Speed of adaptation in structured populations

Adaptation of populations takes place with the occurrence and subsequent fixation of mutations that confer some selective advantage to the individuals which acquire it. For this reason, the study of the process of fixation of advantageous mutations has a long history in the population genetics literature. Particularly, the previous investigations aimed to find out the main evolutionary forces affecting the strength of natural selection in the populations. In the current work, we investigate the dynamics of fixation of beneficial mutations in a subdivided population. The subpopulations (demes) can exchange migrants among their neighbors, in a migration network which is assumed to have either a random graph or a scale-free topology. We have observed that the migration rate drastically affects the dynamics of mutation fixation, despite of the fact that the probability of fixation is invariant on the migration rate, accordingly to Maruyama's conjecture. In addition, we have noticed a topological dependence of the adaptive evolution of the population when clonal interference becomes effective.     

Evolution of Weighted Networks by Duplication-Divergence Mechanism

The duplication and divergence process is ubiquitous in nature and man-made networks. Motivated by the duplication-divergence mechanism which depicts the growth of protein networks, we propose a weighted network model in which topological evolution is coupled with weight dynamics. Large scale numerical results indicate that our model can naturally generate networks with power-law-like distributions of degree, strength and weight. The degree-strength correlation is illustrated as well. These properties are in agreement well with empirical data observed in real-world systems. Furthermore, by altering the retention probability δ, weighted, structured exponential networks are realized.     

From unweighted to weighted networks with local information

In this paper, we analyze an evolving model with local information which can generate a class of networks by choosing different values of the parameter p. The model introduced exhibits the transition from unweighted networks to weighted networks because the distribution of the edge weight can be widely tuned. With the increase in the local information, the degree correlation of the network transforms from assortative to disassortative. We also study the distribution of the degree, strength and edge weight, which all show crossover between exponential and scale-free. Finally, an application of the proposed model to the study of the synchronization is considered. It is concluded that the synchronizability is enhanced when the heterogeneity of the edge weight is reduced.     

Role of Yb and Tm ions in upconversion emission of Tb under 798 and 980 nm laser excitations in Tb-Tm-Yb doped tellurite glass

Upconversion emission and energy transfer processes in singly, doubly and triply doped tellurite glasses have been studied under 798 and 980 nm laser excitations. Emissions have been observed at 482, 544, 584, 655 nm and at 477, 655, 698, 800 nm corresponding to Tb³⁺: ⁵D₄ → ⁷F₆, ⁷F₅, ⁷F₄, ⁷F_J (J = 0, 1, 2, 3) and Tm³⁺: ¹G₄ → ³H₆, ¹G₄ → ³F₄, ³F₃ → ³H₆, ³H₄ → ³H₆ transitions, respectively. Among Tm³⁺, Yb³⁺and Tb³⁺ ions only Tm³⁺ has a ground state absorption at 798 nm excitation due to ³H₄ ← ³H₆ transition. For 980 nm excitation only Yb³⁺ can absorb the incident radiation. However, for both types of excitations, emission from all the three ions Tb, Yb and Tm has been observed. Possible mechanisms are proposed as follows: under 798 nm excitation Tm³⁺ ions are excited which excite Yb³⁺ ions through energy transfer. Finally “cooperative energy transfer” from a pair of Yb³⁺ ions to Tm³⁺ and Tb³⁺ ions takes place. Under 980 nm excitation Yb³⁺ ions absorb the incident energy and excite Tm³⁺ and Tb³⁺ ions via cooperative energy transfer. Variation of emission intensity with the ion concentrations of Yb³⁺, Tm³⁺ and Tb³⁺ has been studied. The lifetime of the ¹G₄ level has also been measured.     

On the construction of complex networks with optimal Tsallis entropy

In this work, we first formulate the Tsallis entropy in the context of complex networks. We then propose a network construction whose topology maximizes the Tsallis entropy. The growing network model has two main ingredients: copy process and random attachment mechanism (C-R model). We show that the resulting degree distribution exactly agrees with the required degree distribution that maximizes the Tsallis entropy. We also provide another example of network model using a combination of preferential and random attachment mechanisms (P-R model) and compare it with the distribution of the Tsallis entropy. In this case, we show that by adequately identifying the exponent factor q, the degree distribution can also be written in the q-exponential form. Taken together, our findings suggest that both mechanisms, copy process and preferential attachment, play a key role for the realization of networks with maximum Tsallis entropy. Finally, we discuss the interpretation of q parameter of the Tsallis entropy in the context of complex networks.     

Other-regarding preference and the evolutionary prisoner’s dilemma on complex networks

Prevailing models of the evolutionary prisoner’s game on networks always assume that agents are pursuing their own profit maximization. But the results from experimental games show that many agents have other-regarding preference. In this paper, we study the emergence of cooperation from the prisoner’s dilemma game on complex networks while some agents exhibit other-regarding preference such as inequality aversion, envious and guilty emotions. Contrary to common ideas, the simulation results show that the existence of inequality aversion agents does not promote cooperation emergence on a BA (Barabási and Albert) scale-free network in most situations. If the defection attraction is big and agents exhibit strong preference for inequality aversion, the frequency of cooperators will be lower than in situations where no inequality aversion agents exist. In some cases, the existence of the inequality agents will even induce the frequency of cooperators to zero, a feature which is not observed in previous research on the prisoner’s dilemma game when the underlying interaction topology is a BA scale-free network. This means that if an agent cares about equality too much, it will be difficult for cooperation to emerge and the frequency of cooperators will be low on BA networks. The research on the effect of envy or guilty emotions on the emergence of cooperation in the prisoner’s dilemma game on BA networks obtains similar results, though some differences exist. However, simulation results on a WS (Watts and Strogatz) small-world network display another scenario. If agents care about the inequality of agents very much, the WS network favors cooperation emergence in the prisoners’ dilemma game when other-regarding agents exist. If the agent weight on other-regarding is lowered, the cooperation frequencies emerging on a WS network are not much different from those in situations without other-regarding agents, although the frequency of cooperators is lower than those of the situation without other-regarding preference agents sometimes. All the simulation results imply that inequality aversion and its variations can have important effects on cooperation emergence in the prisoner’s dilemma game, and different network topologies have different effects on cooperation emergence in the prisoner’s dilemma game played on complex networks.     

Automatic alignment of a high-power CO2 laser resonator

An on-line, electronic feedback approach for automatic alignment of a laser resonator is described. A measure of the resonator alignment error is derived by partitioning a 1% sample of the laser output into quadrants, and using standard electronic circuits to compare the average power present in each quadrant. A motorized X — Y alignment system is then used to implement the alignment changes required to maintain optimum output quality from the laser. The system is shown capable of stabilizing the output power in each quadrant of the beam to within about 2% of the optimum level.     

Monte Carlo simulations of parapatric speciation

Parapatric speciation is studied using an individual-based model with sexual reproduction. We combine the theory of mutation accumulation for biological ageing with an environmental selection pressure that varies according to the individuals geographical positions and phenotypic traits. Fluctuations and genetic diversity of large populations are crucial ingredients to model the features of evolutionary branching and are intrinsic properties of the model. Its implementation on a spatial lattice gives interesting insights into the population dynamics of speciation on a geographical landscape and the disruptive selection that leads to the divergence of phenotypes. Our results suggest that assortative mating is not an obligatory ingredient to obtain speciation in large populations at low gene flow.     

Interrelation of intrinsic and extrinsic defects – congruent, stoichiometric, and regularly ordered lithium niobate

A new approach to the intrinsic and extrinsic defect subsystems, considering them as one integrated functional system, is developed for complex oxides. The strong interrelation of these subsystems becomes especially apparent when concentrations of both defect classes are comparable. A new parameter, reflecting the total intrinsic defect concentration, is introduced to determine the degree of crystal imperfection. The necessity of at least two calibrated samples – congruent and regularly ordered crystals – is substantiated. The accuracy by which the parameter can be determined can serve as a criterion in comparison of different methods of crystal characterisation. A procedure for crystal calibration by several physical characteristics is illustrated. Such a heuristic generalisation of the concept of mutual correlation of the whole defect system and the material only became possible on the basis of experimental and theoretical study of large numbers of LiNbO₃ crystals. The spectra of EPR, NMR, ENDOR, and other properties of crystals with different compositions, diverse modifiers (K, Mg,...), and various probe impurities have been analysed. It is found that crystals with vanishingly small concentration of intrinsic defects offer extraordinary informative opportunities. Their micro- and macroscopic properties are discussed. The developed ideas have a general character; therefore they should also be valid for other non-stoichiometric complex oxides. Received: 3 December 1998 / Revised version: 15 February 1999 / Published online: 12 April 1999     

Precision measurement of theg-factor of the 2+ γ-vibrational state of160Dy and test of theF-spin purity

The rotations of the 392-966 keV and the 299–966 keV directional correlations of¹⁶⁰Dy in the static hyperfine field of DyTb have been measured with high precision. A neutron activated Tb single crystal was used which was magnetized to saturation at 4.2 K by an external magnetic field of 4.5 T applied in the direction of theb-axis. The agreement of both rotations proves that the relaxation of the 4f-shell after the-decay takes less than 10 ps. A quantum mechanical calculation within the magnon model gave a relaxation time of the order of 1 ps. Theg-factor of the 2⁺ -vibrational state was derived as