polygenic adaptation

Campbell RB 1984 The manifestation of phenotypic selection at constituent loci. I. Stabilizing selection. Evolution 38:1033-1038.

• perhaps Kimura overstated the importance of selection when the population mean deviates from the optimum
• rather, any significant selection at individual loci occurs when the population mean is at its optimum
• we agree with the general consensus that selection at individual loci is often imperceptible

• we are ultimately interested in the nature of selection at individual loci
• this will depend on the background phenotypic value of the population

• regardless of linkage, stabilizing selection should entail extensive negative correlation between loci
• the genetic variance is much less than 2 Σ p(1 − p)

• Δx = 2Δp = sp(1 − p)
• a factor of two between the phenotypic and allele frequency changes ensues from our scaling of the phenotype
• all segregating loci have the same selection coefficient, s
• the total phenotypic change should satisfy Δx = s Σ p(1 − p)

• our conclusions differ from Kimura's intuition
• we find no periods of intense selection entailing extensive shifts of allele frequencies
• if the population size (N) is sufficiently large, disruptive selection at individual loci becomes manifest