scholarly journals The Rate of Molecular Evolution When Mutation May Not Be Weak

2018 ◽  
Author(s):  
A.P. Jason de Koning ◽  
Bianca D. De Sanctis
Keyword(s):  
Natural Selection ◽  
Molecular Evolution ◽  
Population Size ◽  
Mutation Rate ◽  
Molecular Clock ◽  
Neutral Theory ◽  
Neutral Evolution ◽  
Systematic Bias ◽  
Effective Population ◽  
Sequence Comparisons

AbstractOne of the most fundamental rules of molecular evolution is that the rate of neutral evolution equals the mutation rate and is independent of effective population size. This result lies at the heart of the Neutral Theory, and is the basis for numerous analytic approaches that are widely applied to infer the action of natural selection across the genome and through time, and for dating divergence events using the molecular clock. However, this result was derived under the assumption that evolution is strongly mutation-limited, and it has not been known whether it generalizes across the range of mutation pressures or the spectrum of mutation types observed in natural populations. Validated by both simulations and exact computational analyses, we present a direct and transparent theoretical analysis of the Wright-Fisher model of population genetics, which shows that some of the most important rules of molecular evolution are fundamentally changed by considering recurrent mutation’s full effect. Surprisingly, the rate of the neutral molecular clock is found to have population-size dependence and to not equal the mutation rate in general. This is because, for increasing values of the population mutation rate parameter (θ), the time spent waiting for mutations quickly becomes smaller than the cumulative time mutants spend segregating before a substitution, resulting in a net deceleration compared to classical theory that depends on the population mutation rate. Furthermore, selection exacerbates this effect such that more adaptive alleles experience a greater deceleration than less adaptive alleles, introducing systematic bias in a wide variety of methods for inferring the strength and direction of natural selection from across-species sequence comparisons. Critically, the classical weak mutation approximation performs well only when θ< 0.1, a threshold that many biological populations seem to exceed.

scholarly journals Does effective population size affect rates of molecular evolution: mitochondrial data for host/parasite species pairs in bees suggests not

2021 ◽  
Author(s):  
Nahid Shokri Bousjein ◽  
Simon Tierney ◽  
Michael Gardner ◽  
Michael Schwarz
Keyword(s):  
Molecular Evolution ◽  
Population Size ◽  
Effective Population Size ◽  
Parasite Species ◽  
Mitochondrial Protein ◽  
Neutral Theory ◽  
Mitochondrial Genes ◽  
Social Parasite ◽  
Effective Population ◽  
Rates Of Molecular Evolution

Adaptive evolutionary theory argues that organisms with larger effective population size (Ne) should have higher rates of adaptive evolution and therefore greater capacity to win evolutionary arm races. However, in some certain cases species with much smaller Ne may be able to survive beside their opponents for an extensive evolutionary time. Neutral theory predicts that accelerated rates of molecular evolution in organisms with exceedingly small Ne is due to the effects of genetic drift and fixation of slightly deleterious mutations. We test this prediction in two obligate social parasite species and their respective host species from the bee tribe Allodapini. The parasites (genus Inquilina) have been locked into a tight coevolutionary arm races with their exclusive hosts (genus Exoneura) for ~15 million years, even though Inquilina exhibit Ne that are an order of magnitude smaller than their host. In this study, we compared rates of molecular evolution between host and parasite using nonsynonymous to synonymous substitution rate ratios (dN/dS) of eleven mitochondrial protein coding genes sequenced from transcriptomes. Tests of selection on mitochondrial genes indicated no significant differences between host and parasite dN/dS, with evidence for purifying selection acting on all mitochondrial genes of host and parasite species. Several potential factors which could weaken the inverse relationship between Ne and rate of molecular evolution are discussed.

scholarly journals Somatic drift and rapid loss of heterozygosity suggest small effective population size of stem cells and high somatic mutation rate in asexual planaria

2019 ◽  
Author(s):  
Hosseinali Asgharian ◽  
Joseph Dunham ◽  
Paul Marjoram ◽  
Sergey V. Nuzhdin
Keyword(s):  
Stem Cells ◽  
Population Size ◽  
Effective Population Size ◽  
Somatic Mutation ◽  
Mutation Rate ◽  
Nucleotide Diversity ◽  
Genetic Model ◽  
Neutral Evolution ◽  
Multiple Time ◽  
Effective Population

AbstractPlanarian flatworms have emerged as highly promising models of body regeneration due to the many stem cells scattered through their bodies. Currently, there is no consensus as to the number of stem cells active in each cycle of regeneration or the equality of their relative contributions. We approached this problem with a population genetic model of somatic genetic drift. We modeled the fissiparous life cycle of asexual planarians as an asexual population of cells that goes through repeated events of splitting into two subpopulations followed by population growth to restore the original size. We sampled a pedigree of obligate asexual clones of Girardia cf. tigrina at multiple time points encompassing 14 generations. Effective population size of stem cells was inferred from the magnitude of temporal fluctuations in the frequency of somatic variants and under most of the examined scenarios was estimated to be in the range of a few hundreds. Average genomic nucleotide diversity was 0.00398. Assuming neutral evolution and mutation-drift equilibrium, the somatic mutation rate was estimated in the 10−5 − 10−7 range. Alternatively, we estimated Ne and somatic μ from temporal changes in nucleotide diversity π without the assumption of equilibrium. This second method suggested even smaller Ne and larger μ. A key unknown parameter in our model on which estimates of Ne and μ depend is g, the ratio of cellular to organismal generations determined by tissue turnover rate. Small effective number of propagating stem cells might contribute to reducing reproductive conflicts in clonal organisms.

scholarly journals The Behavior of Molecular Measures of Natural Selection after a Change in Population Size

2020 ◽  
Author(s):  
Rebekka Müller ◽  
Ingemar Kaj ◽  
Carina F. Mugal
Keyword(s):  
Natural Selection ◽  
Population Size ◽  
Genetic Drift ◽  
Empirical Studies ◽  
Neutral Theory ◽  
Negative Relationship ◽  
Recent Change ◽  
Effective Population ◽  
Nearly Neutral Theory ◽  
Molecular Measures

ABSTRACTA common model to describe natural selection at the molecular level is the nearly neutral theory, which emphasizes the importance of mutations with slightly deleterious fitness effects as they have a chance to get fixed due to genetic drift. Since genetic drift is stronger in smaller than in larger populations, a negative relationship between molecular measures of selection and population size is expected within the nearly neutral theory. Originally, this hypothesis was formulated under equilibrium conditions. A change in population size, however, pushes the selection-drift balance off equilibrium leading to alterations in the efficacy of selection. To investigate the nonequilibrium behavior, we relate measures of natural selection and genetic drift to each other, considering both, measures of micro- and macroevolution. Specifically, we use a Poisson random field framework to model πN/πS and ω as time-dependent measures of selection and assess genetic drift by an effective population size. This analysis reveals a clear deviation from the expected equilibrium selection-drift balance during nonequilibrium periods. Moreover, we find that microevolutionary measures quickly react to a change in population size and reflect a recent change well, at the same time as they quickly lose the knowledge about it. Macroevolutionary measures, on the other hand, react more slowly to a change in population size but instead capture the influence of ancient changes longer. We therefore conclude that it is important to be aware of the different behaviors of micro- and macroevo- lutionary measures when making inference in empirical studies, in particular when comparing results between studies.

scholarly journals Demography and Natural Selection Have Shaped Genetic Variation in the Widely Distributed Conifer Norway Spruce (Picea abies)

2020 ◽  
Vol 12 (2) ◽  
pp. 3803-3817 ◽  
Author(s):  
Xi Wang ◽  
Carolina Bernhardsson ◽  
Pär K Ingvarsson
Keyword(s):  
Genetic Diversity ◽  
Natural Selection ◽  
Picea Abies ◽  
Population Size ◽  
Effective Population Size ◽  
Norway Spruce ◽  
Demographic History ◽  
Neutral Theory ◽  
Effective Population ◽  
Low Genetic Diversity

Abstract Under the neutral theory, species with larger effective population size are expected to harbor higher genetic diversity. However, across a wide variety of organisms, the range of genetic diversity is orders of magnitude more narrow than the range of effective population size. This observation has become known as Lewontin’s paradox and although aspects of this phenomenon have been extensively studied, the underlying causes for the paradox remain unclear. Norway spruce (Picea abies) is a widely distributed conifer species across the northern hemisphere, and it consequently plays a major role in European forestry. Here, we use whole-genome resequencing data from 35 individuals to perform population genomic analyses in P. abies in an effort to understand what drives genome-wide patterns of variation in this species. Despite having a very wide geographic distribution and an corresponding enormous current population size, our analyses find that genetic diversity of P. abies is low across a number of populations (π = 0.0049 in Central-Europe, π = 0.0063 in Sweden-Norway, π = 0.0063 in Finland). To assess the reasons for the low levels of genetic diversity, we infer the demographic history of the species and find that it is characterized by several reoccurring bottlenecks with concomitant decreases in effective population size can, at least partly, provide an explanation for low polymorphism we observe in P. abies. Further analyses suggest that recurrent natural selection, both purifying and positive selection, can also contribute to the loss of genetic diversity in Norway spruce by reducing genetic diversity at linked sites. Finally, the overall low mutation rates seen in conifers can also help explain the low genetic diversity maintained in Norway spruce.

scholarly journals A phylogenetic estimator of effective population size or mutation rate.

Genetics ◽  
1994 ◽  
Vol 136 (2) ◽  
pp. 685-692 ◽  
Author(s):  
Y X Fu
Keyword(s):  
Population Size ◽  
Effective Population Size ◽  
Mutation Rate ◽  
Dna Sequences ◽  
High Efficiency ◽  
Minimum Variance ◽  
Population Subdivision ◽  
Effective Population ◽  
Fisher Model ◽  
Mitochondrial Sequences

Abstract A new estimator of the essential parameter theta = 4Ne mu from DNA polymorphism data is developed under the neutral Wright-Fisher model without recombination and population subdivision, where Ne is the effective population size and mu is the mutation rate per locus per generation. The new estimator has a variance only slightly larger than the minimum variance of all possible unbiased estimators of the parameter and is substantially smaller than that of any existing estimator. The high efficiency of the new estimator is achieved by making full use of phylogenetic information in a sample of DNA sequences from a population. An example of estimating theta by the new method is presented using the mitochondrial sequences from an American Indian population.

Combinatorics of Genotype-Phenotype Maps: An RNA Case Study

2005 ◽  
Author(s):  
Christian M. Reidys
Keyword(s):  
Natural Selection ◽  
Genetic Drift ◽  
Optimization Problem ◽  
Neutral Theory ◽  
Biological Evolution ◽  
Neutral Evolution ◽  
Evolutionary Search ◽  
Random Drift ◽  
Molecular Change ◽  
Biological Phenomena

The fundamental mechanisms of biological evolution have fascinated generations of researchers and remain popular to this day. The formulation of such a theory goes back to Darwin (1859), who in the The Origin of Species presented two fundamental principles: genetic variability caused by mutation, and natural selection. The first principle leads to diversity and the second one to the concept of survival of the fittest, where fitness is an inherited characteristic property of an individual and can basically be identified with its reproduction rate. Wright [530, 531] first recognized the importance of genetic drift in evolution in improving the evolutionary search capacity of the whole population. He viewed genetic drift merely as a process that could improve evolutionary search. About a decade later, Kimura proposed [317] that the majority of changes that are observed in evolution at the molecular level are the results of random drift of genotypes. The neutral theory of Kimura does not deny that selection plays a role, but claims that no appreciable fraction of observable molecular change can be caused by selective forces: mutations are either a disadvantage or, at best, neutral in present day organisms. Only negative selection plays a major role in the neutral evolution, in that deleterious mutants die out due to their lower fitness. Over the last few decades, there has been a shift of emphasis in the study of evolution. Instead of focusing on the differences in the selective value of mutants and on population genetics, interest has moved to evolution through natural selection as an abstract optimization problem. Given the tremendous opportunities that computer science and the physical sciences now have for contributing to the study of biological phenomena, it is fitting to study the evolutionary optimization problem in the present volume. In this chapter, we adopt the following framework: assuming that selection acts exclusively upon isolated phenotypes, we introduce the following compositum of mappings . . . Genotypes→ Phenotypes →Fitness . . . . We will refer to the first map as to the genotype-phenotype map and call the preimage of a given phenotype its neutral network. Clearly, the main ingredients here are the phenotypes and genotypes and their respective organization. In the following we will study various combinatorial properties of phenotypes and genotypes for RNA folding maps.

scholarly journals Effective Population Size Predicts Local Rates but Not Local Mitigation of Read-through Errors

2020 ◽  
Vol 38 (1) ◽  
pp. 244-262
Author(s):  
Alexander T Ho ◽  
Laurence D Hurst
Keyword(s):  
Population Size ◽  
Effective Population Size ◽  
Stop Codon ◽  
Neutral Theory ◽  
Error Rates ◽  
Species Variation ◽  
Effective Population ◽  
Weak Selection ◽  
Error Mitigation ◽  
Good Proxy

Abstract In correctly predicting that selection efficiency is positively correlated with the effective population size (Ne), the nearly neutral theory provides a coherent understanding of between-species variation in numerous genomic parameters, including heritable error (germline mutation) rates. Does the same theory also explain variation in phenotypic error rates and in abundance of error mitigation mechanisms? Translational read-through provides a model to investigate both issues as it is common, mostly nonadaptive, and has good proxy for rate (TAA being the least leaky stop codon) and potential error mitigation via “fail-safe” 3′ additional stop codons (ASCs). Prior theory of translational read-through has suggested that when population sizes are high, weak selection for local mitigation can be effective thus predicting a positive correlation between ASC enrichment and Ne. Contra to prediction, we find that ASC enrichment is not correlated with Ne. ASC enrichment, although highly phylogenetically patchy, is, however, more common both in unicellular species and in genes expressed in unicellular modes in multicellular species. By contrast, Ne does positively correlate with TAA enrichment. These results imply that local phenotypic error rates, not local mitigation rates, are consistent with a drift barrier/nearly neutral model.

scholarly journals Selection in a growing bacterial/yeast colony biases results of mutation accumulation experiments

2021 ◽  
Author(s):  
Anjali Mahilkar ◽  
Sharvari Kemkar ◽  
Supreet Saini
Keyword(s):  
Natural Selection ◽  
Population Size ◽  
Effective Population Size ◽  
Colony Growth ◽  
Mutation Accumulation ◽  
Rate Of Change ◽  
Raw Material ◽  
Effective Population ◽  
The Face ◽  
Mean Fitness

AbstractMutations provide the raw material for natural selection to act. Therefore, understanding the variety and relative frequency of different type of mutations is critical to understanding the nature of genetic diversity in a population. Mutation accumulation (MA) experiments have been used in this context to estimate parameters defining mutation rates, distribution of fitness effects (DFE), and spectrum of mutations. MA experiments performed with organisms such asDrosophilahave an effective population size of one. However, in MA experiments with bacteria and yeast, a single founder is allowed to grow to a size of a colony (~108). The effective population size in these experiments is of the order of 10. In this scenario, while it is assumed that natural selection plays a minimal role in dictating the dynamics of colony growth and therefore, the MA experiment; this effect has not been tested explicitly. In this work, we simulate colony growth and perform an MA experiment, and demonstrate that selection ensures that, in an MA experiment, fraction of all mutations that are beneficial is over represented by a factor greater than two. The DFE of beneficial and deleterious mutations are accurately captured in an MA experiment. We show that the effect of selection in a growing colony varies non-monotonically and that, in the face of natural selection dictating an MA experiment, estimates of mutation rate of an organism is not trivial. We perform experiments with 160 MA lines ofE. coli, and demonstrate that rate of change of mean fitness is a non-monotonic function of the colony size, and that selection acts differently in different sectors of a growing colony. Overall, we demonstrate that the results of MA experiments need to be revisited taking into account the action of selection in a growing colony.

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