A theory of oligogenic adaptation of a quantitative trait.

This article develops an analytical framework to characterize the genetic architecture underlying rapid phenotypic adaptation of an additive quantitative trait (QT) under Gaussian stabilizing selection following an environmental shift. The main focus is on how allele frequency changes at multiple loci collectively contribute to adaptation, bridging the classical dichotomy between monogenic selective sweeps and highly polygenic subtle shifts. The authors identify the population-scaled background mutation rate (Θ_bg), defined as Θ_bg = 2N_eμ(d−1) for d loci carrying beneficial alleles, as the key parameter determining the mode of adaptation: low Θ_bg leads to single-locus sweeps, intermediate Θ_bg to oligogenic partial sweeps, and high Θ_bg to polygenic frequency shifts. Their model, assuming equal locus effects and linkage equilibrium, accurately predicts joint allele frequency distributions at arbitrary phenotypic states ("pheno-time") and reveals a quasi-stable limit architecture during adaptation. Simulations show that linkage and diploidy have limited qualitative effects except under complete linkage. The framework highlights that adaptation mode depends more on mutational redundancy than on selection strength or trait basis size, providing a unified theory of oligogenic adaptation that complements existing monogenic and polygenic models.