Hybridization, where distinguishable populations meet and interbreed resulting in hybrid offspring, is useful for understanding the mechanisms involved in the evolution of reproductive isolation. Hybridization can range from interactions with near-complete reproductive isolation, to stable zones, to complete intergradation. Several models explain how stable hybrid zones are maintained, but each model emphasizes a special case and some zones fit more than one model. The models also do not emphasize moving hybrid zones or occasional hybridization events that add genetic variation to the parental populations. This essay unifies the current models along a continuum of selection pressures organized by geography. Current models differ in assumed hybrid fitness and whether selection is intrinsic or extrinsic. The type of selection is a less general category than the direction of selection (for or against hybrids); multiple types of selection can exist in each zone depending on trait. Thus, the diversity and combinations of selection types make selection direction the most general category for sorting hybrid zones, and variation in selection over geography shapes the spatial extent (in concert with dispersal), movement, and stability of a hybrid zone. This framework is useful for considering zone structure both at the whole organism level and at individual genes or traits. Unifying hybrid zones models by the geographic range of selection pressure organizes each zone into context along the continuum regardless of its fit within one of the classic models, helping us explicitly consider what assumptions are made about the zone and providing direction for further research.
This is a preview of subscription content, access via your institution.
Buy single article
Instant access to the full article PDF.
Tax calculation will be finalised during checkout.
Subscribe to journal
Immediate online access to all issues from 2019. Subscription will auto renew annually.
Tax calculation will be finalised during checkout.
Abbott, R. J., Albach, D., Ansell, S., Arntzen, J. W., Baird, S. J. E., Bierne, N., et al. (2013). Hybridization and speciation. Journal of Evolutionary Biology, 26(2), 229–246.
Abbott, R. J., & Brennan, A. C. (2014). Altitudinal gradients, plant hybrid zones and evolutionary novelty. Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences, 369, 20130346.
Arnold, M. L. (1997). Natural hybridization and evolution. New York: Oxford University Press.
Arntzen, J. W., & Wallis, G. P. (1991). Restricted gene flow in a moving hybrid zone of the newts Triturus cristatus and T. marmoratus in western France. Evolution, 45(4), 805–826.
Barton, N. H. (1992). On the spread of new gene combinations in the third phase of Wright’s shifting-balance. Evolution, 46(2), 551–557.
Barton, N. H., & Hewitt, G. M. (1981). The genetic basis of hybrid inviability in the grasshopper Podisma pedestris. Heredity, 47(3), 367–383.
Barton, N. H., & Hewitt, G. M. (1985). Analysis of hybrid zones. Annual Review of Ecology and Systematics, 16, 113–148.
Barton, N. H., & Hewitt, G. M. (1989). Adaptation, speciation and hybrid zones. Nature, 341, 497–503.
Behm, J. E., Ives, A. R., & Boughman, J. W. (2010). Breakdown in postmating isolation and the collapse of a species pair through hybridization. American Naturalist, 175(1), 11–26.
Bert, T. M., & Arnold, W. S. (1995). An empirical test of predictions of two competing models for the maintenance and fate of hybrid zones: Both models are supported in a hard-clam hybrid zone. Evolution, 49(2), 276–289.
Björklund, M. (2013). The unpredictable impact of hybridization. Journal of Evolutionary Biology, 26(2), 274–275.
Brumfield, R. T., Jernigan, R. W., McDonald, D. B., & Braun, M. J. (2001). Evolutionary implications of divergent clines in an avian (Manacus: Aves) hybrid zone. Evolution, 55(10), 2070–2087.
Buggs, R. J. A. (2007). Empirical study of hybrid zone movement. Heredity, 99(3), 301–312.
Butlin, R. K., Galindo, J., & Grahame, J. W. (2008). Sympatric, parapatric or allopatric: The most important way to classify speciation? Philosophical Transactions of the Royal Society B: Biological Sciences, 363(1506), 2997–3007.
Campbell, D. R., & Waser, N. M. (2007). Evolutionary dynamics of an Ipomopsis hybrid zone: Confronting models with lifetime fitness data. American Naturalist, 169(3), 298–310.
Campbell, D. R., Waser, N. M., Aldridge, G., & Wu, C. A. (2008). Lifetime fitness in two generations of Ipomopsis hybrids. Evolution, 60(10), 2616–2627.
Carling, M. D., & Zuckerberg, B. (2011). Spatio-temporal changes in the genetic structure of the Passerina bunting hybrid zone. Molecular Ecology, 20, 1166–1175.
Curry, C. M., & Patten, M. A. (2014). Current and historical extent of phenotypic variation in the Tufted and Black-crested Titmouse (Paridae) hybrid zone in the southern Great Plains. American Midland Naturalist, 171, 271–300.
Dasmahapatra, K. K., Blum, M. J., Aiello, A., Hackwell, S., Davies, N., Bermingham, E. P., & Mallet, J. (2002). Inferences from a rapidly moving hybrid zone. Evolution, 56(4), 741–753.
Donovan, L. A., Rosenthal, D. R., Sanchez-Velenosi, M., Rieseberg, L. H., & Ludwig, F. (2010). Are hybrid species more fit than ancestral parent species in current hybrid species habitats? Journal of Evolutionary Biology, 23, 805–816.
Ellegren, H., Smeds, L., Burri, R., Olason, P. I., Backström, N., Kawakami, T., et al. (2012). The genomic landscape of species divergence in Ficedula flycatchers. Nature, 491(7426), 756–760.
Endler, J. A. (1973). Gene flow and population differentiation. Science, 179(4070), 243–250.
Endler, J. A. (1977). Geographic variation, speciation, and clines. Monographs in Population Biology, 10, 1–246.
Fitzpatrick, B. M., & Shaffer, H. B. (2004). Environment-dependent admixture dynamics in a tiger salamander hybrid zone. Evolution, 58(6), 1282–1293.
Gompert, Z., Lucas, L. K., Nice, C. C., Fordyce, J. A., Forister, M. L., & Buerkle, C. A. (2012). Genomic regions with a history of divergent selection affect fitness of hybrids between two butterfly species. Evolution, 66(7), 2167–2181.
Good, T. P., Ellis, J. C., Annett, C. A., & Pierotti, R. (2000). Bounded hybrid superiority in an avian hybrid zone: Effects of mate, diet, and habitat choice. Evolution, 54(5), 1774–1783.
Gow, J. L., Peichel, C. L., & Taylor, E. B. (2007). Ecological selection against hybrids in natural populations of sympatric threespine sticklebacks. Journal of Evolutionary Biology, 20, 2173–2180.
Grant, P. R. (1993). Hybridization of Darwin’s finches on Isla Daphne Major, Galapagos. Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences, 340(1291), 127–139.
Grant, P. R., & Grant, B. R. (1994). Phenotypic and genetic effects of hybridization in Darwin’s finches. Evolution, 48(2), 297–316.
Grant, P. R., & Grant, B. R. (2002). Unpredictable evolution in a 30-year study of Darwin’s finches. Science, 296(5568), 707–711.
Haavie, J., Borge, T., Bures, S., Garamszegi, L. Z., Lampe, H. M., Moreno, J., et al. (2004). Flycatcher song in allopatry and sympatry—Convergence, divergence and reinforcement. Journal of Evolutionary Biology, 17, 227–237.
Hedrick, P. W. (2013). Adaptive introgression in animals: Examples and comparison to new mutation and standing variation as sources of adaptive variation. Molecular Ecology, 22, 4606–4618.
Howard, D. J., Waring, G. L., Tibbets, C. A., & Gregory, P. G. (1993). Survival of hybrids in a mosaic hybrid zone. Evolution, 47(3), 789–800.
Huerta-Sánchez, E., DeGiorgio, M., Pagani, L., Tarekegn, A., Ekong, R., Antao, T., et al. (2013). Genetic signatures reveal high-altitude adaptation in a set of Ethiopian populations. Molecular Biology and Evolution, 30(8), 1877–1888.
Ingleby, F. C., Hunt, J., & Hosken, D. J. (2010). The role of genotype-by-environment interactions in sexual selection. Journal of Evolutionary Biology, 23(10), 2031–2045.
Kawakami, T., Butlin, R. K., Adams, M., Paull, D. J., & Cooper, S. J. B. (2009). Genetic analysis of a chromosomal hybrid zone in the Australian morabine grasshoppers (Vandiemenella viatica species group). Evolution, 63(1), 139–152.
Kirk, H., Vrieling, K., & Klinkhamer, P. G. L. (2005). Maternal effects and heterosis influence the fitness of plant hybrids. New Phytologist, 166, 685–694.
Kisel, Y., & Barraclough, T. G. (2010). Speciation has a spatial scale that depends on levels of gene flow. American Naturalist, 175(3), 316–334.
Kleindorfer, S., O’Connor, J. A., Dudaniec, R. Y., Myers, S. A., Robertson, J., & Sulloway, F. J. (2014). Species collapse via hybridization in Darwin’s tree finches. American Naturalist, 183(3), 325–341.
Krosby, M., & Rohwer, S. (2010). Ongoing movement of the Hermit Warbler X Townsend’s Warbler hybrid zone. PLoS ONE, 5(11), e14164.
Mallet, J. (2008). Hybridization, ecological races and the nature of species: Empirical evidence for the ease of speciation. Philosophical Transactions of the Royal Society B: Biological Sciences, 363(1506), 2971–2986.
Mallet, J., & Barton, N. H. (1989). Strong natural selection in a warning-color hybrid zone. Evolution, 43(2), 421–431.
Marshall, J. L., Arnold, M. L., & Howard, D. J. (2002). Reinforcement: The road not taken. Trends in Ecology and Evolution, 17(12), 558–563.
Martin, L. J., & Cruzan, M. B. (1999). Patterns of hybridization in the Piriqueta caroliniana complex in central Florida: Evidence for an expanding hybrid zone. Evolution, 53(4), 1037–1049.
Martin, C. H., & Wainwright, P. C. (2013a). Multiple fitness peaks on the adaptive landscape drive adaptive radiation in the wild. Science, 339(6116), 208–211.
Martin, C. H., & Wainwright, P. C. (2013b). On the measurement of ecological novelty: Scale-eating pupfish are separated by 168 my from other scale-eating fishes. PLoS ONE, 8(8), e71164.
Matute, D. M., Novak, C. J., & Coyne, J. A. (2009). Temperature-based extrinsic reproductive isolation in two species of Drosophila. Evolution, 63(3), 595–612.
Miglia, K. J., McArthur, E. D., Redman, R. S., Rodriguez, R. J., Zak, J. C., & Freeman, D. C. (2007). Genotype, soil type, and locale effects on reciprocal transplant vigor, endophyte growth, and microbial functional diversity of a narrow sagebrush hybrid zone in Salt Creek Canyon, Utah. American Journal of Botany, 94(3), 425–436.
Moore, W. S. (1977). An evaluation of narrow hybrid zones in vertebrates. Quarterly Review of Biology, 52, 263–277.
Moriarty Lemmon, E., & Lemmon, A. R. (2010). Reinforcement in chorus frogs: Lifetime fitness estimates including intrinsic natural and sexual selection against hybrids. Evolution, 64(6), 1748–1761.
Nolte, A. W., Gompert, Z., & Buerkle, C. A. (2009). Variable patterns of introgression in two sculpin hybrid zones suggest that genomic isolation differs among populations. Molecular Ecology, 18, 2615–2627.
Noor, M. A. F. (1999). Reinforcement and other consequences of sympatry. Heredity, 83, 503–508.
Nosil, P. (2013). Degree of sympatry affects reinforcement in Drosophila. Evolution, 67(3), 868–872.
Pardo-Diaz, C., Salazar, C., Baxter, S. W., Merot, C., Figueiredo-Ready, W., Joron, M., et al. (2012). Adaptive introgression across species boundaries in Heliconius butterflies. PLoS Genetics, 8(6), e1002752.
Parsons, T. J., Olson, S. L., & Braun, M. J. (1993). Unidirectional trait spread of secondary sexual plumage traits across an avian hybrid zone. Science, 260, 1643–1646.
Patten, M. A., & Campbell, K. F. (2000). Typological thinking and the conservation of subspecies: The case of the San Clemente Island Loggerhead Shrike. Diversity and Distributions, 6(4), 177–188.
Perry, W. L., Feder, J. L., Dwyer, G., & Lodge, D. M. (2001). Hybrid zone dynamics and species replacement between Orconectes crayfishes in a northern Wisconsin lake. Evolution, 55(6), 1153–1166.
Pfennig, K. S. (2007). Facultative mate choice drives adaptive hybridization. Science, 318, 965–967.
Rand, D. M., & Harrison, R. G. (1989). Ecological genetics of a mosaic hybrid zone: Mitochondrial, nuclear, and reproductive differentiation of crickets by soil type. Evolution, 43(2), 432–449.
Reudink, M. W., Mech, S. G., Mullen, S. P., & Curry, R. L. (2007). Structure and dynamics of the hybrid zone between Black-capped Chickadee (Poecile atricapillus) and Carolina Chickadee (P. carolinensis) in southeastern Pennsylvania. Auk, 124(2), 463–478.
Rieseberg, L. H., Raymond, O., Rosenthal, D. M., Lai, Z., Livingstone, K., Nakazato, T., et al. (2003). Major ecological transitions in wild sunflowers facilitated by hybridization. Science, 301, 1211–1216.
Rolán-Alvarez, E., Johannesson, K., & Erlandsson, J. (1997). The maintenance of a cline in the marine snail Littorina saxatilis: The role of home site advantage and hybrid fitness. Evolution, 51(6), 1838–1847.
Rosenfield, J. A., & Kodric-Brown, A. (2003). Sexual selection promotes hybridization between Pecos pupfish, Cyprinodon pecosensis and sheepshead minnow, C. variegatus. Journal of Evolutionary Biology, 16, 595–606.
Rowher, S., & Martin, P. R. (2007). Time since contact and gene flow may explain variation in hybrid frequencies among three Dendroica townsendi × D. occidentalis (Parulidae) hybrid zones. Auk, 124(4), 1347–1358.
Sætre, G.-P., Moum, T., Bureš, S., Král, M., Adamjan, M., & Moreno, J. (1997). A sexually selected character displacement in flycatchers reinforces premating isolation. Nature, 387(6633), 589–592.
Sambetti, J. M. B., Strasburg, J. L., Ortiz-Barrientos, D., Baack, E. J., & Rieseberg, L. H. (2012). Reconciling extremely strong barriers with high levels of gene exchange in annual sunflowers. Evolution, 66(5), 1459–1473.
Scascitelli, M., Whitney, K. D., Randell, R. A., King, M., Buerkle, C. A., & Rieseberg, L. H. (2010). Genome scan of hybridizing sunflowers from Texas (Helianthus annus and H. debilis) reveals asymmetric patterns of introgression and small islands of genomic differentiation. Molecular Ecology, 19, 521–541.
Servedio, M. R., & Noor, M. A. F. (2003). The role of reinforcement in speciation: Theory and data. Annual Review of Ecology Evolution and Systematics, 34, 339–364.
Shaw, K. L., & Mendelson, T. C. (2013). The targets of selection during reinforcement. Journal of Evolutionary Biology, 26(2), 286–287.
Song, Y., Endepols, S., Klemann, N., Richter, D., Matuschka, F.-R., Shih, C.-H., et al. (2011). Adaptive introgression of anticoagulant rodent poison resistance by hybridization between Old World mice. Current Biology, 21, 1296–1301.
Vallender, R., Robertson, R. J., Friesen, V. L., & Lovette, I. J. (2007). Complex hybridization dynamics between golden-winged and blue-winged warblers (Vermivora chrysoptera and Vermivora pinus) revealed by AFLP, microsatellite, intron and mtDNA markers. Molecular Ecology, 16(10), 2017–2029.
Whitney, K. D., Randell, R. A., & Rieseberg, L. H. (2006). Adaptive introgression of herbivore resistance traits in the weedy sunflower Helianthus annuus. American Naturalist, 167(6), 794–807.
Whitney, K. D., Randell, R. A., & Rieseberg, L. H. (2010). Adaptive introgression of abiotic tolerance traits in the sunflower Helianthus annuus. New Phytologist, 187(1), 230–239.
Whittemore, A. T., & Schall, B. A. (1991). Interspecific gene flow in sympatric oaks. Proceedings of the National Academy of Sciences USA, 88, 2540–2544.
Woodruff, D. S. (1973). Natural hybridization and hybrid zones. Systematic Zoology, 22(3), 213–218.
Woodruff, D. S. (1979). Postmating reproductive isolation in Pseudophryne and the evolutionary significance of hybrid zones. Science, 203(4380), 561–563.
Thanks to M. A. Patten, D. Landoll, A. J. Contina, G. Wellborn, W. T. Honeycutt, and four anonymous reviewers for insightful discussion and comments which have much improved this manuscript. The author was supported by a University of Oklahoma Alumni Fellowship and a U.S. Department of Education GAANN Fellowship.
Conflict of interest
The author declares she has no conflict of interest.
About this article
Cite this article
Curry, C.M. An Integrated Framework for Hybrid Zone Models. Evol Biol 42, 359–365 (2015). https://doi.org/10.1007/s11692-015-9332-9