Population differentiation in Pacific salmons: local adaptation genetic drift, or the environment?

1995 ◽  
Vol 52 (12) ◽  
pp. 2762-2777 ◽  
Author(s):  
Milo D. Adkison
Keyword(s):  
Local Adaptation ◽  
Genetic Drift ◽  
Pacific Salmon ◽  
Genetic Homogeneity ◽  
Biological Research ◽  
Weak Selection ◽  
Selective Pressures ◽  
Population Sizes ◽  
Founder Events ◽  
Stable Populations

Morphological, behavioral, and life-history differences between Pacific salmon (Oncorhynchus spp.) populations are commonly thought to reflect local adaptation, and it is likewise common to assume that salmon populations separated by small distances are locally adapted. Two alternatives to local adaptation exist: random genetic differentiation owing to genetic drift and founder events, and genetic homogeneity among populations, in which differences reflect differential trait expression in differing environments. Population genetics theory and simulations suggest that both alternatives are possible. With selectively neutral alleles, genetic drift can result in random differentiation despite many strays per generation. Even weak selection can prevent genetic drift in stable populations; however, founder effects can result in random differentiation despite selective pressures. Overlapping generations reduce the potential for random differentiation. Genetic homogeneity can occur despite differences in selective regimes when straying rates are high. In sum, localized differences in selection should not always result in local adaptation. Local adaptation is favored when population sizes are large and stable, selection is consistent over large areas, selective differentials are large, and straying rates are neither too high nor too low. Consideration of alternatives to adaptation would improve both biological research and salmon conservation efforts.

scholarly journals Polygenic local adaptation in metapopulations: a stochastic eco-evolutionary model

2020 ◽  
Author(s):  
Enikő Szép ◽  
Himani Sachdeva ◽  
Nick Barton
Keyword(s):  
Population Size ◽  
Local Adaptation ◽  
Genetic Drift ◽  
Diffusion Approximation ◽  
Joint Distribution ◽  
Evolutionary Model ◽  
Ecological Niches ◽  
Demographic Stochasticity ◽  
Linkage Disequilibria ◽  
Population Sizes

AbstractThis paper analyses the conditions for local adaptation in a metapopulation with infinitely many islands under a model of hard selection, where population size depends on local fitness. Each island belongs to one of two distinct ecological niches or habitats. Fitness is influenced by an additive trait which is under habitat-dependent directional selection. Our analysis is based on the diffusion approximation and accounts for both genetic drift and demographic stochasticity. By neglecting linkage disequilibria, it yields the joint distribution of allele frequencies and population size on each island. We find that under hard selection, the conditions for local adaptation in a rare habitat are more restrictive for more polygenic traits: even moderate migration load per locus at very many loci is sufficient for population sizes to decline. This further reduces the efficacy of selection at individual loci due to increased drift and because smaller populations are more prone to swamping due to migration, causing a positive feedback between increasing maladaptation and declining population sizes. Our analysis also highlights the importance of demographic stochasticity, which exacerbates the decline in numbers of maladapted populations, leading to population collapse in the rare habitat at significantly lower migration than predicted by deterministic arguments.

scholarly journals The Effects of Hill-Robertson Interference Between Weakly Selected Mutations on Patterns of Molecular Evolution and Variation

Genetics ◽  
2000 ◽  
Vol 155 (2) ◽  
pp. 929-944 ◽  
Author(s):  
Gilean A T McVean ◽  
Brian Charlesworth
Keyword(s):  
Codon Bias ◽  
Molecular Adaptation ◽  
Weak Selection ◽  
Multiple Loci ◽  
Locus Model ◽  
Fitness Consequences ◽  
Novel Variants ◽  
Population Sizes ◽  
The Hill ◽  
Reversible Mutation

Abstract Associations between selected alleles and the genetic backgrounds on which they are found can reduce the efficacy of selection. We consider the extent to which such interference, known as the Hill-Robertson effect, acting between weakly selected alleles, can restrict molecular adaptation and affect patterns of polymorphism and divergence. In particular, we focus on synonymous-site mutations, considering the fate of novel variants in a two-locus model and the equilibrium effects of interference with multiple loci and reversible mutation. We find that weak selection Hill-Robertson (wsHR) interference can considerably reduce adaptation, e.g., codon bias, and, to a lesser extent, levels of polymorphism, particularly in regions of low recombination. Interference causes the frequency distribution of segregating sites to resemble that expected from more weakly selected mutations and also generates specific patterns of linkage disequilibrium. While the selection coefficients involved are small, the fitness consequences of wsHR interference across the genome can be considerable. We suggest that wsHR interference is an important force in the evolution of nonrecombining genomes and may explain the unexpected constancy of codon bias across species of very different census population sizes, as well as several unusual features of codon usage in Drosophila.

scholarly journals Recombination Can Evolve in Large Finite Populations Given Selection on Sufficient Loci

Genetics ◽  
2003 ◽  
Vol 165 (4) ◽  
pp. 2249-2258 ◽  
Author(s):  
Mark M Iles ◽  
Kevin Walters ◽  
Chris Cannings
Keyword(s):  
Experimental Evidence ◽  
Genetic Drift ◽  
Finite Population ◽  
Selective Advantage ◽  
Indirect Selection ◽  
Small Populations ◽  
Finite Populations ◽  
Population Sizes ◽  
Population Recombination

AbstractIt is well known that an allele causing increased recombination is expected to proliferate as a result of genetic drift in a finite population undergoing selection, without requiring other mechanisms. This is supported by recent simulations apparently demonstrating that, in small populations, drift is more important than epistasis in increasing recombination, with this effect disappearing in larger finite populations. However, recent experimental evidence finds a greater advantage for recombination in larger populations. These results are reconciled by demonstrating through simulation without epistasis that for m loci recombination has an appreciable selective advantage over a range of population sizes (am, bm). bm increases steadily with m while am remains fairly static. Thus, however large the finite population, if selection acts on sufficiently many loci, an allele that increases recombination is selected for. We show that as selection acts on our finite population, recombination increases the variance in expected log fitness, causing indirect selection on a recombination-modifying locus. This effect is enhanced in those populations with more loci because the variance in phenotypic fitnesses in relation to the possible range will be smaller. Thus fixation of a particular haplotype is less likely to occur, increasing the advantage of recombination.

Establishment of local adaptation in partly self-fertilizing populations

Genetics ◽  
2021 ◽  
Author(s):  
Bogi Trickovic ◽  
Sylvain Glémin
Keyword(s):  
Local Adaptation ◽  
Pollen Dispersal ◽  
Theoretical Background ◽  
Weak Selection ◽  
Dominance Level ◽  
Self Fertilization ◽  
Protected Polymorphism ◽  
And Migration ◽  
Closed Form Approximation ◽  
Selfing Species

Abstract Populations often inhabit multiple ecological patches and thus experience divergent selection, which can lead to local adaptation if migration is not strong enough to swamp locally adapted alleles. Conditions for the establishment of a locally advantageous allele have been studied in randomly mating populations. However, many species reproduce, at least partially, through self-fertilization, and how selfing affects local adaptation remains unclear and debated. Using a two-patch branching process formalism, we obtained a closed-form approximation under weak selection for the probability of establishment of a locally advantageous allele (P) for arbitrary selfing rate and dominance level, where selection is allowed to act on viability or fecundity, and migration can occur via seed or pollen dispersal. This solution is compared to diffusion approximation and used to investigate the consequences of a shift in a mating system on P, and the establishment of protected polymorphism. We find that selfing can either increase or decrease P, depending on the patterns of dominance in the two patches, and has conflicting effects on local adaptation. Globally, selfing favors local adaptation when locally advantageous alleles are (partially) recessive, when selection between patches is asymmetrical and when migration occurs through pollen rather than seed dispersal. These results establish a rigorous theoretical background to study heterogeneous selection and local adaptation in partially selfing species.

scholarly journals Uncorrelated genetic drift of gene frequencies and linkage disequilibrium in some models of linked overdominant polymorphisms

1974 ◽  
Vol 24 (3) ◽  
pp. 281-294 ◽  
Author(s):  
Joseph Felsenstein
Keyword(s):  
Linkage Disequilibrium ◽  
Covariance Matrix ◽  
Genetic Drift ◽  
Conditional Distribution ◽  
Large Population ◽  
Multivariate Normal ◽  
Gene Frequencies ◽  
Constant Variance ◽  
Population Sizes ◽  
Multiplicative Family

SUMMARYFor large population sizes, gene frequencies p and q at two linked over-dominant loci and the linkage disequilibrium parameter D will remain close to their equilibrium values. We can treat selection and recombination as approximately linear forces on p, q and D, and we can treat genetic drift as a multivariate normal perturbation with constant variance-covariance matrix. For the additive-multiplicative family of two-locus models, p, q and D are shown to be (approximately) uncorrelated. Expressions for their variances are obtained. When selection coefficients are small the variances of p and q are those previously given by Robertson for a single locus. For small recombination fractions the variance of D is that obtained for neutral loci by Ohta & Kimura. For larger recombination fractions the result differs from theirs, so that for unlinked loci r2 ≃ 2/(3N) instead of 1/(2N). For the Lewontin-Kojima and Bodmer symmetric viability models, and for a model symmetric at only one of the loci, a more exact argument is possible. In the asymptotic conditional distribution in these cases, various of p, q and D are uncorrelated, depending on the type of symmetiy in the model.

scholarly journals Martingales and fixation probabilities of evolutionary graphs

2014 ◽  
Vol 470 (2165) ◽  
pp. 20130730 ◽  
Author(s):  
T. Monk ◽  
P. Green ◽  
M. Paulin
Keyword(s):  
Graph Theory ◽  
Random Walk ◽  
Large Population ◽  
Initial Population ◽  
Weak Selection ◽  
Simplifying Assumptions ◽  
Population Sizes ◽  
Fixation Probabilities ◽  
Evolutionary Graph Theory ◽  
Complete Bipartite Graphs

Evolutionary graph theory is the study of birth–death processes that are constrained by population structure. A principal problem in evolutionary graph theory is to obtain the probability that some initial population of mutants will fixate on a graph, and to determine how that fixation probability depends on the structure of that graph. A fluctuating mutant population on a graph can be considered as a random walk. Martingales exploit symmetry in the steps of a random walk to yield exact analytical expressions for fixation probabilities. They do not require simplifying assumptions such as large population sizes or weak selection. In this paper, we show how martingales can be used to obtain fixation probabilities for symmetric evolutionary graphs. We obtain simpler expressions for the fixation probabilities of star graphs and complete bipartite graphs than have been previously reported and show that these graphs do not amplify selection for advantageous mutations under all conditions.

scholarly journals Most cancers carry a substantial deleterious load due to Hill-Robertson interference

2019 ◽  
Author(s):  
Susanne Tilk ◽  
Christina Curtis ◽  
Dmitri A Petrov ◽  
Christopher D McFarland
Keyword(s):  
Positive Selection ◽  
Negative Selection ◽  
Selection Model ◽  
Fitness Cost ◽  
Deleterious Mutations ◽  
Weak Selection ◽  
Selective Pressures ◽  
Mutational Burden ◽  
Genome Wide ◽  
Average Fitness

AbstractCancer genomes exhibit surprisingly weak signatures of negative selection1,2. This may be because tumors evolve under weak selective pressures (‘weak selection’) or because genome-wide linkage in cancer prevents most deleterious mutations from being removed due to Hill-Robertson interference3 (‘inefficient selection’). The weak selection model argues that most genes are only important for multicellular function and that selection acts only on a subset of essential genes. In contrast, the inefficient selection model predicts that only cancers with low mutational burdens, where linkage effects are minimal, will exhibit strong signals of negative selection against deleterious passengers and positive selection for beneficial drivers. We leverage the 10,000-fold variation in mutational burden across cancer subtypes to stratify tumors by their genome-wide mutational burden and used a normalized ratio of nonsynonymous to synonymous substitutions (dN/dS) to quantify the extent that selection varies with mutation rate. We find that appreciable negative selection (dN/dS ~ 0.4) is present in tumors with a low mutational burden, while the remaining cancers (96%) exhibit dN/dS ratios approaching 1, suggesting that the majority of tumors do not remove deleterious passengers. A parallel pattern is seen in drivers, where positive selection attenuates as the mutational burden of cancers increases. Both trends persist across tumor-types, are not exclusive to essential or housekeeping genes, are present in clonal and subclonal mutations, and persist in Copy Number Alterations. A consequence of this inability to remove deleterious passengers is that tumors with elevated mutational burdens, which are expected to harbor substantial protein folding stress, upregulate heat shock pathways. Finally, using evolutionary modeling, we find that Hill-Robertson interference alone can reproduce the patterns of attenuated selection observed in both drivers and passengers if the average fitness cost of passengers is 1.0% and the average fitness benefit of drivers is 19%. As a result, despite the weak individual fitness effects of passengers, most cancers harbor a large mutational load (median ~40% total fitness cost). Collectively, our findings suggest that the lack of observed negative selection in most tumors is not due to relaxed selective pressures, but rather the inability of selection to remove individual deleterious mutations in the presence of genome-wide linkage.

scholarly journals The relative strength of selection on modifiers of genetic architecture under migration load

2021 ◽  
Author(s):  
Stephen R Proulx ◽  
Henrique Teotonio
Keyword(s):  
Local Adaptation ◽  
Genetic Architecture ◽  
Absolute Magnitude ◽  
Relative Strength ◽  
Demographic Stochasticity ◽  
Recombination Rates ◽  
Relative Population ◽  
Population Sizes ◽  
The Mean ◽  
Selection For

Gene flow between populations adapting to differing local environmental conditions creates a "migration load" because individuals might disperse to habitats where their survival is low or because they might reproduce with locally maladapted individuals. The amount by which the mean relative population fitness is kept below one creates an opportunity for modifiers of the genetic architecture to spread due to selection. Prior work that separately considered modifiers changing dispersal or recombination rates, or altering dominance or epistasis, has typically focused on the direction of selection rather than its absolute magnitude. We here develop methods to determine the strength of selection on modifiers of the genetic architecture, including modifiers of the dispersal rate, after populations evolved local adaptation. We consider scenarios with up to five loci contributing to local adaptation and derive a matrix model for the deterministic spread of modifiers. We find that selection for modifiers of epistasis and dominance is stronger than selection for decreased recombination, and that selection for partial reductions in recombination are extremely weak, regardless of the number of loci contributing to local adaptation. The spread of modifiers for a reduction in dispersal depends on the number of loci, pre-existing epistasis and extent of migration load. We identify a novel effect, that modifiers of dominance are more strongly selected when they are unlinked to the locus that they modify. Overall, these results help explain population differentiation and reproductive isolation and provide a benchmark to compare selection on genetic architecture modifiers in finite population sizes and under demographic stochasticity.

scholarly journals Disentangling adaptation from drift in bottlenecked and reintroduced populations of Alpine ibex

2021 ◽  
Author(s):  
Deborah Leigh ◽  
Heidi Lischer ◽  
Frédéric Guillaume ◽  
Christine Grossen ◽  
Torsten Günther
Keyword(s):  
Local Adaptation ◽  
Genetic Drift ◽  
False Positive ◽  
Conservation Management ◽  
Detection Methods ◽  
True Positive ◽  
Alpine Ibex ◽  
Snp Data ◽  
Outlier Loci ◽  
Management Plans

Identifying local adaptation in bottlenecked species is essential for conservation management. Selection detection methods have an important role in species management plans, assessments of adaptive capacity, and looking for responses to climate change. Yet, the allele frequency changes exploited in selection detection methods are similar to those caused by the strong neutral genetic drift expected during a bottleneck. Consequently, it is often unclear what accuracy selection detection methods have across bottlenecked populations. In this study, simulations were used to explore if signals of selection could be confidently distinguished from genetic drift across 23 bottlenecked and reintroduced populations of Alpine ibex (Capra ibex). The meticulously recorded demographic history of the Alpine ibex was used to generate comprehensive simulated SNP data. The simulated SNPs were then used to benchmark the confidence we could place in outliers identified in empirical Alpine ibex SNP data. Within the simulated dataset, the false positive rates were high for all selection detection methods but fell substantially when two or more methods were combined. True positive rates were consistently low and became negligible with increased stringency. Despite finding many outlier loci in the empirical Alpine ibex SNPs, none could be distinguished from genetic drift-driven false positives. Unfortunately, the low true positive rate also prevents the exclusion of recent local adaptation within the Alpine ibex. The baselines and stringent approach outlined here should be applied to other bottlenecked species to ensure the risk of false positive, or negative, signals of selection are accounted for in conservation management plans.

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