Partial protection from cyclical selection generates a high level of polymorphism at multiple non‐neutral sites

Abstract

Temporally varying selection is known to maintain genetic polymorphism under certain restricted conditions. However, if part of a population can escape from selective pressure, a condition called the “storage effect” is produced, which greatly promotes balanced polymorphism. We investigate whether seasonally fluctuating selection can maintain polymorphism at multiple loci, if cyclically fluctuating selection is not acting on a subpopulation called a “refuge.” A phenotype with a seasonally oscillating optimum is determined by alleles at multiple sites, across which the effects of mutations on phenotype are distributed randomly. This model resulted in long‐term polymorphism at multiple sites, during which allele frequencies oscillate heavily, greatly increasing the level of nonneutral polymorphism. The level of polymorphism at linked neutral sites was either higher or lower than expected for unlinked neutral loci. Overall, these results suggest that for a protein‐coding sequence, the nonsynonymous‐to‐synonymous ratio of polymorphism may exceed one. In addition, under randomly perturbed environmental oscillation, different sets of sites may take turns harboring long‐term polymorphism, thus making trans‐species polymorphism (which has been predicted as a classical signature of balancing selection) less likely.

Differences in Thermal Tolerance between Parental Species Could Fuel Thermal Adaptation in Hybrid Wood Ants

“…However, the conditions under which genetic polymorphism can be maintained over long periods via temporally varying selection are still debated because many models require very strong selection (Bergland et al, 2014, but see Buffalo and Coop, 2020). Recent theoretical studies have outlined biologically realistic scenarios for the maintenance of polymorphisms (Bertram and Masel, 2019;Park and Kim, 2019;Wittmann et al, 2017) several of which are compatible with the ant system. The first mechanism is the storage effect, where a portion of the population is protected from selection because of a protective life history trait (Chesson, 1985;Chesson and Warner, 1981).…”

“…Indeed, the reproductive queens are buried deep in the ground in the warmth of the nest as they do not forage and hence are buffered from fluctuating outside temperatures. The storage effect requires overlapping generations and strong selection to maintain long-term genetic polymorphism (Park and Kim, 2019), both compatible with the ant system. Queens are long-lived and reproduce over multiple years, allowing them to potentially produce offspring of similar genetic background independently of the temperature.…”

Modeling the genetic footprint of fluctuating balancing selection: From the local to the genomic scale

“…Chesson and Warner 1981; Gulisija et al . 2016; Park and Kim 2019; Reinhold 2000) or relative nonlinearity (e.g. Armstrong and McGehee 1980; Chesson 2000).…”

“…In particular, fluctuating selection itself can maintain genetic polymorphism under appropriate conditions (e.g. Gillespie 1973, 1978; Haldane and Jayakar 1963; Park and Kim 2019; Wittmann et al . 2017; Yi and Dean 2013).…”

Partial protection from fluctuating selection leads to evolution toward wider population size fluctuation and a novel mechanism of balancing selection

“…2001), even though in each subpopulation fitness is not given as the function of allele frequency. It can generate stable long-term polymorphism at multiple loci (P ark and K im 2019), although it requires strong selection (a large magnitude of fitness oscillation; B ertram and M asel (2019)).…”