In This Article Expand or collapse the "in this article" section Hybridization and Diversification

  • Introduction
  • General Overviews
  • Books
  • Journals
  • Hybridization and Radiation
  • Hybridization and Net Diversification Rates
  • Alternative Patterns and Explanations
  • Hybridization and Loss of Diversity

Evolutionary Biology Hybridization and Diversification
Nora Mitchell
  • LAST REVIEWED: 27 November 2023
  • LAST MODIFIED: 27 November 2023
  • DOI: 10.1093/obo/9780199941728-0152


There are many acknowledged and documented evolutionary outcomes of hybridization, ranging from extinction or “dead ends” to the idea that hybridization provides evolutionary fuel and can promote diversification. Although any lineages capable of hybridizing would be classified as one species under the biological species concept, many well defined taxonomic species are capable of hybridizing. The definition of “diversification” in this context is varied—it can mean an increase in genetic diversity within a population or species, hybrid speciation, an increase in the number of lineages within a clade at many taxonomic levels, or net diversification rates. The literature connecting hybridization and diversification reflects these varied definitions. The topic of hybridization and speciation is the focus of the separate Oxford Bibliographies in Evolutionary Biology article “Hybrid Speciation,” and that bibliography also discusses hybridization and adaptive radiations. Although speciation can fall under the overall umbrella of diversification, it is not explicitly covered here. A large body of work has been dedicated to connecting hybridization to evolutionary radiation, either through phylogenetic evidence of ancient hybridization in lineages that have undergone rapid diversification (the “hybrid swarm” hypothesis) or through documentation of reticulate evolution. Other studies have examined associations between hybridization and net diversification rates in macroevolutionary contexts. In addition to diversification in terms of taxa, hybridization can also result in other aspects of diversification, such as an increase in phenotypic variation, ecological niche, or geographic range expansion. Hybridization can also increase genome diversification, through increased allelic diversity, genome-wide diversity, larger changes in chromosome structure, and even polyploidy. Though many hypotheses suggest that hybridization can facilitate or increase diversification in its varied forms, other factors are also at play, and faster diversification can also potentially result in increased opportunities for hybridization via incomplete reproductive isolation. Hybridization can also result in a loss of diversity through extinction or genome swamping under other circumstances. Finally, phylogenetic comparative methods can be used to detect associations between diversification and other characters or character states, including hybridization.

General Overviews

There are several foundational works or review papers that introduce or synthesize evidence for the idea that hybridization can promote evolution or radiation, although these may or may not explicitly address “diversification.” During the modern synthesis, botanists put forth ideas about hybridization and evolution. Anderson and Stebbins 1954 is one of the first to introduce this concept, while Stebbins 1959 reviews and synthesizes evidence for the role of hybridization in generating genetic diversity (using both plant and animal examples). Dowling and Secor 1997 addresses the frequency of hybridization in animals. Seehausen 2004 explicitly addresses the role of hybridization in evolutionary radiation through the hybrid swarm hypothesis. Marques, et al. 2019 takes a broader approach, defining “combinatorial mechanisms” to include any mixing of genetic variation into new combinations, and discusses evidence for speciation and radiation arising via these mechanisms.

  • Anderson, E., and G. L. Stebbins. 1954. Hybridization as an evolutionary stimulus. Evolution 8.4: 378–388.

    DOI: 10.2307/2405784

    Introduces the power of hybridization as a potential force of “creative evolution” of new “adaptive systems,” either through allopolyploidy or introgressive hybridization, with a focus on plants. Includes examples from human domestication and disturbed habitats with paleontological/geological evidence, as well as contemporary examples in plants.

  • Dowling, T. E., and C. L. Secor. 1997. The role of hybridization and introgression in the diversification of animals. Annual Review of Ecology and Systematics 28.1: 593–619.

    DOI: 10.1146/annurev.ecolsys.28.1.593

    Reviews historical thinking about the role hybridization in evolution in animals, including diversification through allopolyploidy and introgression. Hybridization in animals may be rare but this does not imply insignificance, and the authors call for changes in attitude and increased study.

  • Marques, D. A., J. I. Meier, and O. Seehausen. 2019. A combinatorial view on speciation and adaptive radiation. Trends in Ecology & Evolution 34.6: 531–544.

    DOI: 10.1016/j.tree.2019.02.008

    Reconciles rapid speciation and slow mutation rates by positing that multiple “combinatorial” mechanisms contribute to the production of novel genetic combinations from existing, ancient variants. Focus is on this ancient variation in both rapid speciation and recent radiation, with example study systems. Makes testable predictions regarding phenotypic evolution, reproductive isolation, and haplotypes of large effect and includes outstanding questions in the field.

  • Seehausen, O. 2004. Hybridization and adaptive radiation. Trends in Ecology & Evolution 19.4: 198–207.

    DOI: 10.1016/j.tree.2004.01.003

    Puts forth the “hybrid swarm” hypothesis that hybridization can facilitate adaptive radiation. Hybridization generates variation (such as through transgressive segregation), and ecological conditions that promote hybridization also promote radiation. Also puts forth the “syngameon hypothesis,” where hybridization between members of a radiation further increase genetic variation and can facilitate additional speciation. Statistics and population genetics can disentangle alternate explanations, while crossing experiments, genealogical discordance, and biogeography are needed to test predictions of this hypothesis.

  • Stebbins, G. L. 1959. The role of hybridization in evolution. Proceedings of the American Philosophical Society 103.2: 231–251.

    Reviews the overall hypothesis that hybridization can be a major source of genetic variation to promote evolution. Reviews studies of hybridization in plants and animals, acknowledging that all plant groups studied have evidence for hybridization, while in animals it appears rarer. Counters the idea of hybrid sterility with examples documenting a wide variety of outcomes and the potential for both polyploidy and introgression to stabilize hybrids.

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