sympatry, most would agree that separate species are detected. Similarly, species delimitation by means of the multi-species coalescent (MSC; see Section "The Multispecies Coalescent and the Species Tree") in sympatry would similarly provide a test for the existence of sympatric genotypic clusters.

Genotypic clusters are neither profound nor original; the idea traces the earliest use of the method to Darwin׳s (1859) morphological gap criterion of species (see Darwin׳s Morphological Species Criterion), although earlier sources probably employ similar ideas since acceptance of evolution is not required to identify species on the basis of character distribution. Many similar proposals have been made (Simpson, 1937; Hutchinson, 1968; Sokal and Crovello, 1970; Avise and Ball, 1990; Cohan, 1994; Smith, 1994). The approach is essentially the same in most taxonomic decisions (see Taxonomic Practice), in the phenetic concept (see Subsection "Phenetic Species Concept"), and in a practical application of the biological species concept (see below). Multilocus genotypic clusters are almost universally applied as a criterion of speciation in theoretical models of sympatric speciation (e.g., Dieckmann and Doebeli, 1999; Gavrilets and Waxman, 2002; Kondrashov and Mina, 1986; Kondrashov and Kondrashov, 1999): in these models, a bimodal genotypic distribution evolves via reproductive isolation, but it is the demonstration that a pair of genetically divergent groups of individuals emerge from a single population, rather than the mere existence of hybrid inviability or mate choice, that is required for evidence of speciation.

Asexual forms, unclassifiable under the interbreeding concept, and arbitrarily definable at any level under concepts depending on phylogeny, can be clustered and classified as genotypic clusters in exactly the same way as sexual species. The precise taxonomic level of species clustering for asexuals is somewhat arbitrary, as in the phylogenetic concepts, but at least the method acknowledges this arbitrariness rather than purporting to use some higher evolutionary principle. Many asexual forms such as bdelloid rotifers have clearly distinguishable species taxa (Hutchinson, 1968; Fontaneto et al., 2007), probably due to ecological selection for distinct characteristics. In bacteria, competition is thought to structure largely asexual populations with occasional promiscuous horizontal gene transfer into recognizable genetic clusters (Cohan, 1994, also see Subsection "Ecological Species Concept"). Thus, reproductive isolation may help but is not required to be complete for the existence of a genotypic cluster.

Critics have argued that the genotypic cluster criterion in sexual species is nothing other than a gene flow concept of species under a different guise. This is true for one specialized interpretation of gene flow in sexual populations. If we label gene flow as successful or effective, as opposed to treating gene flow as the per generation rate of hybridization and backcrossing, we can see that a gene flow criterion becomes similar to the genotypic cluster criterion. To find whether a hybridization or gene flow event is successful, we must either follow the fate of every gene through all possible descendants for all time, or we may examine the genotypic state of a population and determine if genes from one form are mixed randomly with genes from another form. Looking for random associations of genes within genotypes in the genotypic cluster approach will be methodologically the same as a genotypic analysis to determine whether a population is interbreeding, but the latter requires additional assumptions. The genotypic cluster criterion in sexual species could thus be looked upon simply as a practical application of the biological species concept. However, one may prefer the genotypic cluster criterion to the interbreeding concept, if only because its name emphasizes that the definition is character-based, rather than actually based on interbreeding, and is thus applicable to asexuals as well as sexual species.

If a single geographic race, which previously intergraded at all its boundaries with other geographic races, were to split into two forms that coexist as separate genotypic clusters, we could have a situation where the original polytypic species became paraphyletic. The new species has been derived from only one of the component subspecies. Thus, paraphyly of species would be permitted under this definition, as in both interbreeding and diagnostic concepts.

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Speciation

E. Mayr , in Encyclopedia of Genetics, 2001

Incomplete Speciation and Hybridization

Incipient species often re-establish contact with the parental population owing to range expansion before their isolating mechanisms had been perfected. In the zone of contact, hybridization will now take place and a more or less extensive hybrid zone will develop. Several different outcomes of such an event have been recorded. If the isolating mechanisms were nearly perfect and only a few hybrids occur, the two species will not fuse and natural selection may even lead to an improvement of the isolating mechanisms.

If there is almost indiscriminate hybridization, a more or less permanent hybrid zone will develop due to the continuing elimination of the hybrids and their descendants, since they are of reduced viability. At the same time new hybrids between the two populations continue to be produced. The isolating mechanisms cannot be improved because of this continuous recolonization of the hybrid belt from the two parental populations.

There is some evidence that a highly isolated small hybrid population may, in time, develop its own isolating mechanisms (owing to an absence of recolonization by the two parental populations) and finally become a separate species. This is the most likely explanation for the occurrence of homoploid species coexisting with one or the other parental species, now without hybridizing with them. Such cases have been described in plants and in animals.

Some authors have claimed that two incipient species connected by a hybrid belt might become full species by a process called parapatric speciation. They postulate that the selection pressure against the hybrids would in due time lead to a reduced frequency of hybridization and ultimately to its disappearance. A careful study of all these cases, however, has convinced me that this is unlikely to occur. This is indicated by the high age of some of the hybrid belts between incipient species which had originated in Pleistocene refugia and had re-established contact with each other as much as 8000 to 10000 years ago.

Cases in which an expanding species begins to overlap the range of a closely related species are different. The most advanced colonists of the expanding species may not be able to find conspecific mates and then mate with individuals of the overlapped species. As the expansion continues, sufficient individuals of their own species become available and the hybridization occurs no longer.

The isolating mechanisms are not always perfect. They are 'leaky' as it is said. The result is occasional hybridization, even between good species. The frequency of such occasional hybridization varies among different kinds of organisms (see Species).

In the prokaryotes, unilateral gene exchange between agamospecies is apparently very frequent, even among such distant groups as the eubacteria and the archaebacteria.

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