Hybrid Speciation


Hybridisation between genetically divergent populations may lead to the formation of new evolutionary lineages. This may occur via the duplication of a hybrid's chromosome complement (allopolyploid speciation) or by the stabilisation of a fertile hybrid segregant (homoploid hybrid speciation). Although more common in plants, both modes of hybrid speciation also occur in animals, including fish, amphibians and insects. The successful origin and establishment of hybrid species is highly dependant on an ability to occupy novel and/or spatially isolated habitats, in order to escape the effects of competition and gene flow from parental species. This ability is favoured by the generation of genetic novelty resulting from the dramatic effects that hybridisation (and genome duplication in allopolyploids) have on modifying genome structure and gene expression. Examples of new allopolyploid and homoploid hybrid species originating within the last 200–300 years are important models for investigating processes involved in hybrid speciation and establishment.

Key Concepts:

  • Hybridisation between genetically divergent populations is promoted by habitat disturbance and may lead to the formation of new evolutionary lineages.

  • Hybrid speciation may occur via the duplication of a hybrid's chromosome complement (allopolyploid speciation) or by the stabilisation of a fertile hybrid segregant (homoploid hybrid speciation).

  • Allopolyploidy leads to instantaneous reproductive isolation between the allopolyploid neospecies and its parents, and is therefore an example of very rapid speciation.

  • Allopolyploidy is very common in higher plants, but less common in animals, although examples are known in insects, crustaceans, molluscs, fish, amphibians, reptiles and mammals.

  • Homoploid hybrid speciation is thought to be relatively rare, although an increasing number of homoploid hybrid species of animals, plants and fungi are being recognised using molecular markers.

  • Homoploid hybrid speciation can be completed within a few generations, although several hundred generations may be required before the genome of the hybrid neospecies is fully stabilised.

  • Considerable genetic novelty is generated in the early stages of allopolyploidy and homoploid hybrid speciation due to the marked effects that hybridisation (and genome duplication in the case of allopolyploidy) have on modifying genome structure and gene expression.

  • Such genetic novelty aids the establishment of hybrid neospecies in ecologically divergent habitats, thus reducing the likelihood of competition and crossing with parents.

  • Several examples are known of new allopolyploid and homoploid hybrid species originating within the last 200–300 years and these have become important models for investigating processes involved in hybrid speciation and establishment.

Keywords: allopolyploidy; hybridisation; genome merger; genome duplication; introgression; habitat disturbance; hybrid fitness; reproductive isolation; speciation

Figure 1.

(a) Allotetraploid formation through genome doubling of a sterile F1 hybrid between two diploid species. (b) Allotetraploid formation through fusion of unreduced gametes produced by two hybridising diploid species. (c) Stable diploid recombinant offspring produced by two hybridising diploid species. If one or more of these offspring or later generation recombinants are reproductively isolated from the parental species by pre‐zygotic and/or post‐zygotic barriers, they may be considered to be a new homoploid hybrid species.

Figure 2.

Origin of the neoallopolyploid Spartina anglica. This occurred in Southampton Water, UK, in the 1870s. Chromosome numbers are in brackets. Photo by Stefan Nehring.

Figure 3.

Origins of the neoallopolyploids, Tragopogon mirus and Tragopogon miscellus. First found in nature in Washington State, USA, in the 1940s. Soltis and Soltis (). Reproduced by permission of Annual Reviews.

Figure 4.

Hybrid origins of new Senecio taxa in the British Isles. Dates of first records are in red. Senecio squalidus (Oxford ragwort) is a homoploid hybrid species originating from hybridisation between S. aethnensis and S. chrysanthemifolius, which occur at high and low altitudes, respectively, on Mount Etna, Sicily. Senecio cambrensis (Welsh groundsel) is an allohexaploid that originated by genome duplication of the sterile triploid hybrid (S. x baxteri) formed between S. squalidus and the native nonradiate form of Groundsel (S. vulgaris (nr)). Senecio eboracensis (York radiate groundsel) is a fertile tetraploid hybrid thought to have originated through fusion of an unreduced gamete of S. squalidus with a normal gamete of S. vulgaris (nr). Senecio vulgaris (r) is a radiate form of Groundsel that originated through backcrossing of S. x baxteri to S. vulgaris (n). Photo of S. squalidus by Enrico Coen and Minsung Kim.

Figure 5.

Recently originated homoploid hybrid animal species. (a) The hybrid invasive sculpin (Cottus sp., top) originated following hybridisation between C. rhenanus (top) and C. perifretum. (b) Italian sparrow (Passer italiae). (c) Female Appalachian tiger swallowtail butterfly (Papilio appalachiensis). (d) Lonicera fly (note: the photo is of Rhagoletis pomonella which is morphologically indistinguishable from the hybrid Lonicera fly). Photos by Andreas Hartl, Elin Hjelle, Krushnamegh Kunte and Guy Bush, respectively.



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Further Reading

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Levin DA (2002) The Role of Chromosomal Change in Plant Evolution. Oxford: Oxford University Press.

Mallet J (2007) Hybrid speciation. Nature 446: 279–283.

Rieseberg LH and Wendel J (1993) Introgression and its consequences in plants. In: Harrison R (ed.) Hybrid Zones and the Evolutionary Process, pp. 70–109. New York: Oxford University Press.

Stebbins GL (1959) The role of hybridization in evolution. Proceedings of the American Philosophical Society 103: 231–251.

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Abbott, Richard J, and Rieseberg, Loren H(Jun 2012) Hybrid Speciation. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0001753.pub2]