Migration is the movement of individuals in geographic space. When that movement has genetic consequences, migration determines gene flow. By introducing new genes where they were previously absent, migration increases genetic diversity within populations and reduces the differences among them. As a rule, groups that dwell close to each other tend to exchange more migrants than distant or isolated groups. But there are exceptions, especially during demographic expansions and when the populations are subdivided by geographic or cultural barriers. Rates and patterns of migration can be reconstructed from the analysis of genetic data.

Keywords: gene flow; genetic diversity; island model; stepping‐stone model; isolation by distance; clines; barriers; demic diffusion

Figure 1.

Model of unidirectional migration. Thick dashed arrows represent migration and thin solid arrows represent the effects of inheritance.

Figure 2.

Scheme of Wright's island model. p1, p2,…, p5 are the (different) frequencies of one allele in the five subpopulations shown, and m is the rate of migration, which is the same between all subpopulations. The black dot represents the temporary gene pool formed by the emigrants, before they immigrate into their final subpopulation.

Figure 3.

Two‐dimensional stepping‐stone model of population structure. Each black dot represents a subpopulation, which exchanges with each of its four neighbors the same fraction (m) of migrants at each generation.

Figure 4.

Examples of simulated genealogies for eight genes from a population subdivided into two isolates; genes sampled in either isolate are indicated by open circles and solid circles respectively. (a, b) Two realizations in which isolates exchange migrants with high probability, approximating panmixia; (c, d) two realizations in which isolates exchange migrants with low probability. In all examples, population sizes were the same initially, and increased exponentially through time. In (d), the second sequence from the left suggests migration from the ‘white’ into the ‘black’ subpopulation.



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Barbujani, Guido(Apr 2008) Migration. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0005450.pub2]