Hybrid Zones

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Abstract Hybrid zones are narrow geographic regions where different populations, races or interbreeding species meet and mate, producing mixed ‘hybrid’ offspring. They are relatively common and can be found in a diverse range of organisms and environments. The study of hybrid zones has played an important role in our understanding of the origin of species, with hybrid zones having been described as ‘natural laboratories’. This is because they allow us to study, in situ , the conditions and evolutionary forces that enable divergent taxa to remain distinct despite some ongoing gene exchange between them. Key Concepts Hybrid zones are narrow geographic regions where different populations, races or interbreeding species meet and mate, producing mixed ‘hybrid’ offspring. They are relevant to understanding local adaptation and speciation as they show us how taxa can remain distinct despite gene flow. Hybrid zones can occur at environmental boundaries between taxa adapted to different habitats or separate taxa with genetic incompatibilities. Many hybrid zones are maintained by a balance between selection and gene flow, creating sharp spatial gradients (clines) in traits and allele frequencies. Extensive theory has been developed that quantitatively relates cline shapes to evolutionary processes in hybrid zones. The strength of selection can vary across the genome and phenome, which causes some loci or phenotypes to show sharper clines than others. Hybrid zones can form in a continuous population (primary origin) or when two populations diverge in isolation and come into secondary contact (secondary origin). The long‐term outcomes of hybridisation can vary: Hybrid zones can collapse, move in space or be maintained as stable phenomena. An array of methods can be applied to empirical data collected from hybrid zones, in particular to identify loci under selection. Emerging methods will ensure that hybrid zones will continue to advance our understanding of speciation in the future.