Most species show geographic variation in allele frequencies and the means of phenotypic traits. Clines, which are smooth changes in an allele frequency or trait mean, are a very common pattern.
Clines and other patterns can result from local adaptation, which results when selection varies in space.
Gene flow is the mixing of alleles from different populations, eroding differences caused by selection and drift. It results from the dispersal of individuals and their gametes. Gene flow is measured by the migration rate (m) when populations are discrete or patchy, and by the migration variance (σm2) when populations are continuously distributed in space.
FST is a statistic commonly used to describe genetic divergence between two or more populations. In many species with broad geographic ranges, FST increases with the distance between two populations, a pattern called isolation-by-distance. FST varies across the genome, and genomic regions with high FST can be used to find loci that are locally adapted.
When both gene flow and local selection are at work, allele frequencies evolve toward a compromise between them. If gene flow is weak relative to selection, allele frequencies will evolve to what selection favors at each location. If gene flow is relatively strong, allele frequencies will be equalized. Strong gene flow can cause gene swamping, which is when a locally favored allele is lost because migration overwhelms local selection.
In continuous habitats, the widths of clines are determined by the ratio of the migration variance to the strength of local selection. When there is a patch of habitat that selects for a different allele than that favored outside the patch, the locally adapted allele will be lost by gene swamping if the size of the patch is smaller than a critical size determined by the relative strengths of migration and selection.
Tension zones are clines in allele frequencies that result from selection against heterozygotes (underdominance) that acts uniformly in space.
Drift can cause allele frequencies at selectively neutral loci to diverge between populations. Very small rates of migration prevent divergence at neutral loci. The amount of divergence can be used to estimate the amount of gene flow.
Dispersal rates evolve. Higher dispersal is favored by habitat disturbance that causes extinction of local populations, competition between related individuals, and inbreeding. Lower dispersal is favored because movement is often risky and energetically expensive. In a species that is expanding its range, there is an automatic increase at the range’s edge of alleles that enhance dispersal.
Species ranges evolve. Factors that prevent ranges from expanding outward include dispersal barriers, genetic constraints and gene flow that prevents adaptation to more extreme environments, and competition with other species that have adjacent ranges. Global climate change is causing shifts in the ranges of many species, but there is little evidence that species can generally avoid extinction by adapting to the new conditions.