Migration-Selection Balance

Migration-selection balance is the evolutionary equilibrium that arises when the homogenizing force of gene flow opposes the differentiating force of local natural selection. Populations adapting to distinct environments accumulate locally advantageous alleles, but migrants from other populations introduce alleles adapted to different conditions. The population's genetic composition settles at a point where the rate of change due to selection equals the rate of change due to migration.

The balance is quantified by the ratio of selection coefficient (s) to migration rate (m). When s >> m, local adaptation persists despite gene flow; when m >> s, the population becomes a genetic average of its sources. The critical threshold depends on population size, the number of loci under selection, and the spatial structure of the landscape.

Migration-selection balance is central to understanding local adaptation, ecological speciation, and the limits of species range expansion. A population at the edge of a species' range may be prevented from adapting to the edge environment because gene flow from the range core swamps local adaptation — a phenomenon known as gene swamping.

The migration-selection balance model treats migration as a scalar rate and selection as a uniform force. Both assumptions are false. Real landscapes are heterogeneous, migration is network-structured, and selection varies across loci and through time. The model is a useful null hypothesis, but treating it as descriptive of real populations is like treating a point mass as descriptive of a galaxy.