Reinforcement is the process, in which traits that increase pre‐zygotic isolation between two differentiated populations are favoured due to natural selection against the production of unfit hybrids or otherwise maladaptive interbreeding. It is a central process in many models of speciation and it is the only known process in which natural selection acts directly to promote speciation. Reinforcement was for long considered a controversial idea despite the fact that early theoretical models showed that the process could work under apparently realistic biological assumptions. Clarification of concepts along with the publications of refined theoretical models as well as convincing comparative and empirical case studies has more or less resolved the controversy; reinforcement works, but it is still debated how important it is in speciation.

Key Concepts:

  • Reinforcement is an increase in pre‐zygotic isolation between differentiated taxa caused by natural selection against maladaptive hybridisation.

  • Reinforcement may contribute to speciation in previously geographically isolated taxa that experience secondary contact as well as to parapatric and sympatric modes of speciation.

  • Reinforcement can lead to a sympatric divergence (character displacement) in any trait that reduces the likelihood of mating or fertilisation between diverging taxa, including secondary sexual traits, mating behaviour and traits affecting enzymatic communication between egg and sperm.

  • Population genetic models show that assortative mating can increase as a response to genetic incompatibilities at other loci, but the process is sensitive to recombination.

  • Factors that reduce recombination, including strong assortative mating, strong incompatibility selection and physical linkage promotes speciation by reinforcement.

  • Comparative evidence suggests that pre‐zygotic isolation is stronger in sympatric species pairs than in allopatric ones, consistent with reinforcement.

  • In well‐documented empirical cases of reinforcement genetic factors that reduce recombination has been identified and incompatibility selection tends to be strong.

Keywords: speciation; reproductive isolation; mate recognition; character displacement; assortative mating

Figure 1.

Conditions for reinforcement based on 30 runs of the Liou and Price simulation. Initial divergence in the male mating trait (measured in phenotypic standard deviations) was paralleled by divergence in female preference. There was free recombination. The possible outcomes were speciation by reinforcement, extinction of one of the starting populations, or merging of the populations due to hybridisation. From Liou and Price . Copyright © 1994 The Society for the Study of Evolution.

Figure 2.

The classic example of increased mating signal divergence between sympatric populations, in frogs of the genus Litoria in Australia. Litoria ewingi occurs in the west and L. verreauxi in the east, with a substantial area of overlap. Oscillograms of L. ewingi (E) and L. verreauxi (V) calls show how a marked difference in pulse number and rate is maintained throughout the area of sympatry despite the similarity between the calls of allopatric populations. From Littlejohn . Copyright © 1965 The Society for the Study of Evolution.

Figure 3.

The relationship between pre‐zygotic isolation and genetic distance (Nei's D) in (a) allopatric and (b) sympatric pairs of Drosophila species showing the markedly greater isolation among sympatric pairs at genetic distances of less than .5. From Coyne and Orr . Copyright © 1989 The Society for the Study of Evolution.

Figure 4.

Plumage comparisons between European flycatcher species from allopatric and sympatric localities (Pied flycatcher, Ficedula hypolenca and collared flycatcher, Ficedula albicollis). The pattern of sympatric character divergence has been attributed to reinforcement selection on female mate preferences (Sætre et al., ).



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Sætre G‐P and Sæther SA (2010) Ecology and genetics of speciation in Ficedula flycatchers. Molecular Ecology 19: 1091–1106.

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Sætre, Glenn‐Peter(Sep 2012) Reinforcement. In: eLS. John Wiley & Sons Ltd, Chichester. [doi: 10.1002/9780470015902.a0001754.pub3]