Population Genetics of Polymorphism and Divergence Under Fluctuating Selection
Emilia Huerta-Sanchez, Rick Durrett and Carlos D. Bustamante
Abstract.
Current methods for detecting fluctuating selection require time series data on genotype frequencies.
Here, we propose an alternative approach that makes use of DNA polymorphism data from a sample of
individuals collected at a single point in time. Our method uses classical diffusion approximations to model
temporal fluctuations in the selection coefficients to find the expected distribution of mutation frequencies
in the population. Using the Poisson random-field setting we derive the site-frequency spectrum (SFS) for
three different models of fluctuating selection. We find that the general effect of fluctuating selection is to
produce a more U-shaped site-frequency spectrum with an excess of high-frequency derived mutations at
the expense of middle-frequency variants. We present likelihood-ratio tests, comparing the fluctuating
selection models to the neutral model using SFS data, and use Monte Carlo simulations to assess their power.
We find that we have sufficient power to reject a neutral hypothesis using samples on the order of a few
hundred SNPs and a sample size of 20 and power to distinguish between selection that varies in time and
constant selection for a sample of size 20.We also find that fluctuating selection increases the probability of
fixation of selected sites even if, on average, there is no difference in selection among a pair of alleles
segregating at the locus. Fluctuating selection will, therefore, lead to an increase in the ratio of divergence to polymorphism similar to that observed under positive directional selection.
Reprint of paper as PDF
Genetics 178 (2008), 325-337
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