Reinforcement

Abstract

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.

Crossing species boundaries : the hybrid histories of the true geese

“…The analyses of gene flow and their position on the speciation continuum suggest that the diversification of True Geese may have been influenced by reinforcement, i.e. "the process, in which traits that increase prezygotic isolation between two differentiated populations are favoured due to natural selection against the production of unfit hybrids or otherwise maladaptive interbreeding" (Saetre, 2012). The exact selective pressures against hybrids remain to be determined by experiments and field observations.…”