scholarly journals Accumulation of mutations in sexual and asexual populations

1987 ◽  
Vol 49 (2) ◽  
pp. 135-146 ◽  
Author(s):  
Pekka Pamilo ◽  
Masatoshi Nei ◽  
Wen-Hsiung Li
Keyword(s):  
Weak Selection ◽  
Population Variance ◽  
Asexual Population ◽  
Term Evolution ◽  
A Genome ◽  
Analytical Formulae ◽  
The Mean ◽  
Asexual Populations ◽  
Mutant Genes

SummaryThe accumulation of beneficial and harmful mutations in a genome is studied by using analytical methods as well as computer simulation for different modes of reproduction. The modes of reproduction examined are biparental (bisexual, hermaphroditic), uniparental (selfing, automictic, asexual) and mixed (partial selfing, mixture of hermaphroditism and parthenogenesis). It is shown that the rates of accumulation of both beneficial and harmful mutations with weak selection depend on the within-population variance of the number of mutant genes per genome. Analytical formulae for this variance are derived for neutral mutant genes for hermaphroditic, selfing and asexual populations; the neutral variance is largest in a selfing population and smallest in an asexual population. Directional selection reduces the population variance in most cases, whereas recombination partially restores the reduced variance. Therefore, biparental organisms accumulate beneficial mutations at the highest rate and harmful mutations at the lowest rate. Selfing organisms are intermediate between biparental and asexual organisms. Even a limited amount of outcrossing in largely selfing and parthenogenetic organisms markedly affects the accumulation rates. The accumulation of mutations is likely to affect the mean population fitness only in long-term evolution.

scholarly journals The Local Mass Conservation in SPH

2003 ◽  
Vol 208 ◽  
pp. 411-412
Author(s):  
Yusuke Imaeda ◽  
Shu-ichiro Inutsuka
Keyword(s):  
Fluid Velocity ◽  
Mass Conservation ◽  
Local Mass ◽  
Term Evolution ◽  
Conservation Property ◽  
Local Mass Conservation ◽  
Particle Hydrodynamics ◽  
Smoothed Particle ◽  
The Mean

Smoothed particle hydrodynamics (SPH) is one of the widely used methods to calculate the various astrophysical fluid dynamics. However, standard SPH cannot accurately describe the long-term evolution of shear flows: The large density error emerges within a dynamical timescale, and the amplitude of the error becomes larger than the value of density itself (Δρ ≳ ρ), when we take the mean separation of the particles as the smoothing length. The origin of error is due to the inaccurate description of the continuity equation in the standard SPH formalism. To ensure the local mass conservation property, we have reformulated SPH, in which we distinguish the particle velocity and the fluid velocity for the updation of the particle positions. We find that the present modification provides an accurate description of the density evolution in SPH.

scholarly journals King's formula for the mutation load with epistasis.

Genetics ◽  
1988 ◽  
Vol 120 (3) ◽  
pp. 853-856
Author(s):  
A S Kondrashov ◽  
J F Crow
Keyword(s):  
Mutation Rate ◽  
Haploid Genome ◽  
Relative Fitness ◽  
Mutation Load ◽  
Asexual Population ◽  
Deleterious Alleles ◽  
Correct Formula ◽  
The Mean ◽  
Mutant Genes ◽  
New Mutations

Abstract A formula by J. L. King gives the equilibrium mutation load as L = 2 sigma ui(1 - qi)/z - x) in which ui is the mutation rate to deleterious alleles at the ith locus, qi is the frequency of mutant alleles at this locus, x is the mean number of such mutant genes per individual before selection, z is the mean number in individuals eliminated by selection, and the summation is over all relevant loci. We show that this rule is inaccurate for intense selection and that a correct formula is L = 2 sigma ui(1 - qi) w/(z - x) = 2U w/(z - x) = 2U/(z - x + 2U) in which U is the mean number of new mutations per haploid genome in the population and w is the mean relative fitness before selection. If w/(z - x) less than 1/2, the mutation load is less than the Haldane value (U less than or equal to L less than or equal to 2U) and can be considerably less. In a diploid asexual population, however, with independent occurrence of mutations, L = 1 - e-2U regardless of the mode of selection.

scholarly journals Nova Outbursts and the Secular Evolution of Cataclysmic Variables

1996 ◽  
Vol 158 ◽  
pp. 447-448
Author(s):  
K. Schenker ◽  
U. Kolb ◽  
H. Ritter
Keyword(s):  
Mass Transfer ◽  
Angular Momentum ◽  
Transfer Rate ◽  
Mass Transfer Rate ◽  
Secular Evolution ◽  
Term Evolution ◽  
Angular Momentum Loss ◽  
The Mean ◽  
Nova Outbursts

AbstractWe present calculations of the long-term evolution of CVs which include the influence of nova outbursts. In particular we investigate the consequences of the discontinuous mass loss due to recurring outburst events and the effects of frictional angular momentum loss (FAML), i.e. the interaction of the expanding nova envelope with the secondary. We show that a description assuming continuous mass loss – averaged over a complete nova cycle – is applicable for determining the mean mass transfer rate and the secular evolution both with and without FAML. Between two subsequent outbursts, deviations from the mean evolution depend on the strength of FAML and on the mass ejected during the outburst. Formally FAML is a consequential angular momentum loss [1] and therefore increases the mean mass transfer rate by pushing the systems closer to mass transfer instability. Depending on the actual strenghth of FAML the long-term evolution of CVs could be significantly different from the standard model predictions.

The Long‐Term Evolution of AR 7978: The Scalings of the Coronal Plasma Parameters with the Mean Photospheric Magnetic Field

2003 ◽  
Vol 586 (1) ◽  
pp. 579-591 ◽  
Author(s):  
L. van Driel‐Gesztelyi ◽  
P. Demoulin ◽  
C. H. Mandrini ◽  
L. Harra ◽  
J. A. Klimchuk
Keyword(s):  
Magnetic Field ◽  
Long Term Evolution ◽  
Photospheric Magnetic Field ◽  
Coronal Plasma ◽  
Plasma Parameters ◽  
Term Evolution ◽  
The Mean

scholarly journals Selection against harmful mutations in large sexual and asexual populations

1982 ◽  
Vol 40 (3) ◽  
pp. 325-332 ◽  
Author(s):  
Alexey S. Kondrashov
Keyword(s):  
Reasonable Assumption ◽  
New Mutation ◽  
Asexual Population ◽  
Sexual Population ◽  
A Genome ◽  
Large Populations ◽  
Asexual Populations ◽  
Average Fitness ◽  
Equilibrium Number ◽  
New Mutations

SUMMARYSelection against harmful mutations in large populations is studied assuming that the rate of fitness decrease grows with every new mutation added to a genome. Under this reasonable assumption (Mayr, 1970) the average fitness of a sexual population, without linkage between the loci, is higher, and the average equilibrium number of harmful mutations per individual lower, than in an asexual population. If a gamete contains on the average one or more new mutations, the resulting advantage of sexual reproduction and recombination seems to be sufficient to counterbalance the double advantage of parthenogenesis. Moreover, selection against harmful mutations is probably the most powerful factor preventing linkage disequilibrium even with epistatic interaction between the loci.

scholarly journals The effects of weak selection on neutral diversity at linked sites

2021 ◽  
Author(s):  
Brian Charlesworth
Keyword(s):  
Theoretical Models ◽  
Population Variation ◽  
Deleterious Mutations ◽  
Weak Selection ◽  
The Mean ◽  
Frequency Changes ◽  
Site Diversity ◽  
Neutral Mutations ◽  
Approximate Expressions

The effects of selection on variability at linked sites have an important influence on levels and patterns of within-population variation across the genome. Most theoretical models of these effects have assumed that selection is sufficiently strong that allele frequency changes at the loci concerned are largely deterministic. These models have led to the conclusion that directional selection for new selectively favorable mutations, or against recurrent deleterious mutations, reduces nucleotide site diversity at linked neutral sites. Recent work has shown, however, that fixations of weakly selected mutations, accompanied by significant stochastic changes in allele frequencies, can sometimes cause higher diversity at linked sites when compared with the effects of fixations of neutral mutations. The present paper extends this work by deriving approximate expressions for the mean times to loss and fixation of mutations subject to selection, and analysing the conditions under which selection increases rather than reduces these times. Simulations are used to examine the relations between diversity at a neutral site and the fixation and loss times of mutations at a linked site subject to selection. It is shown that the long-term level of neutral diversity can be increased over the equilibrium expectation in the absence of selection by recurrent fixations and losses of linked, weakly selected dominant or partially dominant favorable mutations, and by linked recessive or partially recessive deleterious mutations. The results are used to examine the conditions under which associative overdominance, as opposed to background selection, is likely to operate.

scholarly journals Fitness-valley crossing in subdivided asexual populations

2016 ◽  
Author(s):  
Michael R. McLaren
Keyword(s):  
Quantitative Description ◽  
Natural Populations ◽  
Asexual Population ◽  
Migration Rates ◽  
Large Populations ◽  
Subdivided Populations ◽  
The Mean ◽  
And Migration ◽  
Asexual Populations ◽  
Mean Time

AbstractAdaptations may require multiple mutations that are beneficial only in combination. To adapt, a lineage must acquire mutations that are individually neutral or deleterious before gaining the beneficial combination, thereby crossing a plateau or valley, respectively, in the mapping from genotype to fitness. Spatial population structure can facilitate plateau and valley crossing by allowing neutral and deleterious lineages to survive longer and produce more beneficial mutants. Here, we analyze adaptation across a two-mutation plateau or valley in an asexual population that is subdivided into discrete subpopulations, or demes, connected by migration. We describe how subdivision alters the dynamics of adaptation from those in an equally sized unstructured population and give a complete quantitative description of these dynamics for the island migration model. Subdivision can significantly decrease the waiting time for the adaptation if demes and migration rates are small enough that single-mutant lineages fix in one or more demes before producing the beneficial double mutant. But, the potential decrease is small in very large populations and may also be limited by the slow spread of the beneficial mutant in extremely subdivided populations. Subdivision has a smaller effect on the probability that the population adapts very quickly than on the mean time to adapt, which has important consequences in some applications, such as the development of cancer. Our results provide a general and intuitive framework for predicting the effects of spatial structure in other models and in natural populations.

scholarly journals Parameters and determinants of responses to selection in antibody libraries

2021 ◽  
Vol 17 (3) ◽  
pp. e1008751
Author(s):  
Steven Schulz ◽  
Sébastien Boyer ◽  
Matteo Smerlak ◽  
Simona Cocco ◽  
Rémi Monasson ◽  
...  
Keyword(s):  
Affinity Maturation ◽  
Response To Selection ◽  
Evolutionary Potential ◽  
Term Evolution ◽  
A Genome ◽  
Antibody Libraries ◽  
Selection For ◽  
The Impact ◽  
Selection Of

The sequences of antibodies from a given repertoire are highly diverse at few sites located on the surface of a genome-encoded larger scaffold. The scaffold is often considered to play a lesser role than highly diverse, non-genome-encoded sites in controlling binding affinity and specificity. To gauge the impact of the scaffold, we carried out quantitative phage display experiments where we compare the response to selection for binding to four different targets of three different antibody libraries based on distinct scaffolds but harboring the same diversity at randomized sites. We first show that the response to selection of an antibody library may be captured by two measurable parameters. Second, we provide evidence that one of these parameters is determined by the degree of affinity maturation of the scaffold, affinity maturation being the process by which antibodies accumulate somatic mutations to evolve towards higher affinities during the natural immune response. In all cases, we find that libraries of antibodies built around maturated scaffolds have a lower response to selection to other arbitrary targets than libraries built around germline-based scaffolds. We thus propose that germline-encoded scaffolds have a higher selective potential than maturated ones as a consequence of a selection for this potential over the long-term evolution of germline antibody genes. Our results are a first step towards quantifying the evolutionary potential of biomolecules.

Acute and Chronic Changes in Platelet Volume and Count After Cardiopulmonary Bypass Induced Thrombocytopenia in Man

1987 ◽  
Vol 57 (01) ◽  
pp. 55-58 ◽  
Author(s):  
J F Martin ◽  
T D Daniel ◽  
E A Trowbridge
Keyword(s):  
Platelet Count ◽  
Mean Platelet Volume ◽  
Artery Bypass ◽  
Bypass Graft ◽  
Control Group ◽  
Artery Bypass Graft ◽  
Platelet Volume ◽  
Young Males ◽  
The Mean

SummaryPatients undergoing surgery for coronary artery bypass graft or heart valve replacement had their platelet count and mean volume measured pre-operatively, immediately post-operatively and serially for up to 48 days after the surgical procedure. The mean pre-operative platelet count of 1.95 ± 0.11 × 1011/1 (n = 26) fell significantly to 1.35 ± 0.09 × 1011/1 immediately post-operatively (p <0.001) (n = 22), without a significant alteration in the mean platelet volume. The average platelet count rose to a maximum of 5.07 ± 0.66 × 1011/1 between days 14 and 17 after surgery while the average mean platelet volume fell from preparative and post-operative values of 7.25 ± 0.14 and 7.20 ± 0.14 fl respectively to a minimum of 6.16 ± 0.16 fl by day 20. Seven patients were followed for 32 days or longer after the operation. By this time they had achieved steady state thrombopoiesis and their average platelet count was 2.44 ± 0.33 × 1011/1, significantly higher than the pre-operative value (p <0.05), while their average mean platelet volume was 6.63 ± 0.21 fl, significantly lower than before surgery (p <0.001). The pre-operative values for the platelet volume and counts of these patients were significantly different from a control group of 32 young males, while the chronic post-operative values were not. These long term changes in platelet volume and count may reflect changes in the thrombopoietic control system secondary to the corrective surgery.

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