102,336 research outputs found
Exploring the effect of sex on empirical fitness landscapes
The nature of epistasis has important consequences for the evolutionary significance of sex and recombination. Recent efforts to find negative epistasis as a source of negative linkage disequilibrium and associated long-term advantage to sex have yielded little support. Sign epistasis, where the sign of the fitness effects of alleles varies across genetic backgrounds, is responsible for the ruggedness of the fitness landscape, with several unexplored implications for the evolution of sex. Here, we describe fitness landscapes for two sets of strains of the asexual fungus Aspergillus niger involving all combinations of five mutations. We find that 30% of the single-mutation fitness effects are positive despite their negative effect in the wild-type strain and that several local fitness maxima and minima are present. We then compare adaptation of sexual and asexual populations on these empirical fitness landscapes by using simulations. The results show a general disadvantage of sex on these rugged landscapes, caused by the breakdown by recombination of genotypes on fitness peaks. Sex facilitates movement to the global peak only for some parameter values on one landscape, indicating its dependence on the landscape’s topography. We discuss possible reasons for the discrepancy between our results and the reports of faster adaptation of sexual population
Fundamental vs. Solitonic Description of D3 branes
Type IIB string theory expanded around D3 brane backgrounds describes the
dynamics of D3 branes as solitonic objects. On the other hand, there is a
fundamental description of them via Polchinski's open strings with Dirichlet
boundary conditions. Since these two descriptions describe the dynamics of the
same objects, D3 branes, it is natural to believe that they are dual. Therefore
at this level, we have a string-string duality as opposed to a string-field
theory duality. Once we take the same limits in both descriptions, Maldacena
Conjecture in its weaker form follows. We try to make this viewpoint precise
and study the implication of it for the stronger form of Maldacena Conjecture.Comment: 11 pages, 3 figures, typos correcte
Rotation of Cosmic Voids and Void-Spin Statistics
We present a theoretical study of void spins and their correlation
properties. The concept of the spin angular momentum for an unbound void is
introduced to quantify the effect of the tidal field on the distribution of
matter that make up the void. Both the analytical and numerical approaches are
used for our study. Analytically, we adopt the linear tidal torque model to
evaluate the void spin-spin and spin-density correlations, assuming that a void
forms in the initial region where the inertia momentum and the tidal shear
tensors are maximally uncorrelated with each other. Numerically, we use the
Millennium run galaxy catalog to find voids and calculate their spin
statistics. The numerical results turn out to be in excellent agreement with
the analytic predictions, both of which consistently show that there are strong
spatial alignments between the spin axes of neighbor voids and strong
anti-alignments between the void spin axes and the directions to the nearest
voids. We expect that our work will provide a deeper insight into the origin
and properties of voids and the large scale structure.Comment: accepted version, ApJ in press, the concept of void spins explained,
typos correcte
Hybrid-functional and quasi-particle calculations of band structures of Mg2Si, Mg2Ge, and Mg2Sn
We perform hybrid functional and quasi-particle band structure calculations
with spin-orbit interaction to investigate the band structures of Mg2Si, Mg2Ge,
and Mg2Sn. For all Mg2X materials, where X = Si, Ge, and Sn, the
characteristics of band edge states, i.e., band and valley degeneracies, and
orbital characters, are found to be conserved, independent of the computational
schemes such as density functional generalized gradient approximation, hybrid
functionals, or quasi-particle calculations. However, the magnitude of the
calculated band gap varies significantly with the computational schemes. Within
density-functional calculations, the one-particle band gaps of Mg2Si, Mg2Ge,
and Mg2Sn are 0.191, 0.090, and -0.346 eV, respectively, and thus severely
underestimated compared to the experimental gaps, due to the band gap error in
the density functional theory and the significant relativistic effect on the
low-energy band structures. By employing hybrid-functional calculations with a
35% fraction of the exact Hartree-Fock exchange energy (HSE-35%), we overcame
the negative band gap issue in Mg2Sn. Finally, in quasi-particle calculations
on top of the HSE-35% Hamiltonians, we obtained band gaps of 0.835, 0.759, and
0.244 eV for Mg2Si, Mg2Ge, and Mg2Sn, respectively, consistent with the
experimental band gaps of 0.77, 0.74, and 0.36 eV, respectively.Comment: 23 pages, including 84 references, 5 tables, 3 figure
Massive Integrable Soliton Theories
Massive integrable field theories in dimensions are defined at the
Lagrangian level, whose classical equations of motion are related to the
``non-abelian'' Toda field equations. They can be thought of as generalizations
of the sine-Gordon and complex sine-Gordon theories. The fields of the theories
take values in a non-abelian Lie group and it is argued that the coupling
constant is quantized, unlike the situation in the sine-Gordon theory, which is
a special case since its field takes values in an abelian group. It is further
shown that these theories correspond to perturbations of certain coset
conformal field theories. The solitons in the theories will, in general, carry
non-abelian charges.Comment: 18 pages, no figures, plain tex with macro include
Propagator for spinless and spin-1/2 Aharonov-Bohm-Coulomb systems
The propagator of the spinless Aharonov-Bohm-Coulomb system is derived by
following the Duru-Kleinert method. We use this propagator to explore the
spin-1/2 Aharonov-Bohm-Coulomb system which contains a point interaction as a
Zeeman term. Incorporation of the self-adjoint extension method into the
Green's function formalism properly allows us to derive the finite propagator
of the spin-1/2 Aharonov-Bohm-Coulomb system. As a by-product, the relation
between the self-adjoint extension parameter and the bare coupling constant is
obtained. Bound-state energy spectra of both spinless and spin-1/2
Aharonov-Bohm-Coulomb systems are examined.Comment: Slightly changed, to appear in Annals of Physics. 18 pages, Late
Splitting of Surface Plasmon Frequencies of Metal Particles in a Nematic Liquid Crystal
We calculate the effective dielectric function for a suspension of small
metallic particles immersed in a nematic liquid crystal (NLC) host. For a
random suspension of such particles in the dilute limit, we calculate the
effective dielectric tensor exactly and show that the surface plasmon
(SP)resonance of such particles splits into two resonances, polarized parallel
and perpendicular to the NLC director. At higher concentrations, we calculate
this splitting using a generalized Maxwell-Garnett approximation, which can
also be applied to a small metal particle coated with NLC. To confirm the
accuracy of the MGA for NLC-coated spheres, we also use the Discrete Dipole
Approximation. The calculated splitting is comparable to that observed in
recent experiments on NLC-coated small metal particlesComment: 11 pages, 2 figures. To be published in Appl. Phys. Let
The Properties of Type Ia Supernova Host Galaxies from the Sloan Digital Sky Survey
We investigate the properties and environments of Type Ia Supernova (SN Ia)
host galaxies in the Stripe 82 of the Sloan Digital Sky Survey-II Supernova
Survey centered on the celestial equator. Host galaxies are defined as the
galaxy nearest to the supernova (SN) in terms of angular distance whose
velocity difference from the SN is less than 1000 km s^{-1}. Eighty seven SN Ia
host galaxies are selected from the SDSS Main galaxy sample with the apparent
r-band magnitude m_r < 17.77, and compared with the SDSS Main galaxies. The SN
Ia rates for early and late-type galaxies are 0.81 +- 0.19 SN (100yr)^{-1} and
0.99 +- 0.21 SN (100yr)^{-1}, respectively. We find that the host galaxies have
a color distribution consistent with that of the Main galaxies, regardless of
their morphology. However, host galaxies are on average brighter than the Main
galaxies by ~ 0.3 mag over the range of -18.3 > M_r > -21.3. But the brighter
ends of their luminosity distributions are similar. The distribution of the
distance to the nearest neighbor galaxy shows that SNe Ia are more likely to
occur in isolated galaxies without close neighbors. We also find that the SN Ia
host galaxies are preferentially located in a region close to massive galaxy
clusters compared to the Main galaxies.Comment: 10 pages, 8 figures, accepted for publication on Ap
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