polygenic adaptation - senecio7の日記

Chevin L-M & Hospital F 2008 Selective sweep at a quantitative trait locus in the presence of background genetic variation. Genetics 180:1645-1660.

there are two main views of how adaptation occurs as a genetic mechanism
the first one, periodic selection (Atwood et al. 1951), considers that adaptation proceeds by the successive fixations of beneficial mutations that sweep through the population one after the other
in microbes, the large population sizes make purifying selection very efficient
which strongly limits the amount of slightly deleterious polymorphisms
as a consequence, phenotypic traits have little genetic variance in such organisms, except the variance generated anew by mutation
moreover, asexuality induces clonal interference
which is expected to prevent simultaneous segregation at several loci

the second view of adaptation is that of quantitative genetics, in which there is simultaneous selection at many loci that contribute to adaptive traits (polygenic selection)
it is more relevant when linkage is loose, such that selective interferences between loci are low − allowing for simultaneous sweeps at many loci − and when selection is not very efficient, so that phenotypic traits can accumulate substantial genetic variation
it is intended for sexual organisms of reasonably low population sizes

the periodic selection and quantitative genetics views are probably two extremes of a continuum
it is striking that the theory behind signatures of positive selection, namely genetic hitchhiking (Maynard Smith and Haigh 1974), is clearly one of periodic selection
whereas such signatures are searched in sexual species (since hitchhiking mapping is dependent on the recombination rate) that often exhibit substantial genetic variation for many traits
for such species (essentially plants or animals), the quantitative genetic approach is widely used and recognized as efficient to predict the response to selection, at least in the short term, for cultivated as well as natural populations
here, our aim is to extend the theory of hitchhiking to the context of a locus affecting a quantitative trait that also harbors background genetic variation due to other loci

contrary to the usual approach for selective sweeps (Maynard Smith and Haigh 1974; Stephan et al. 1992), we do not define a constant selection coefficient a priori for A₁
instead, we focus on a mutation affecting a quantitative trait and define selective pressure at the level of the trait only
our aim is to describe the dynamics of the beneficial mutation in the presence of background genetic variation
we mainly use a deterministic argument as in Maynard Smith and Haigh (1974), since it allows us to characterize major trends analytically
our results may be good approximations for stochastic populations when the frequency of the beneficial mutation is neither very low nor very high

Lande's (1983) model assumes normally distributed genetic values in the background, with mean m and variance σ²
it also assumes a large population size (that is, the model is deterministic), no linkage, and no interaction of the focal locus with the genetic background
such that the distribution of genetic values within each genotypic class at locus A is also normal