Abstract
Polygenic adaptation in response to selection on quantitative traits has become an important topic in evolutionary biology. Here we review the recent literature on models of polygenic adaptation. In particular, we focus on a model that includes mutation and both directional and stabilizing selection on a highly polygenic trait in a population of finite size (thus experiencing random genetic drift). Assuming that a sudden environmental shift of the fitness optimum occurs while the population is in a stochastic equilibrium, we analyze the adaptation of the trait to the new optimum. When the shift is not too large relative to the equilibrium genetic variance and this variance is determined by loci with mostly small effects, the approach of the mean phenotype to the optimum can be approximated by a rapid exponential process (whose rate is proportional to the genetic variance). During this rapid phase the underlying changes to allele frequencies, however, may depend strongly on genetic drift. While trait-increasing alleles with intermediate equilibrium frequencies are dominated by selection and contribute positively to changes of the trait mean (i.e., are aligned with the direction of the optimum shift), alleles with low or high equilibrium frequencies show more of a random dynamics, which is expected when drift is dominating. A strong effect of drift is also predicted for population size bottlenecks. Our simulations show that the presence of a bottleneck results in a larger deviation of the population mean of the trait from the fitness optimum, which suggests that more loci experience the influence of drift.
Highlights
Evolutionary adaptation is the process that natural populations experience to become suited to their environment
In contrast to the single- and multi-locus cases, in polygenic models, selection acts on a phenotypic trait, and a genotype-phenotype map is assumed to bridge the gap to population genetics
When the shift of the fitness optimum is not too large relative to the genetic variance in the trait and the variance is mostly due to loci with small effects, we found that the new optimum is approached exponentially at a rate proportional to the equilibrium genetic variance before the optimum shift
Summary
Evolutionary adaptation is the process that natural populations experience to become suited to their environment. (2010) [17] and Pritchard and Di Rienzo (2010) [18] drew the attention of population geneticists to this type of selection These papers predicted that allele frequencies change by small amounts when a large number of genetic loci of minor effect sizes control a phenotypic trait. This led to problems in defining the values of the allele frequencies at the time of the environmental change, as the equilibrium allele frequencies of the deterministic model do not agree with the frequencies typically observed in GWAS Recognizing this problem, the studies published in the past few years [33,34,35,36,37] include genetic drift (due to finite population size) into their polygenic models. Of the additional factors influencing a natural population (listed above), we study the effect of varying population size by computer simulation
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