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

Entanglement and Frustration in Ordered Systems

This article reviews and extends recent results concerning entanglement and frustration in multipartite systems which have some symmetry with respect to the ordering of the particles. Starting point of the discussion are Bell inequalities: their relation to frustration in classical systems and their satisfaction for quantum states which have a symmetric extension. It is then discussed how more general global symmetries of multipartite systems constrain the entanglement between two neighboring particles. We prove that maximal entanglement (measured in terms of the entanglement of formation) is always attained for the ground state of a certain nearest neighbor interaction Hamiltonian having the considered symmetry with the achievable amount of entanglement being a function of the ground state energy. Systems of Gaussian states, i.e. quantum harmonic oscillators, are investigated in more detail and the results are compared to what is known about ordered qubit systems.Comment: 13 pages, for the Proceedings of QIT-EQIS'0

Do school inspections improve primary school performance?

Inspectors from the Dutch Inspectorate of Education inspect primary schools, write inspection reports on each inspected school, and make recommendations as to how each school can improve. We test whether these inspections result in better school performance. Using a fixed-effects model, we find evidence that school inspections do lead to measurably better school performance. Our assessment of school performance is based on the Cito test scores of pupils in their final year of primary school. Therefore school improvement means increased Cito test scores. The results indicate that the Cito test scores improve by 2% to 3% of a standard deviation of the test score in the two years following an inspection. The arithmetic component shows the largest improvement. Our estimates are the result of an analysis of two types of school inspections performed between 1999 and 2002, where one type was more intensive than the other. In one fixed-effects model, we assume that the effect of the two types of school inspections was the same. We cannot, however, be sure that the estimates from this model are free from the problem of endogeneity bias. Therefore, we also obtain estimates for a less restrictive fixed-effects model. In this less restrictive model, we make use of the fact that a subset of the more intensive school inspections occurs at a representative selection of primary schools. Based on this smaller, essentially randomly drawn sample of schools, we can be confident that these estimates of the effect of school inspections are free from endogeneity bias. Due to the limited number of inspections at randomly selected schools, these estimates are not significantly different from zero. These estimates are, however, consistent with the effects found based on all inspections. The less restrictive model also allows for the effect of the more intensive inspections to differ from that for the less intensive ones. We find evidence that the more intensive inspections are responsible for larger increases in the Cito test scores than the less intensive ones.

Anisotropic magnetoresistance and anisotropic tunneling magnetoresistance due to quantum interference in ferromagnetic metal break junctions

We measure the low-temperature resistance of permalloy break junctions as a function of contact size and the magnetic field angle, in applied fields large enough to saturate the magnetization. For both nanometer-scale metallic contacts and tunneling devices we observe large changes in resistance with angle, as large as 25% in the tunneling regime. The pattern of magnetoresistance is sensitive to changes in bias on a scale of a few mV. We interpret the effect as a consequence of conductance fluctuations due to quantum interference.Comment: 4 pages, 4 figures. Changes in response to reviewer comments. New data provide information about the mechanism causing the AMR and TAM