17,573 research outputs found
On the feasibility of saltational evolution
Is evolution always gradual or can it make leaps? We examine a mathematical
model of an evolutionary process on a fitness landscape and obtain analytic
solutions for the probability of multi-mutation leaps, that is, several
mutations occurring simultaneously, within a single generation in one genome,
and being fixed all together in the evolving population. The results indicate
that, for typical, empirically observed combinations of the parameters of the
evolutionary process, namely, effective population size, mutation rate, and
distribution of selection coefficients of mutations, the probability of a
multi-mutation leap is low, and accordingly, the contribution of such leaps is
minor at best. However, we show that, taking sign epistasis into account, leaps
could become an important factor of evolution in cases of substantially
elevated mutation rates, such as stress-induced mutagenesis in microbes. We
hypothesize that stress-induced mutagenesis is an evolvable adaptive strategy.Comment: Extended version, in particular, the section is added on
non-equilibrium model of stress-induced mutagenesi
Integrable quadratic Hamiltonians on so(4) and so(3,1)
We investigate a special class of quadratic Hamiltonians on so(4) and so(3,1)
and describe Hamiltonians that have additional polynomial integrals. One of the
main results is a new integrable case with an integral of sixth degree.Comment: 16 page
Chiral criticality in doped MnFeSi compounds
The critical spin fluctuations in doped compounds MnFeSi have
been studied by means of ac-susceptibility measurements, polarized neutron
small angle scattering and spin echo spectroscopy. It is shown that these
compounds undergo the transition from the paramagnetic to helimagnetic phase
through continuous, yet well distinguishable crossovers: (i) from paramagnetic
to partially chiral, (ii) from partially chiral to highly chiral fluctuating
state. The crossover points are identified on the basis of combined analysis of
the temperature dependence of ac-susceptibility and polarized SANS data. The
whole transition is marked by two inflection point of the temperature
dependence of ac-susceptibility: the upper one corresponds to the crossover to
partially chiral state at , where the inverse correlation length , the lower one corresponds to the transition to the spin helix
structure. The intermediate crossover to the highly chiral phase is observed at
the inflection point of the first derivative of ac-susceptibility, where
. The temperature crossovers to the highly chiral fluctuating
state is associated with the enhancing influence of the Dzyaloshinskii-Moria
interaction close to .Comment: 5 pages, 5 figures, 1 table, 13 cite
Comparable contributions of structural-functional constraints and expression level to the rate of protein sequence evolution
<p>Abstract</p> <p>Background</p> <p>Proteins show a broad range of evolutionary rates. Understanding the factors that are responsible for the characteristic rate of evolution of a given protein arguably is one of the major goals of evolutionary biology. A long-standing general assumption used to be that the evolution rate is, primarily, determined by the specific functional constraints that affect the given protein. These constrains were traditionally thought to depend both on the specific features of the protein's structure and its biological role. The advent of systems biology brought about new types of data, such as expression level and protein-protein interactions, and unexpectedly, a variety of correlations between protein evolution rate and these variables have been observed. The strongest connections by far were repeatedly seen between protein sequence evolution rate and the expression level of the respective gene. It has been hypothesized that this link is due to the selection for the robustness of the protein structure to mistranslation-induced misfolding that is particularly important for highly expressed proteins and is the dominant determinant of the sequence evolution rate.</p> <p>Results</p> <p>This work is an attempt to assess the relative contributions of protein domain structure and function, on the one hand, and expression level on the other hand, to the rate of sequence evolution. To this end, we performed a genome-wide analysis of the effect of the fusion of a pair of domains in multidomain proteins on the difference in the domain-specific evolutionary rates. The mistranslation-induced misfolding hypothesis would predict that, within multidomain proteins, fused domains, on average, should evolve at substantially closer rates than the same domains in different proteins because, within a mutlidomain protein, all domains are translated at the same rate. We performed a comprehensive comparison of the evolutionary rates of mammalian and plant protein domains that are either joined in multidomain proteins or contained in distinct proteins. Substantial homogenization of evolutionary rates in multidomain proteins was, indeed, observed in both animals and plants, although highly significant differences between domain-specific rates remained. The contributions of the translation rate, as determined by the effect of the fusion of a pair of domains within a multidomain protein, and intrinsic, domain-specific structural-functional constraints appear to be comparable in magnitude.</p> <p>Conclusion</p> <p>Fusion of domains in a multidomain protein results in substantial homogenization of the domain-specific evolutionary rates but significant differences between domain-specific evolution rates remain. Thus, the rate of translation and intrinsic structural-functional constraints both exert sizable and comparable effects on sequence evolution.</p> <p>Reviewers</p> <p>This article was reviewed by Sergei Maslov, Dennis Vitkup, Claus Wilke (nominated by Orly Alter), and Allan Drummond (nominated by Joel Bader). For the full reviews, please go to the Reviewers' Reports section.</p
Non-perturbative gluon evolution, squeezing, correlations and chaos in jets
We study evolution of colour gluon states in isolated QCD jet at the
non-perturbative stage. Fluctuations of gluons are less than those for coherent
states under specific conditions. This fact suggests that there gluon squeezed
states can arise. The angular and rapidity dependencies of the normalized
second-order correlation function for present gluon states are studied at this
stage of jet evolution. It is shown that these new gluon states can have both
sub-Poissonian and super-Poissonian statistics corresponding to, respectively,
antibunching and bunching of gluons by analogy with squeezed photon states.
We investigate the possibility of coexisting both squeezing and chaos using
Toda criterion and temporal correlator analysis. It is shown that these effects
may coexist under some conditions.Comment: 18 pages, 3 figures, Reported on IPPP Workshop on Multiparticle
Production in QCD Jets (University of Durham, Durham, UK, 12-15 December
2001
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