Página 1 dos resultados de 3549 itens digitais encontrados em 0.155 segundos

## "Avaliação de diferentes marcadores para a determinação da digestibilidade e taxa de passagem do alimento em suínos" ; Use of different markers for determining apparent digestibility and rate of passage in swine

Oetting, Liliana Lotufo
Fonte: Biblioteca Digitais de Teses e Dissertações da USP Publicador: Biblioteca Digitais de Teses e Dissertações da USP
Tipo: Dissertação de Mestrado Formato: application/pdf
Relevância na Pesquisa
65.5%

## Does Diploidy Increase the Rate of Adaptation?

Orr, H. A.; Otto, S. P.
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
65.69%
Explanations of the evolution of diploidy have focused on the advantages gained from masking deleterious alleles. Recent theory has shown, however, that masking does not always provide an advantage to diploidy and would never favor diploidy in predominantly asexual organisms. We explore a neglected alternative theory which posits that, by doubling the genome size, diploids double the rate at which favorable mutations arise. Consequently, the rate of adaptation in diploids is presumed to be faster than in haploids. The rate of adaptation, however, depends not only on the rate of appearance of new favorable mutations but also on the rate at which these mutations are incorporated (which depends on the population size and on the dominance of favorable mutations). We show that, in both asexuals and sexuals, doubling the mutation rate via diploidy often does not accelerate the rate of adaptation. Indeed, under many conditions, diploidy slows adaptation.

## The rate of adaptation in asexuals.

Orr, H A
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
65.67%
I study the population genetics of adaptation in asexuals. I show that the rate of adaptive substitution in an asexual species or nonrecombining chromosome region is a bell-shaped function of the mutation rate: at some point, increasing the mutation rate decreases the rate of substitution. Curiously, the mutation rate that maximizes the rate of adaptation depends solely on the strength of selection against deleterious mutations. In particular, adaptation is fastest when the genomic rate of mutation, U, equals the harmonic mean of selection coefficients against deleterious mutations, where we assume that selection for favorable alleles is milder than that against deleterious ones. This simple result is independent of the shape of the distribution of effects among favorable and deleterious mutations, population size, and the action of clonal interference. In the course of this work, I derive an approximation to the probability of fixation of a favorable mutation in an asexual genome or nonrecombining chromosome region in which both favorable and deleterious mutations occur.

## Keeping up with a warming world; assessing the rate of adaptation to climate change

Visser, Marcel E
Fonte: The Royal Society Publicador: The Royal Society
Tipo: Artigo de Revista Científica
EN
Relevância na Pesquisa
65.54%
The pivotal question in the debate on the ecological effects of climate change is whether species will be able to adapt fast enough to keep up with their changing environment. If we establish the maximal rate of adaptation, this will set an upper limit to the rate at which temperatures can increase without loss of biodiversity.

## How much do genetic covariances alter the rate of adaptation?

Agrawal, Aneil F.; Stinchcombe, John R.
Fonte: The Royal Society Publicador: The Royal Society
Tipo: Artigo de Revista Científica
EN
Relevância na Pesquisa
65.71%
Genetically correlated traits do not evolve independently, and the covariances between traits affect the rate at which a population adapts to a specified selection regime. To measure the impact of genetic covariances on the rate of adaptation, we compare the rate fitness increases given the observed G matrix to the expected rate if all the covariances in the G matrix are set to zero. Using data from the literature, we estimate the effect of genetic covariances in real populations. We find no net tendency for covariances to constrain the rate of adaptation, though the quality and heterogeneity of the data limit the certainty of this result. There are some examples in which covariances strongly constrain the rate of adaptation but these are balanced by counter examples in which covariances facilitate the rate of adaptation; in many cases, covariances have little or no effect. We also discuss how our metric can be used to identify traits or suites of traits whose genetic covariances to other traits have a particularly large impact on the rate of adaptation.

## Rate of Adaptation in Large Sexual Populations

Neher, R. A.; Shraiman, B. I.; Fisher, D. S.
Fonte: Genetics Society of America Publicador: Genetics Society of America
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
75.63%
Adaptation often involves the acquisition of a large number of genomic changes that arise as mutations in single individuals. In asexual populations, combinations of mutations can fix only when they arise in the same lineage, but for populations in which genetic information is exchanged, beneficial mutations can arise in different individuals and be combined later. In large populations, when the product of the population size N and the total beneficial mutation rate Ub is large, many new beneficial alleles can be segregating in the population simultaneously. We calculate the rate of adaptation, v, in several models of such sexual populations and show that v is linear in NUb only in sufficiently small populations. In large populations, v increases much more slowly as log NUb. The prefactor of this logarithm, however, increases as the square of the recombination rate. This acceleration of adaptation by recombination implies a strong evolutionary advantage of sex.

## Characterizing the Influence of Effective Population Size on the Rate of Adaptation: Gillespie’s Darwin Domain

Jensen, Jeffrey D.; Bachtrog, Doris
Fonte: Oxford University Press Publicador: Oxford University Press
Tipo: Artigo de Revista Científica
EN
Relevância na Pesquisa
65.63%
Characterizing the role of effective population size in dictating the rate of adaptive evolution remains a major challenge in evolutionary biology. Depending on the underlying distribution of fitness effects of new mutations, populations of different sizes may differ vastly in their rate of adaptation. Here, we collect polymorphism data at over 100 loci for two closely related Drosophila species with different current effective population sizes (Ne), Drosophila miranda and D. pseudoobscura, to evaluate the prevalence of adaptive evolution versus genetic drift in molecular evolution. Utilizing these large and consistently sampled data sets, we obtain greatly improved estimates of the demographic histories of both species. Specifically, although current Ne differs between these species, their ancestral sizes were much more similar. We find that statistical approaches capturing recent adaptive evolution (using patterns of polymorphisms) detect higher rates of adaptive evolution in the larger D. pseudoobscura population. In contrast, methods aimed at detecting selection over longer time periods (i.e., those relying on divergence data) estimate more similar rates of adaptation between the two species. Thus, our results suggest an important role of effective population size in dictating rates of adaptation and highlight how complicated population histories—as is probably the case for most species—can effect rates of adaptation. Additionally...

## Distribution of fixed beneficial mutations and the rate of adaptation in asexual populations

Good, Benjamin H.; Rouzine, Igor M.; Balick, Daniel J.; Hallatschek, Oskar; Desai, Michael M.
Tipo: Artigo de Revista Científica
EN
Relevância na Pesquisa
75.68%
When large asexual populations adapt, competition between simultaneously segregating mutations slows the rate of adaptation and restricts the set of mutations that eventually fix. This phenomenon of interference arises from competition between mutations of different strengths as well as competition between mutations that arise on different fitness backgrounds. Previous work has explored each of these effects in isolation, but the way they combine to influence the dynamics of adaptation remains largely unknown. Here, we describe a theoretical model to treat both aspects of interference in large populations. We calculate the rate of adaptation and the distribution of fixed mutational effects accumulated by the population. We focus particular attention on the case when the effects of beneficial mutations are exponentially distributed, as well as on a more general class of exponential-like distributions. In both cases, we show that the rate of adaptation and the influence of genetic background on the fixation of new mutants is equivalent to an effective model with a single selection coefficient and rescaled mutation rate, and we explicitly calculate these effective parameters. We find that the effective selection coefficient exactly coincides with the most common fixed mutational effect. This equivalence leads to an intuitive picture of the relative importance of different types of interference effects...

## Limits to the Rate of Adaptive Substitution in Sexual Populations

Weissman, Daniel B.; Barton, Nicholas H.
Fonte: Public Library of Science Publicador: Public Library of Science
Tipo: Artigo de Revista Científica
EN
Relevância na Pesquisa
65.58%
In large populations, many beneficial mutations may be simultaneously available and may compete with one another, slowing adaptation. By finding the probability of fixation of a favorable allele in a simple model of a haploid sexual population, we find limits to the rate of adaptive substitution, , that depend on simple parameter combinations. When variance in fitness is low and linkage is loose, the baseline rate of substitution is , where is the population size, is the rate of beneficial mutations per genome, and is their mean selective advantage. Heritable variance in log fitness due to unlinked loci reduces by under polygamy and under monogamy. With a linear genetic map of length Morgans, interference is yet stronger. We use a scaling argument to show that the density of adaptive substitutions depends on , , , and only through the baseline density: . Under the approximation that the interference due to different sweeps adds up, we show that , implying that interference prevents the rate of adaptive substitution from exceeding one per centimorgan per 200 generations. Simulations and numerical calculations confirm the scaling argument and confirm the additive approximation for ; for higher , the rate of adaptation grows above ...

## Rate of Adaptation in Sexuals and Asexuals: A Solvable Model of the Fisher–Muller Effect

Park, Su-Chan; Krug, Joachim
Fonte: Genetics Society of America Publicador: Genetics Society of America
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
65.64%
The adaptation of large asexual populations is hampered by the competition between independently arising beneficial mutations in different individuals, which is known as clonal interference. In classic work, Fisher and Muller proposed that recombination provides an evolutionary advantage in large populations by alleviating this competition. Based on recent progress in quantifying the speed of adaptation in asexual populations undergoing clonal interference, we present a detailed analysis of the Fisher–Muller mechanism for a model genome consisting of two loci with an infinite number of beneficial alleles each and multiplicative (nonepistatic) fitness effects. We solve the deterministic, infinite population dynamics exactly and show that, for a particular, natural mutation scheme, the speed of adaptation in sexuals is twice as large as in asexuals. This result is argued to hold for any nonzero value of the rate of recombination. Guided by the infinite population result and by previous work on asexual adaptation, we postulate an expression for the speed of adaptation in finite sexual populations that agrees with numerical simulations over a wide range of population sizes and recombination rates. The ratio of the sexual to asexual adaptation speed is a function of population size that increases in the clonal interference regime and approaches 2 for extremely large populations. The simulations also show that the imbalance between the numbers of accumulated mutations at the two loci is strongly suppressed even by a small amount of recombination. The generalization of the model to an arbitrary number L of loci is briefly discussed. If each offspring samples the alleles at each locus from the gene pool of the whole population rather than from two parents...

## The Rate of Adaptation in Large Sexual Populations with Linear Chromosomes

Weissman, Daniel B.; Hallatschek, Oskar
Fonte: Genetics Society of America Publicador: Genetics Society of America
Tipo: Artigo de Revista Científica
EN
Relevância na Pesquisa
65.45%
In large populations, multiple beneficial mutations may be simultaneously spreading. In asexual populations, these mutations must either arise on the same background or compete against each other. In sexual populations, recombination can bring together beneficial alleles from different backgrounds, but tightly linked alleles may still greatly interfere with each other. We show for well-mixed populations that when this interference is strong, the genome can be seen as consisting of many effectively asexual stretches linked together. The rate at which beneficial alleles fix is thus roughly proportional to the rate of recombination and depends only logarithmically on the mutation supply and the strength of selection. Our scaling arguments also allow us to predict, with reasonable accuracy, the fitness distribution of fixed mutations when the mutational effect sizes are broad. We focus on the regime in which crossovers occur more frequently than beneficial mutations, as is likely to be the case for many natural populations.

## Courtship Song Does Not Increase the Rate of Adaptation to a Thermally Stressful Environment in a Drosophila melanogaster Laboratory Population

Cabral, Larry G.; Holland, Brett
Fonte: Public Library of Science Publicador: Public Library of Science
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
75.54%
Courtship song in D. melanogaster contributes substantially to male mating success through female selection. We used experimental evolution to test whether this display trait is maintained through adaptive female selection because it indicates heritable male quality for thermal stress tolerance. We used non-displaying, outbred populations of D. melanogaster (nub1) mutants and measured their rate of adaptation to a new, thermally stressful environment, relative to wild-type control populations that retained courtship song. This design retains sexually selected conflict in both treatments. Thermal stress should select across genomes for newly beneficial alleles, increasing the available genetic and phenotypic variation and, therefore, the magnitude of female benefit derived from courtship song. Following introduction to the thermally stressful environment, net reproductive rate decreased 50% over four generations, and then increased 19% over the following 16 generations. There were no differences between the treatments. Possible explanations for these results are discussed.

## Economics of Adaptation to Climate Change : Ethiopia

World Bank
Fonte: Washington, DC Publicador: Washington, DC
EN_US
Relevância na Pesquisa
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The report is part of a broader study, the Economics of Adaptation to Climate Change (EACC), which has two objectives: (a) to develop a global estimate of adaptation costs for informing international climate negotiations; and (b) to help decision makers in developing countries assess the risks posed by climate change and design national strategies for adapting to it. This paper is one of a series of country-level studies, where national data were disaggregated to more local and sector levels, helping to understand adaptation from a bottom-up perspective. Ethiopia is heavily dependent on rainfed agriculture. Its geographical location and topography in combination with low adaptive capacity entail a high vulnerability to the impacts of climate change. Historically the country has been prone to extreme weather variability. Rainfall is highly erratic, most rain falls with high intensity, and there is a high degree of variability in both time and space. Since the early 1980s, the country has suffered seven major droughts five of which have led to famines in addition to dozens of local droughts. Major floods also occurred in different parts of the country in 1988...

## Economics of Adaptation to Climate Change : Mozambique

World Bank
Fonte: Washington, DC Publicador: Washington, DC
EN_US
Relevância na Pesquisa
65.57%
This report is part of a broader global study, the Economics of Adaptation to Climate Change (EACC), which has two principal objectives: (a) to develop a global estimate of adaptation costs for informing international climate negotiations; and (b) to help decision makers in developing countries assess the risks posed by climate change and design national strategies for adapting to it. The purpose of this study is to assist the Government of Mozambique in its efforts to understand the potential economic impacts of climate change and to support its efforts to develop sound policies and investments in response to these potential impacts. The Mozambique Economics of Adaptation to Climate Change (EACC) study selected four sectors that are believed to be vulnerable to climate change: (1) agriculture, which employs over 70 percent of the population; (2) energy, particularly hydropower generation, which is dependent on water runoff; (3) transport infrastructure, notably roads; and (4) coastal areas, which do not conform to a "sector" but characterize specific geographical areas vulnerable to floods and storm surges directly and indirectly related to sea level rise. The report ends with a discussion of seven lessons learned from the study. Volume 1contains the final report...

## Economics of Adaptation to Climate Change : Samoa

World Bank
Fonte: Washington, DC Publicador: Washington, DC
EN_US
Relevância na Pesquisa
65.63%
Over the last two decades Samoa has suffered major damage from two cyclones in 1990-91, minor damage from a third cyclone in 2004, and an earthquake tsunami in 2009. Changes in the scale and impact of these types of natural disasters are likely to be important consequences of climate change for the country because the increases in sea level and in average sea surface temperatures will increase theintensity and damage from major storms. Other potential impacts are linked to changes in the weather patterns associated with El Niño Southern Oscillation (ENSO) events. The primary concern focuses on the impact on agriculture, especially in periods of lower precipitation following strong El Niño episodes.This study examines the consequences of an increase in average temperatures of up to 1°C by 2050 and up to 2.75°C by 2100 for the frequency and intensity of major cyclones that hit the islands. Estimates of the economic damage caused by storms in the past have been used to calibrate a damage function that yields an estimated increase in the expected value of economic damage as the peak wind speeds for storms with return periods of 10...

## Adaptation of asexual populations under Muller’s ratchet

Bachtrog, D.; Gordo, I.
Fonte: Fundação Calouste Gulbenkian Publicador: Fundação Calouste Gulbenkian
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
65.66%
We study the population genetics of adaptation in nonequilibrium haploid asexual populations. We find that the accumulation of deleterious mutations, due to the operation of Muller’s ratchet, can considerably reduce the rate of fixation of advantageous alleles. Such reduction can be approximated reasonably well by a reduction in the effective population size. In the absence of Muller’s ratchet, a beneficial mutation can only become fixed if it creates the best possible genotype; if Muller’s ratchet operates, however, mutations initially arising in a nonoptimal genotype can also become fixed in the population, since the loss of the least-loaded class implies that an initially nonoptimal background can become optimal. We show that, while the rate at which adaptive mutations become fixed is reduced, the rate of fixation of deleterious mutations due to the ratchet is not changed by the presence of beneficial mutations as long as the rate of their occurrence is low and the deleterious effects of mutations (sd) are higher than the beneficial effects (sa). When sa . sd, the advantage of a beneficial mutation can outweigh the deleterious effects of associated mutations. Under these conditions, a beneficial allele can drag to fixation deleterious mutations initially associated with it at a higher rate than in the absence of advantageous alleles. We propose analytical approximations for the rates of accumulation of deleterious and beneficial mutations. Furthermore...

## Asymptotic behavior of the rate of adaptation

Yu, Feng; Etheridge, Alison; Cuthbertson, Charles
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
65.65%
We consider the accumulation of beneficial and deleterious mutations in large asexual populations. The rate of adaptation is affected by the total mutation rate, proportion of beneficial mutations and population size $N$. We show that regardless of mutation rates, as long as the proportion of beneficial mutations is strictly positive, the adaptation rate is at least $\mathcal{O}(\log^{1-\delta}N)$ where $\delta$ can be any small positive number, if the population size is sufficiently large. This shows that if the genome is modeled as continuous, there is no limit to natural selection, that is, the rate of adaptation grows in $N$ without bound.; Comment: Published in at http://dx.doi.org/10.1214/09-AAP645 the Annals of Applied Probability (http://www.imstat.org/aap/) by the Institute of Mathematical Statistics (http://www.imstat.org)

## Rate of Adaptation in Large Sexual Populations

Neher, Richard A.; Shraiman, Boris I.; Fisher, Daniel S.
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
65.64%
Adaptation often involves the acquisition of a large number of genomic changes which arise as mutations in single individuals. In asexual populations, combinations of mutations can fix only when they arise in the same lineage, but for populations in which genetic information is exchanged, beneficial mutations can arise in different individuals and be combined later. In large populations, when the product of the population size N and the total beneficial mutation rate U_b is large, many new beneficial alleles can be segregating in the population simultaneously. We calculate the rate of adaptation, v, in several models of such sexual populations and show that v is linear in NU_b only in sufficiently small populations. In large populations, v increases much more slowly as log NU_b. The prefactor of this logarithm, however, increases as the square of the recombination rate. This acceleration of adaptation by recombination implies a strong evolutionary advantage of sex.

## Muscle Adaptation and Challenges to Motor Stability During Walking in Young Healthy Individuals

Schoenfeld, Jason
Fonte: University of Delaware Publicador: University of Delaware
EN
Relevância na Pesquisa
65.5%
Katherine Rudolph; It is well established that humans adapt to sudden or unexpected perturbations through a posture control system that has vestibular, visual, proprioceptive, and sensory components. Older adults, however, are not able to generate postural responses as quickly as young adults when environmental conditions or sensory cues change rapidly. The slower postural responses may arise from progressive degeneration of the nervous system as a result of normal aging. To investigate the neuromuscular responses at the knee to perturbations during walking, ten ???young healthy??? (mean= 21.9 years; range= 20-23; sex= 4 males) and ten healthy ???older adults??? (mean= 63 years; range= 52-73; sex= 7 males) walked for 10 level trials across a 13 m walkway. Next, they walked for 30 trials across a custom-built, movable platform (NSK Ltd, Tokyo, Japan) that translated laterally when stepped upon. EMG and knee kinematic data were collected. Subjects also underwent knee proprioception and stiffness testing on a custom built Stiffness and Proprioception Assessment Device (SPAD). There were no group differences in the magnitude of muscle activation in response to the first perturbation, or in the rate of adaptation over the first five trials. However...

## RATES OF FITNESS DECLINE AND REBOUND SUGGEST PERVASIVE EPISTASIS

Perfeito, L.; Sousa, A.; Bataillon, T.; Gordo, I.