Hybrid Zones

Abstract

A 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.

Keywords: hybrid zones; divergence; speciation; selection; dispersal; population density; phylogeography

Figure 1.

The hybrid zone in M. m. musculus and M. m. domesticus. (a) The solid purple line indicates the location of the hybrid zone. An approximate position of the Norwegian/Swedish zone is shown as a dashed line. Six extensively studied transects are shown as yellow rectangles. (b) The detailed location of the hybrid zone at the Czech transect (D). Red circles indicate 228 sampling sites. The yellow dashed line depicts the course of the zone based on consensus of six autosomal and nine X‐linked loci. Map by courtesy of Miloš Macholán.

Figure 2.

Effects of environment on hybrid zone structure and position. Two genotypes strongly adapted to two distinct habitats with sample transects taken across (a) a linear zone and (b) a mosaic zone. (c, d) The respective character clines resulting from these transects. Note differences in width and shape. (e) The distribution of individuals through a low‐density region, where the zone is not at the density trough and (f) the dispersal of parental types (arrows) to bring the hybrid zone to rest in the centre of the density trough.

Figure 3.

Shape of sigmoid and stepped clines (blue and red lines, respectively). Widths of each cline (w1 and w2) are measured as the inverse of the maximum slope (dashed lines) at the centre of the zone (c) (i.e. w1>w2). A sigmoid cline consists only of a hyperbolic tangent function (tanh), whereas a stepped cline consists of a steep central region, defined by a tanh function, and shallow tails of introgression, defined by an exponential function. Note that the extent of introgression at the left and right tails can be similar to (symmetric stepped cline) or different from each other (asymmetric stepped cline).

Figure 4.

Phylogeography of European hedgehogs. (a) The ranges of Erinaceus europeus (west) and Erinaceus concolor (east), with (b) an mtDNA‐cytb phylogeny superimposed. Note the deep divergence (12%) between the nominate species, and their cryptic subdivision into further distinct (6%) clades. This indicates separate survival in Mediterranean refugia over several ice ages.

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Further Reading

Arnold ML (2006) Evolution Through Genetic Exchange. Oxford: Oxford University Press.

Buggs RJA (2007) Empirical study of hybrid zone movement. Heredity 99: 301–312.

Coyne JA and Orr HA (2004) Speciation. Sunderland, MA: Sinauer Associates.

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Macholán M, Baird SJE, Munclinger P and Piálek J (eds) (2012) Evolution of the House Mouse. Cambridge Studies in Morphology and Molecules: New Paradigms in Evolutionary Biology. Cambridge: Cambridge University Press.

Noor MAF and Feder JL (2006) Speciation genetics: evolving approaches. Nature Reviews Genetics 7: 851–861.

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How to Cite close
Kawakami, Takeshi, and Butlin, Roger K(Jan 2012) Hybrid Zones. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0001752.pub2]