Simulation code for: Effects of population size change on the genetics of adaptation following an abrupt change in environment

Since the rediscovery of Mendelian genetics over a century ago, there has
been much debate about the evolutionary importance of mutations with large
phenotypic effects. While population genetic models predict that
large-effect mutations will typically contribute to adaptation following
an abrupt change in environment, the prediction applies to populations of
stable size and overlooks effects of population size change on adaptation
(e.g., population decline following habitat loss; growth during range
expansion). We evaluate the phenotypic and fitness effects of mutations
contributing to adaptation immediately following an abrupt environmental
shift that alters both selection and population size dynamics. We show
that large-effect mutations are likely to contribute to adaptation in
populations declining to a new carrying capacity, somewhat smaller-effect
mutations contribute to evolutionary rescue, and small-effect mutations
predominate in growing populations. We also show that the relative
contributions of positively selected and overdominant mutations to
adaptation depend on interactions between the phenotypic effect size
distribution for new mutations and the specific form of population size
change during adaptation (i.e., growth, decline, or evolutionary rescue).
Our results illustrate how population size dynamics can shape the genetic
basis of adaptation, which should motivate empirical comparisons of
populations adapting in different demographic contexts.