402 research outputs found
Stability and response of polygenic traits to stabilizing selection and mutation
When polygenic traits are under stabilizing selection, many different
combinations of alleles allow close adaptation to the optimum. If alleles have
equal effects, all combinations that result in the same deviation from the
optimum are equivalent. Furthermore, the genetic variance that is maintained by
mutation-selection balance is per locus, where is the mutation
rate and the strength of stabilizing selection. In reality, alleles vary in
their effects, making the fitness landscape asymmetric, and complicating
analysis of the equilibria. We show that that the resulting genetic variance
depends on the fraction of alleles near fixation, which contribute by , and on the total mutational effects of alleles that are at intermediate
frequency. The interplay between stabilizing selection and mutation leads to a
sharp transition: alleles with effects smaller than a threshold value of
remain polymorphic, whereas those with larger effects are
fixed. The genetic load in equilibrium is less than for traits of equal
effects, and the fitness equilibria are more similar. We find that if the
optimum is displaced, alleles with effects close to the threshold value sweep
first, and their rate of increase is bounded by . Long term
response leads in general to well-adapted traits, unlike the case of equal
effects that often end up at a sub-optimal fitness peak. However, the
particular peaks to which the populations converge are extremely sensitive to
the initial states, and to the speed of the shift of the optimum trait value.Comment: Accepted in Genetic
Zero-bias anomalies on SrLaCuO thin films
High-impedance contacts made on the surface of SrLaCuO
superconducting thin films systematically display a zero-bias anomaly. We
consider two-level systems (TLS) as the origin of this anomaly. We observe that
the contribution of some TLS to the contact resistance is weakened by a
magnetic field. We show that this could result from the increase of the TLS
relaxation rate in the superconducting state, due to its ability to create
pairs of quasiparticles out of the condensate, when located close to the
surface of the film
Determinism, Noise, and Spurious Estimations in a Generalised Model of Population Growth
We study a generalised model of population growth in which the state variable
is population growth rate instead of population size. Stochastic parametric
perturbations, modelling phenotypic variability, lead to a Langevin system with
two sources of multiplicative noise. The stationary probability distributions
have two characteristic power-law scales. Numerical simulations show that noise
suppresses the explosion of the growth rate which occurs in the deterministic
counterpart. Instead, in different parameter regimes populations will grow with
``anomalous'' stochastic rates and (i) stabilise at ``random carrying
capacities'', or (ii) go extinct in random times. Using logistic fits to
reconstruct the simulated data, we find that even highly significant
estimations do not recover or reflect information about the deterministic part
of the process. Therefore, the logistic interpretation is not biologically
meaningful. These results have implications for distinct model-aided
calculations in biological situations because these kinds of estimations could
lead to spurious conclusions.Comment: Accepted in Physica A. Updated with [minor] observations from the
reffere
Advances in Modeling of Scanning Charged-Particle-Microscopy Images
Modeling artificial scanning electron microscope (SEM) and scanning ion
microscope images has recently become important. This is because of the need to
provide repeatable images with a priori determined parameters. Modeled
artificial images are highly useful in the evaluation of new imaging and
metrological techniques, like image-sharpness calculation, or drift-corrected
image composition (DCIC). Originally, the NIST-developed artificial image
generator was designed only to produce the SEM images of gold-on-carbon
resolution sample for image-sharpness evaluation. Since then, the new improved
version of the software was written in C++ programming language and is in the
Public Domain. The current version of the software can generate arbitrary
samples, any drift function, and many other features. This work describes
scanning in charged-particle microscopes, which is applied both in the
artificial image generator and the DCIC technique. As an example, the
performance of the DCIC technique is demonstrated.Comment: 9 pages, 6 figure
Keijsers, Shklyarevskii and van Kempen Reply
Answer to the Comment on ``Point-Contact Study of Fast and Slow Two-Level
Fluctuators in Metallic Glasses'' by Jan von Delft et al.Comment: 3 pages, no figures, accepted Phys. Rev. Letter
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