polygenic adaptation - senecio7の日記

Goldstein DB & Holsinger KE 1992 Maintenance of polygenic variation in spatially structured populations: roles for local mating and genetic redundancy. Evolution 46:412-429.

mutation-selection balance play a role, but there is considerable doubt over whether it can account for heritabilities as high as 0.5, which are commonly found in natural populations
we use Monte Carlo simulations to examine the effect of isolation by distance on the variation maintained by mutation in a polygenic trait subject to optimizing selection

isolation by distance can substantially increase the total variation maintained in continuous populations over a wide range of dispersal patterns
but only if more than one genotype produces the optimal phenotype
(genetic redundancy)

isolation by distance alone has only a slight effect on the variation maintained in the total population for neutral alleles.
the combined effect of isolation by distance and genetic redundancy, however, allows the maintenance of substantial variation despite strong stabilizing selection
because of their independent evolutionary histories, different parts of the population independently draw from the available set of redundant genotypes
because the genotypes are redundant, selection does not discriminate among them
they will persist until eliminated by drift
the population as a whole maintains many distinct genotypes
over a moderate range of dispersal patterns the amount of variation maintained in the entire population is independent of both the strength of selection and the variance of the dispersal distance

for computational convenience, most of our simulations assume an additive mapping from the genotype to the phenotype
when the phenotype is additively determined, and the optimum is intermediate, many different genotypes produce the optimum
if the optimum is exactly intermediate, no loci will be polymorphic at equilibrium (Barton, 1986)

when dispersal is very restricted, genetic variation is hidden within redundant genotypes in different parts of the population
distinct, redundant genotypes drift in and out, just as in a mutation-drift balance
the strength of selection has little impact on the amount of variation maintained in the population as a whole, even though it has a definite impact on the amount of variation maintained locally

in many cases compiled by Cohan (1984a) it seems that the alternative genetic solutions result in little, if any, fitness difference
the standard additive mapping assumed in many population and quantitative genetics models, where the phenotype is taken as the sum of the allelic effects across all loci, leads to perfect redundancy
as Wright (1935) and Barton (1986) have shown, when such traits are subject to optimizing selection, a maximum of one locus can be polymorphic at equilibrium, all other loci being fixed
the redundancy present in additive mappings does not depend on equal effects among alleles

local mating and genetic redundancy may be a general and powerful mechanism promoting the maintenance of polygenic variation