Search PubMed⌕ Search

SEARCH · Search PubMed

Results for “Fluctuating selection”

Search indexed PubMed citations on genomics, clinical trials, systematic reviews and public health. Explore titles, authors and supplied subject terms, then open the PubMed record.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 325 records · Page 18Linked to original sources
PubMed2005

An evolutionary relationship between genetic variation and phenotypic fluctuation.

The relevance of phenotype fluctuations among clones (i.e., organisms with identical genes) to evolution has recently been recognized both theoretically and experimentally. By considering the stability of the distributions of genetic variations and phenotype fluctuations, we derive a general inequality between the phenotype variance due to genetic differences and the intrinsic phenotype variance of clones. For a given mutation rate, an approximately linear relationship between the two is obtained which elucidates the consistency between the fundamental theorem of natural selection by Fisher and the evolutionary fluctuation-response relationship (fluctuation dissipation theorem) proposed recently. A general condition for the error catastrophe is also derived as the violation of the inequality, which sets up the limit to the speed of stable evolution. All of these theoretical results are confirmed by a numerical evolution experiment of a cell that consists of a catalytic reaction network. Based on the relationships proposed here, relevance of the phenotypic plasticity to evolution as well as the genetic assimilation is discussed.

Animals↗
PubMed1985

[Tinnitus therapy with lidocaine and tocainide].

A randomised clinical cross-over study was performed to evaluate the therapeutic value of lidocaine, tocainide infusion and oral application of tocainide against placebo in the treatment of tinnitus. 40 patients with sensory hearing loss of different origin and otherwise not treatable constant tinnitus were included. Lidocaine infusion showed a tinnitus suppression of more than 50 per cent in about 70 per cent of patients; in comparison, tocainide infusion in 55 per cent of the patients. Oral tocainide therapy was successful only in about 35 per cent of patients. This fact depends partially on the high proportion of side effects. Therefore, tocainide therapy should be used in such patients only, who suffer severely from their tinnitus. Patient selection can be done by the lidocaine test. Oral tocainide therapy is advisable only if the lidocaine test result is positive. The methodical problems in the assessment of the value of drugs in tinnitus treatment are discussed: Evaluation and elimination of placebo effects and spontaneous fluctuations of tinnitus intensity, as well as selection of patients included in the study.

Administration, Oral↗
PubMed2004

The 1- to 2-Hz oscillations in muscle force are exacerbated by stress, especially in older adults.

Although force fluctuations during a steady contraction are often heightened in old adults compared with young adults and are enhanced in young adults during the stress response, the mechanisms underlying the augmentation are uncertain. The purpose of the study was to compare the effect of a stressor on the plasma concentrations of selected stress hormones and on the force fluctuations experienced by young and old adults during the performance of a precision grip. Thirty-six men and women (19-86 yr) participated in a protocol that comprised anticipatory (30 min), stressor (15 min), and recovery periods (25 min). The stressor was a series of noxious electrical stimuli applied to the dorsal surface of the left hand. Subjects sustained a pinch-grip force with the right hand at 2% of the maximal voluntary contraction force. The fluctuations in pinch-grip force, the interference electromyogram (EMG) of six muscles, and the spectra for the force and EMG were quantified across the 70-min protocol. The stressor increased the force fluctuations, largely due to an enhancement of the power at 1-2 Hz in the force spectrum (r(2) = 0.46). The effect was greatest for the old adults compared with young and middle-aged adults. The plasma concentrations of the stress hormones (adrenocorticotropin, epinephrine, and norepinephrine) were elevated to similar levels for all three age groups, and the changes were not associated with modulation of the force fluctuations. Furthermore, the heightened EMG activity exhibited by the old adults during all periods was not related to the changes in the force fluctuations or the 1- to 2-Hz force oscillations. The absence of a change in the mean pinch-grip force during the protocol and the lack of an association between elevation of the plasma concentrations for the stress hormones and modulation of the force fluctuations suggest that the enhanced force fluctuations caused by the stressor was due to an increase in the low-frequency output of the spinal motor neurons.

Adolescent↗
PubMed2005

Creating order from random fluctuations in small spin ensembles.

We demonstrate the ability to create spin order by using a magnetic resonance force microscope to harness the naturally occurring statistical fluctuations in small ensembles of electron spins. In one method, we hyperpolarized the spin system by selectively capturing the transient spin order created by the statistical fluctuations. In a second method, we took a more active approach and rectified the spin fluctuations by applying real-time feedback to the entire spin ensemble. The created spin order can be stored in the laboratory frame for a period on the order of the longitudinal relaxation time of 30 seconds and then read out.

Journal Article↗
PubMed1996

Is routine use of episiotomy justified?

Episiotomy, one of the most common surgical procedures, was introduced in clinical practice in the eighteenth century without having strong scientific evidence of its benefits. Its use was justified by the prevention of severe perineal tears, better future sexual function, and a reduction of urine and fecal incontinence. With regard to the first assumption, the evidence that is based on five randomized controlled trials shows a 9% reduction in severe perineal tears in the selective use of episiotomy, but this effect fluctuates between a 40% reduction and a 38% increase. In relation to long-term effects, women in whom management includes routine use of episiotomy have shown poorer future sexual function, similar pelvic floor muscle strength, and similar urinary incontinence in comparison with women in whom episiotomy is used in a selective manner. In summary, there is no reliable evidence that routine use of episiotomy has any beneficial effect; on the contrary, there is clear evidence that it may cause harm such as a greater need for surgical repair and a poorer future sexual capability. In view of the available evidence the routine use of episiotomy should be abandoned and episiotomy rates > 30% do not seem justified.

Coitus↗
PubMed1991

[Adaptability of experimental populations of Drosophila melanogaster in directed and stabilizing selection].

Stabilizing selection for a set of morphometric wing traits was combined with directional selection for increased expression of radius incompletus (ri) mutation of Drosophila melanogaster. Three experimental regimens were used: directional and stabilizing selection (stabilized lines); directional selection (unstabilized lines); no selection (control lines). The dynamics of two fitness-associated parameters (coefficient of fluctuating asymmetry and population size) was registered during the experiment. At the end of experiment, selection competitive ability test was carried out. The competitive indices did not significantly differ under various treatments, while fluctuating asymmetry was significantly lower in stabilized than in unstabilized lines. Average population size was greater in stabilized lines as compared to the unstabilized ones. The possible causes for these differences are discussed.

Animals↗
PubMed2000

[Effect of chronic irradiation on the genetic structure of natural populations of Arabidopsis thaliana (L.) Heynh].

The genetic structure of Arabidopsis thaliana populations from in the Chernobyl zone of radioactive contamination was studied. The dynamics of genotypic and allele frequencies of eight polymorphic loci were described. Analysis of the results obtained and earlier data revealed selection for loci Per1 and Acp2. Sharp fluctuations of allele frequencies were observed in consecutive generations of chronically irradiated A. thaliana populations. These fluctuations led to replacement of the most frequent alleles, including those close to fixation.

Alleles↗
PubMed1999

Diet selection and animal state: an integrative framework.

In the present paper we deal with the problems of explaining and predicting diet selection of animals under controlled conditions, i.e. conditions that can be described and in which any influences of the environment can be either controlled or at least monitored. Diet selection is considered within an integrative framework of feeding behaviour that views both food intake and diet selection as an outcome of the animal's internal state and knowledge of the feeding environment. Three questions that arise from the framework are considered: (1) how do animals learn about foods available to them as a choice? (2) what changes in internal state affect diet selection? (3) how much time is needed for a change in the animal's internal state to be detected and for it to react to this change through a modification of its diet selection? It is proposed that animals have developed behavioural mechanisms that allow them to recognize foods on the basis of their nutritional as well as other properties. The rate at which animals learn about foods depends largely on the extent of the animal's deficiency and on the extent of the post-ingestive consequences induced by the foods. There is little evidence that animals modify their diet selection in response to short-term systemic fluctuation of their internal environment. On the other hand, long-term changes in the internal state of the animal lead to consequent long-term changes in diet selection. The time needed for a change in diet selection to be observed depends on the deviation created in the animal's internal state, either as a result of a physiological change or as a consequence of feeding. Thus, a more appropriate question to consider is not 'what time period matters to the animal?' but 'how much change or deviation in the internal state is the animal prepared to accept?'

Animals↗
PubMed2006

Mutation, selection and the future of human evolution.

Several recent analyses provide growing evidence of the influence of positive selection acting in the ancestors of modern humans. Additionally, the best way to explain current fluctuations in neutral variation across the genome is by including negative selection against a high rate of deleterious mutants. We suggest that explaining these predicted high deleterious mutation rates in humans could require the inclusion of additional factors, such as inbreeding and prezygotic selection, in addition to rank-order selection and fitness interactions among mutations. We also suggest that some forms of selection, rather than being relaxed in modern humans, are probably still acting and might intensify in the near future, and make some predictions about the next several millennia of human evolution.

Evolution, Molecular↗
PubMed2004

Fluctuation-driven molecular transport through an asymmetric membrane channel.

Channel proteins that selectively conduct molecules across cell membranes often exhibit an asymmetric structure. By means of a stochastic model, we argue that channel asymmetry in the presence of nonequilibrium fluctuations, fueled by the cell's metabolism as observed recently, can dramatically influence the transport through such channels by a ratchetlike mechanism. For an aquaglyceroporin that conducts water and glycerol, we show that a previously determined asymmetric glycerol potential leads to enhanced inward transport of glycerol, but for unfavorably high glycerol concentrations also to enhanced outward transport that protects a cell against poisoning.

Aquaporins↗
PubMed1991

[A new levodopa benserazide preparation for Parkinson's disease with motor fluctuations refractory to standard L-dopa].

As Parkinson's disease worsens, many patients develop motor fluctuations which usually correlate directly or indirectly with L-dopa plasma levels. A new L-dopa-benserazide HBS preparation (Madopar) a control release pharmaceutical formulation which is activated when it contacts gastric fluid thus providing more stable L-dopa plasma levels, was assayed. Ten patients with a diagnosis of idiopathic Parkinson's disease and motor fluctuations otherwise unresponsive to conventional therapy were selected. The average age was 62 years and the duration of the disease 9 years. The motor status was evaluated on an hourly basis with the King's College Parkinson's disease rating scale; in addition, a nocturnal disability scale (Lees) was used. Out of the 10 patients, 2 dropped out within the first month due to worsening of parkinsonian signs, while 7 of the remainders preferred HBS preparation to the previous treatment. The number of off hours in this group was reduced by 58% and motor fluctuation became less severe. In only 3 cases was it possible to use HBS as monotherapy while in the rest standard L-dopa had to be added, specially as morning doses. The average L-dopa daily dose was increased by 36%. Unwanted effects included psychiatric disturbances in two (in one L-dopa dose had to be reduced) and epigastralgia in one. Our findings suggest that this L-dopa-benserazide control release may be considered an able therapeutic formulation in the control of motor fluctuations in Parkinson's disease.

Adult↗
PubMed2025

Haplotype stacking to improve stability of stripe rust resistance in wheat.

Genotype-by-environment interaction analysis and haplotype-level characterisation provide novel insights into the stability of stripe rust resistance. Breeding selection strategies are proposed to achieve rapid and stable genetic gains across environments. This study investigated stripe/yellow rust (YR) responses in the Vavilov wheat diversity panel evaluated across 11 field experiments conducted in Australia and Ethiopia during 2014-2021. Genotype-by-environment interaction (GEI) was analysed using a factor analytic (FA) model. Genotype-level selection was performed with overall performance (OP) and root-mean-square deviation (RMSD), which reflected average performance and stability of YR resistance across environments, respectively. Genomic estimated breeding values (GEBV) for these traits were calculated and compared with those from a multi-trait GBLUP model with average performance represented by the mean GEBV across environments and stability by the standard deviation of GEBV across environments. The FA-based and multi-trait GBLUP GEBV had high correlations. Haplotypes with large effects on OP and RMSD were identified using the local GEBV method. Favourable haplotypes were then used for stacking in breeding simulations, using the Vavilov collection as a base. Compared to truncation selection, optimal haplotype selection (OHS) using an artificial intelligence (AI)-based algorithm achieved longer-term genetic gains for both OP and RMSD (after many generations) by initially selecting founder parents that maximised favourable haplotypes. Simulations using YR responses from diverse environments that mimicked fluctuating environmental conditions across seasons were conducted to evaluate strategies for selection of YR resistance that is stable across years. Strategies which gave most weight to OP, but some weight to RMSD were optimal in these conditions, and substantially reduced variation of performance across years. This study provides useful information for breeding cultivars with both high YR resistance and high stability of resistance across environments.

Triticum↗
PubMed1996

A search for a general phenomenon of adaptive mutability.

The most prominent systems for the study of adaptive mutability depend on the specialized activities of genetic elements like bacteriophage Mu and the F plasmid. Searching for general adaptive mutability, we have investigated the behavior of Salmonella typhimurium strains with chromosomal lacZ mutations. We have studied 30 revertible nonsense, missense, frameshift, and insertion alleles. One-third of the mutants produced > or = 10 late revertant colonies (appearing three to seven days after plating on selective medium). For the prolific mutants, the number of late revertants showed rank correlation with the residual beta-galactosidase activity; for the same mutants, revertant number showed no correlation with the nonselective reversion rate (from fluctuation tests). Leaky mutants, which grew slowly on selective medium, produced late revertants whereas tight nongrowing mutants generally did not produce late revertants. However, the number of late revertants was not proportional to residual growth. Using total residual growth and the nonselective reversion rate, the expected number of late revertants was calculated. For several leaky mutants, the observed revertant number exceeded the expected number. We suggest that excess late revertants from these mutants arise from general adaptive mutability available to any chromosomal gene.

Adaptation, Biological↗
PubMed2004

The effect of selection on genealogies.

The coalescent process can describe the effects of selection at linked loci only if selection is so strong that genotype frequencies evolve deterministically. Here, we develop methods proposed by Kaplan, Darden, and Hudson to find the effects of weak selection. We show that the overall effect is given by an extension to Price's equation: the change in properties such as moments of coalescence times is equal to the covariance between those properties and the fitness of the sample of genes. The distribution of coalescence times differs substantially between allelic classes, even in the absence of selection. However, the average coalescence time between randomly chosen genes is insensitive to the current allele frequency and is affected significantly by purifying selection only if deleterious mutations are common and selection is strong (i.e., the product of population size and selection coefficient, Ns>3). Balancing selection increases mean coalescence times, but the effect becomes large only when mutation rates between allelic classes are low and when selection is extremely strong. Our analysis supports previous simulations that show that selection has surprisingly little effect on genealogies. Moreover, small fluctuations in allele frequency due to random drift can greatly reduce any such effects. This will make it difficult to detect the action of selection from neutral variation alone.

Biological Evolution↗
PubMed1999

Jamaican Symmetry Project: long-term study of fluctuating asymmetry in rural Jamaican children.

Fluctuating asymmetry, small deviations from perfect bilateral symmetry, is negatively correlated with health and positively correlated with sexual selection in human adults, but the accumulation, persistence, and fitness implications of asymmetries during childhood are largely unknown. Here, we introduce the Jamaican Symmetry Project, a long-term study of fluctuating asymmetry and its physical and behavioral correlates in rural Jamaican children. The project is based on an initial sample of 285 children (156 boys and 129 girls), aged 5 to 11 years. We describe the design of the project and the methodology of measuring 10 paired morphometric traits. All traits except hand width showed fluctuating asymmetry. Fluctuating asymmetries of the legs tended to be related and were less than half as great as fluctuating asymmetries of the arms and ears. Therefore the legs may show high developmental stability resulting from selection for mechanical efficiency. A fluctuating asymmetry composite score revealed that boys have significantly lower fluctuating asymmetry than girls and that this effect resides mainly in the elbows. There were significant positive relationships between composite fluctuating asymmetry and age, height, and weight, but multiple regression analyses showed that age was negatively related to fluctuating asymmetry, whereas body size was positively correlated. These findings are compared with results from recent English studies.

Analysis of Variance↗
PubMed2006

Population bottlenecks in quasispecies dynamics.

The characteristics of natural populations result from different stochastic and deterministic processes that include reproduction with error, selection, and genetic drift. In particular, population fluctuations constitute a stochastic process that may play a very relevant role in shaping the structure of populations. For example, it is expected that small asexual populations will accumulate mutations at a higher rate than larger ones. As a consequence, in any population the fixation of mutations is accelerated when environmental conditions cause population bottlenecks. Bottlenecks have been relatively frequent in the history of life and it is generally accepted that they are highly relevant for speciation. Although population bottlenecks can occur in any species, their effects are more noticeable in organisms that form large and heterogeneous populations, such as RNA viral quasispecies. Bottlenecks can also positively select and isolate particles that still keep the ability to infect cells from a disorganized population created by crossing the error threshold.

Base Sequence↗
PubMed2004

Automated variable selection methods for logistic regression produced unstable models for predicting acute myocardial infarction mortality.

OBJECTIVES: Automated variable selection methods are frequently used to determine the independent predictors of an outcome. The objective of this study was to determine the reproducibility of logistic regression models developed using automated variable selection methods. STUDY DESIGN AND SETTING: An initial set of 29 candidate variables were considered for predicting mortality after acute myocardial infarction (AMI). We drew 1,000 bootstrap samples from a dataset consisting of 4,911 patients admitted to hospital with an AMI. Using each bootstrap sample, logistic regression models predicting 30-day mortality were obtained using backward elimination, forward selection, and stepwise selection. The agreement between the different model selection methods and the agreement across the 1,000 bootstrap samples were compared. RESULTS: Using 1,000 bootstrap samples, backward elimination identified 940 unique models for predicting mortality. Similar results were obtained for forward and stepwise selection. Three variables were identified as independent predictors of mortality among all bootstrap samples. Over half the candidate prognostic variables were identified as independent predictors in less than half of the bootstrap samples. CONCLUSION: Automated variable selection methods result in models that are unstable and not reproducible. The variables selected as independent predictors are sensitive to random fluctuations in the data.

Aged↗