Hybrid Zones

AbstractA hybrid zone occurs where two distinct genetic forms meet, mate and produce offspring with mixed genomes. Such zones may vary in width, length and patchiness, and are found between species, subspecies, races or forms. Stable hybrid zones may be maintained by selection against hybrids, environmental selection, or a combination of the two. A hybrid zone can arise either by direct environmental selection in contiguous populations or by renewed contact between previously isolated populations. Hybrid zones act as semi‐permeable barriers, which allow gene exchange for neutral or adaptive characters, whereas restricting introgression of alleles that contribute to local adaptation or reduced hybrid fitness. The study of genomic regions that experience barriers to gene flow can provide an important window for identifying specific genes and mutations that underlie reproductive isolation and local adaptation. With the help of recent technological advances in development of thousands of molecular markers, distributed genome‐wide, identification of such genomic regions is becoming possible in natural hybrid zones.Key Concepts:A hybrid zone is a narrow geographic region where two genetically distinct populations or species are found in close proximity and hybridise to produce offspring of mixed ancestry.Hybrid zones are widespread, both geographically and across animal and plant taxa.A hybrid zone is maintained by a balance between selection and dispersal.Selective forces can be intrinsic (e.g. selection against less‐fit hybrids) or extrinsic (e.g. environment‐dependent selection).Spatial analysis of allele‐frequency clines across a hybrid zone provides estimates of important population genetic parameters (e.g. selection, dispersal and linkage disequilibrium).Surveys of differential gene flow across a hybrid zone have the potential to localise genomic regions that habour genes responsible for barriers to gene exchange.