polygenic adaptation

Thompson KA, Osmond MM & Schluter D 2019 Parallel genetic evolution and speciation from standing variation. Evol Lett 3:129-141.

  • adaptation from standing variation would reduce the evolution of reproductive isolation under parallel selection
  • parental populations would fix more of the same alleles and therefore evolve fewer incompatibilities
  • our simulations consider pairs of populations and multivariate phenotypes determined by multiple additive loci
  • N haploid parents are then randomly sampled with replacement from a multinomial distribution with probabilities proportional to their fitness, W
  • parents then randomly mate and produce two haploid offspring per pair, with free recombination between all loci
  • we assume an effectively infinite number of loci such that all mutations arise at a previously unmutated locus
  • mutational effects are drawn from a multivariate normal distribution ("continuum-of-alleles" sensu Kimura [1965]), with a mean of 0 and an SD of α in all m traits and no correlations among traits
  • i.e., universal pleiotropy
  • our general conclusions hold if the ancestor is under much stronger selection (σanc = 1) that puts it into the multivariate "House-of-Cards" regime
  • a parental population was established by first randomly choosing n polymorphic loci in the ancestor
  • each parental individual received the mutant (i.e., "derived") allele at each of these n loci with a probability equal to the allele's frequency in the ancestor
  • this admittedly artificial sampling procedure allowed us more control over the amount of standing genetic variation across simulations with different parameter values
  • further control was achieved by making the second parental population initially identical to the first
  • each possessed the exact same collection of genotypes
  • there were therefore no founder effects
  • populations adapted from only new (i.e., de novo) mutation when n = 0
  • an unavoidable and important effect of standing variation is that it quickens adaptation because populations do not have to wait for beneficial alleles to arise
  • reproductive isolation evolves rapidly during the initial stages of adaptation
  • after populations reach their respective phenotypic optima, genetic divergence accumulates slowly at a rate proportional to the mutation rate
  • our results reflect quasi-equilibrium conditions rather than transient states and are unaffected by standing variation's influence on the speed of adaptation
  • phenotypic variance in parental populations (i.e., before hybridization) is near zero and does not differ between populations founded with versus without standing variation
  • such low variance is expected because our simulations have fixed optima, frequency-independent selection, no migration, and parameter values corresponding to strong selection and relatively weak mutation
  • we determined the fitness (eq. (1)) of each hybrid in both parental environments and recorded its fitness as the larger of the two values
  • genetic parallelism rarely decreases to zero even under completely divergent selection (θ = 180°), indicating that populations fix some deleterious alleles