A fundamental question in evolutionary biology is what promotes genetic variation at nonneutral loci, a major precursor to adaptation in changing environments. In particular, balanced polymorphism under realistic evolutionary models of temporally varying environments in finite natural populations remains to be demonstrated. Here, we propose a novel mechanism of balancing selection under temporally varying fitnesses. Using forward-in-time computer simulations and mathematical analysis, we show that cyclic selection that spatially varies in magnitude, such as along an environmental gradient, can lead to elevated levels of nonneutral genetic polymorphism in finite populations. Balanced polymorphism is more likely with an increase in gene flow, magnitude and period of fitness oscillations, and spatial heterogeneity. This polymorphism-promoting effect is robust to small systematic fitness differences between competing alleles or to random environmental perturbation. Furthermore, we demonstrate analytically that protected polymorphism arises as spatially heterogeneous cyclic fitness oscillations generate a type of storage effect that leads to negative frequency dependent selection. Our findings imply that spatially variable cyclic environments can promote elevated levels of nonneutral genetic variation in natural populations.

Phenotypic plasticity is known to evolve in perturbed habitats, where it alleviates the deleterious effects of selection. But the effects of plasticity on levels of genetic polymorphism, an important precursor to adaptation in temporally varying environments, are unclear. Here we develop a haploid, two-locus population-genetic model to describe the interplay between a plasticity modifier locus and a target locus subject to periodically varying selection. We find that the interplay between these two loci can produce a "genomic storage effect" that promotes balanced polymorphism over a large range of parameters, in the absence of all other conditions known to maintain genetic variation. The genomic storage effect arises as recombination allows alleles at the two loci to escape more harmful genetic backgrounds and associate in haplotypes that persist until environmental conditions change. Using both Monte Carlo simulations and analytical approximations we quantify the strength of the genomic storage effect across a range of selection pressures, recombination rates, plasticity modifier effect sizes, and environmental periods.KEYWORDS balanced polymorphism; phenotypic plasticity; temporally varying selection; storage effect B ALANCED polymorphism fosters adaptation in changing environments. As populations continuously adapt from one environment to the other, genetic polymorphism provides a readily available reservoir of adaptive alleles that selection can act upon and, thus, promotes population persistence (Lande and Shannon 1996;Barrett and Schluter 2008). Despite their role in persistence, evolutionary mechanisms that help maintain genetic polymorphism in temporally changing environments remain poorly understood.Empirical studies have revealed cases of polymorphism that are subject to temporally varying selection (e.g., Lynch 1987;Cain et al. 1990;Turelli et al. 2001;Bergland et al. 2014). However, the underlying mechanisms maintaining genetic polymorphism in these cases are unclear. Theoretical possibilities that predict balanced polymorphism in varying environments include heterozygous advantage [geometric mean overdominance, which cannot occur in haploids (Dempster 1955;Haldane and Jayakar 1963;Gillespie 1973Gillespie , 1974], overlapping generations with age-/stage-specific selection or seed banks (Ellner and Hairston 1994;Turelli et al. 2001;Svardal et al. 2011), density regulation with resource competition (Dean 2005;Yi and Dean 2013), or these mechanisms in combination with spatial heterogeneity in selection (Gillespie 1974(Gillespie , 1975Ewing 1979;Gulisija and Kim 2015;Svardal et al. 2015). Despite these developments, balancing selection due to temporally varying selection has not been widely accepted in population genetic literature probably because early models were criticized due to their failure to maintain polymorphism under genetic drift (Hedrick 1976).The storage effect, initially recognized in studies of species coexistence in community ecology (Chesson and Warner 1981;Chesson 1985Chesso...

Saline migrants into freshwater habitats constitute among the most destructive invaders in aquatic ecosystems throughout the globe. However, the evolutionary and physiological mechanisms underlying such habitat transitions remain poorly understood. To explore the mechanisms of freshwater adaptation and distinguish between adaptive (evolutionary) and acclimatory (plastic) responses to salinity change, we

The harmful effects of inbreeding can be reduced if deleterious recessive alleles were removed (purged) by selection against homozygotes in earlier generations. If only a few generations are involved, purging is due almost entirely to recessive alleles that reduce fitness to near zero. In this case the amount of purging and allele frequency change can be inferred approximately from pedigree data alone and are independent of the allele frequency. We examined pedigrees of 59,778 U.S. Jersey cows. Most of the pedigrees were for six generations, but a few went back slightly farther. Assuming recessive homozygotes have fitness 0, the reduction of total genetic load due to purging is estimated at 17%, but most of this is not expressed, being concealed by dominant alleles. Considering those alleles that are currently expressed due to inbreeding, the estimated amount of purging is such as to reduce the expressed load (inbreeding depression) in the current generation by 12.6%. That the reduction is not greater is due mainly to (1) generally low inbreeding levels because breeders in the past have tended to avoid consanguineous matings, and (2) there is essentially no information more than six generations back. The methods used here should be applicable to other populations for which there is pedigree information.KEY WORDS: Inbred load, inbreeding depression, Jersey cattle, pedigree analysis, purging.Genetic load and inbreeding depression can be reduced if deleterious recessive alleles are removed (purged) from the population by selection against homozygotes in previous generations. The purpose of this article is to show how to infer the amount of purging from pedigrees. Of course, an actual pedigree does not show the individuals that have been purged. We show that, with a few reasonable assumptions and approximations, we can assess the amount of purging that has occurred despite not having observed the purged individuals. The methods are applied to a pedigreed population of Jersey cattle. A Brief Literature SurveyThere are a number of experimental studies of purging, with varying results. The best data come from plants, as expected, because they are easy to control, can be studied in large numbers, and frequently permit self-fertilization. The large plant literature has been summarized by Byers and Waller (1999), who found that of 34 studies, 14 reported significant evidence of purging, as determined by comparison of inbred individuals with different inbreeding histories.Crnokrak and Barrett (2002) reviewed studies on 28 animal and plant species. They found strong evidence of purging, although the extent and patterns were quite inconsistent. Among other animal studies, the results are similarly variable. Ballou (1997) analyzed data from 25 captive populations and compared inbreeding depression with varying levels of ancestral inbreeding. The combined data support purging, but the effects are small and individually not significant. Although not studying purging per se, Wang and Hill (1999) showed that selection within inbr...