The panels illustrate the allele frequency dynamics of a novel variant (red) in a population, when the variant is advantageous in some locations and disadvantageous in others (A, B) or when local balancing selection (e.g. due to heterozygote advantage) is operating (C, D). These scenarios give rise to a stable polymorphism (where the novel and ancestral variant persist in the population). In these models the novel variant will not replace the ancestral variant - the novel variant will simply become more common in the regions where it is advantageous and can spread to via dispersal. For every selected allele, a representative neutral allele of similar average frequency (blue) is shown for reference.
(A) Allele is favored in some patches and disfavored in others; in this situation, the allele will be fixed in the geographic patches where it is advantageous, and absent in regions where it is disadvantageous, with clines of frequency expected along the contact points between the two regions
(B) Allele is favored in one geographic extreme and disfavoured in the opposite extreme. If the transition from being advantageous and disadvantageous occurs across a geographic range, rather than being abrupt, then broader clines are expected.
(C) Local balancing selection that varies in intensity across space. When selection intensities vary across space, the local equilibrium frequencies will vary across space depending on the environmental factors driving selection and in turn generate correlations of allele frequencies with environmental factors that are not transient. A classic example is the sickle-cell mutation, which is found in high frequency in regions where malarial endemia is high and decreases in frequency as the prevalence of malaria decreases.
(D) Local balancing selection that is constant across space; balancing selection can lead to exceptionally constant allele frequencies over space.
Scenarios A–C will generate correlations between allele frequency and environmental factors underlying variation in selection.