polygenic adaptation - senecio7の日記

Jain K & Stephan W 2017 Modes of rapid polygenic adaptation. Mol Biol Evol 34:3169-3175.

in contrast to the classical model of directional selection (Charlesworth and Charlesworth 2010), here the strength of selection also depends on the distance from the new phenotypic optimum

the directional selection model also allows us to predict the minimum size of the phenotypic effect required for a selective sweep to occur at a major locus
when most effects are small, our analysis shows that an effect size larger than the initial variance is required for a large change in the allele frequency
for exponentially distributed effects, the probability of such events is exceedingly small for large ℓ and therefore selective sweeps are unlikely when a phenotypic trait is controlled by many small-effect loci
when most effects are large, we find that the allele frequency at a locus with an effect size larger than the mean effect may undergo a large shift
as many loci satisfy this condition, selective sweeps occur at several major loci when many large-effect loci determine a trait

when most effects are large, in addition to classical sweeps, occasionally we find large allele frequency shifts that resemble sweeps to some extent but are very slow and thus do not occur within the short-term phase in which the classical sweeps are predicted

to our knowledge, the only method that appears to be suitable for detecting genomic signatures of rapid polygenic adaptation is a new technique that focuses on patterns of variation around each selected SNP to infer recent changes in the relative frequencies of the two alleles (Field et al. 2016)
the terminal branches of the genealogy tend to be shorter for the favored allele than for the other one
hence haplotypes carrying the favored allele tend to carry fewer singletons

in the short-term phase, the response of the allele frequencies to an environmental change is correlated in the sense that the majority of them (if not all) shift in the same direction

fast polygenic adaptation may be caused by two qualitatively very different mechanisms
strong positive directional selection (leading to selective sweeps) at a few loci of large effects
subtle frequency shifts of alleles at many loci of small effects

we ignored the findings of association studies that selection affecting one trait may often affect many other traits (pleiotropy)