arXiv subjects

Henrik Jeldtoft Jensen

Publications and source records attributed to Henrik Jeldtoft Jensen.

At least 91 records · Page 5Linked to original sources

arXiv2005

The Tangled nature model with inheritance and constraint: Evolutionary ecology restricted by a conserved resource

We study a version of the Tangled Nature model of evolutionary ecology redefined in a phenotype space where mutants have properties correlated to their parents. The model has individual-based dynamics whilst incorporating species scale competitive constraints and a system scale resource constraint. Multiple species arise that coexist in a species interaction network with evolving global properties. Both the mean interaction strength and the network connectance increase relative to the null system as mutualism becomes more extensive. From a study of the dependence of average degree on the resource level we extract the diversity-connectance relationship which conforms to the hyperbolic form seen in field data. This is adjudged to arise as a consequence of the evolutionary pressure to achieve positive interactions. The network degree distributions conform more strongly to exponential than to the null binomial distributions in all cases. This effect is believed to be caused by correlations in the reproductive process. We also study how resource availability influences the phenotypical lifetime distribution which is approximately of power law form. We observe that the mean lifetime is inversely related to the resource level.

q-bio.PE↗

arXiv2005

Record dynamics and the observed temperature plateau in the magnetic creep rate of type II superconductors

We use Monte Carlo simulations of a coarse grained three dimensional model to demonstrate that the experimentally observed approximate temperature independence of the magnetic creep rate for a broad range of temperatures may be explained in terms of record dynamics, {\it viz.} the dynamical properties of the times at which a stochastic fluctuating signal establishes records.

cond-mat.supr-con↗

arXiv2005

Diversity as a product of interspecial interactions

We demonstrate diversification rather than optimisation for highly interacting organisms in a well mixed biological system by means of a simple model and reference to experiment, and find the cause to be the complex network of interactions formed, allowing species less well adapted to an environment to flourish by co-interaction over the `best' species. This diversification can be considered as the construction of many co-evolutionary niches by the network of interactions between species. Evidence for this comes from work with the bacteria Escherichia coli, which may coexist with their own mutants under certain conditions. Diversification only occurs above a certain threshold interaction strength, below which competitive exclusion occurs.

q-bio.PE↗

arXiv2005

Evolution in complex systems: record dynamics in models of spin glasses, superconductors and evolutionary ecology

Recent research on the non-stationary nature of the dynamics of complex systems is reviewed through three specific models. The long time dynamics consists of a slow, decelerating but spasmodic release of generalized intrinsic strain. These events are denoted quakes. Between the quakes weak fluctuations occur but no essential change in properties are induced. The accumulated effect of the quakes, however, is to induce a direct change in the probability density functions characterising the system. We discuss how the log-Poisson statistics of record dynamics may be an effective description of the long time evolution and describe how an analysis of the times at which the quakes occur enables one to check the applicability of record dynamics.

cond-mat.stat-mech↗

arXiv2004

Intermittency, aging and record fluctuations

Aging in spin glasses is analyzed via the Probability Density Function (PDF) of the heat transfer between system and bath over a small time $δt$. The PDF contains a Gaussian part, describing reversible fluctuations, and an exponential tail caused by intermittent events. We find that the relative weight of these two parts depends, for fixed $δt$, on the ratio of the total sampling time to the age $t_w$. Fixing this ratio, the intensity of the intermittent events is proportional to $δt/t_w$ and independent of the temperature. The Gaussian part has a variance with the same temperature dependence as the variance of the equilibrium energy in a system with an exponential density of states. All these observations are explained assuming that, for any $t_w$, intermittent events are triggered by local energy fluctuations exceeding those previously occurred.

cond-mat.soft↗

arXiv2004

Evolution in complex systems

What features characterise complex system dynamics? Power laws and scale invariance of fluctuations are often taken as the hallmarks of complexity, drawing on analogies with equilibrium critical phenomena[1-3]. Here we argue that slow, directed dynamics, during which the system's properties change significantly, is fundamental. The underlying dynamics is related to a slow, decelerating but spasmodic release of an intrinsic strain or tension. Time series of a number of appropriate observables can be analysed to confirm this effect. The strain arises from local frustration. As the strain is released through "quakes", some system variable undergoes record statistics with accompanying log-Poisson statistics for the quake event times[4]. We demonstrate these phenomena via two very different systems: a model of magnetic relaxation in type II superconductors and the Tangled Nature model of evolutionary ecology, and show how quantitative indications of ageing can be found.

cond-mat.stat-mech↗

arXiv2003

A new, efficient algorithm for the Forest Fire Model

The Drossel-Schwabl Forest Fire Model is one of the best studied models of non-conservative self-organised criticality. However, using a new algorithm, which allows us to study the model on large statistical and spatial scales, it has been shown to lack simple scaling. We thereby show that the considered model is not critical. This paper presents the algorithm and its parallel implementation in detail, together with large scale numerical results for several observables. The algorithm can easily be adapted to related problems such as percolation.

cond-mat.stat-mech↗

arXiv2003

Who runs fastest in an adaptive landscape: Sexual versus asexual reproduction

We compare the speed with which a sexual, respectively an asexual, population is able to respond to a biased selective pressure. Our model focuses on the Weismann hypothesis that the extra variation caused by crossing-over and recombination during sexual reproduction allows a sexual population to adapt faster. We find, however, that the extra variation amongst the progeny produced during sexual reproduction for most model parameters is unable to overcome the effect that parents with a high individual fitness in general must mate with individuals of lower individual fitness resulting in a moderate reproductive fitness for the pair.

cond-mat↗

arXiv2003

Anisotropy and universality: the Oslo model, the rice pile experiment and the quenched Edwards-Wilkinson equation

We show that any amount of anisotropy moves the Oslo model to another known universality class, the exponents of which can be derived exactly. This amounts to an exact solution of the quenched Edwards-Wilkinson equation with a drift term. We argue that anisotropy is likely to be experimentally relevant and may explain why consistent exponents have not been extracted in the rice pile experiments.

cond-mat.stat-mech↗

arXiv2003

Fluctuation spectrum and size scaling of river flow and level

We describe the statistical properties of two large river systems: the Danube and the Mississippi. The properties of the two rivers are compared qualitatively to the general properties of a critical steady state system. Specifically, we test the recent suggestion by Bramwell, Fennell, Holdsworth and Portelli [{\it Europhys. Lett.} {\bf 57}, 310 (2002)] that a universal probability density function (PDF) exists for the fluctuations in river level, namely the Bramwell-Holdsworth-Pinton (BHP) PDF. The statistical properties investigated in this paper are: the PDF of the river flow and river level; moment scaling with basin area; moment to moment scaling or relative scaling; and power spectral properties of the data. We find that the moments of the flow scale approximately with basin area and that the seasonally adjusted flows exhibit relative moment scaling. Compared to the Mississippi, the Danube shows large deviations from spatial scaling and the power spectra show considerable dependence on system size. This might be due to water use and regulations as well as inhomogeneities in the basin area. We also find that the PDF of level data in some, but not all, cases can be qualitatively approximated by the BHP PDF. We discuss why this coincidence appears to be accidental.

cond-mat.stat-mech↗

arXiv2003

Network properties, species abundance and evolution in a model of evolutionary ecology

We study the evolution of the network properties of a populated network embedded in a genotype space characterised by either a low or a high number of potential links, with particular emphasis on the connectivity and clustering. Evolution produces two distinct types of network. When a specific genotype is only able to influence a few other genotypes, the ecology consists of separate non-interacting clusters in genotype space. When different types may influence a large number of other sites, the network becomes one large interconnected cluster. The distribution of interaction strengths -- but not the number of connections -- changes significantly with time. We find that the species abundance is only realistic for a high level of species connectivity. This suggests that real ecosystems form one interconnected whole in which selection leads to stronger interactions between the different types. Analogies with niche and neutral theory are also considered.

cond-mat.stat-mech↗

arXiv2003

Darwinian Selection and Non-existence of Nash Equilibria

We study selection acting on phenotype in a collection of agents playing local games lacking Nash equilibria. After each cycle one of the agents losing most games is replaced by a new agent with new random strategy and game partner. The network generated can be considered critical in the sense that the lifetimes of the agents is power law distributed. The longest surviving agents are those with the lowest absolute score per time step. The emergent ecology is characterized by a broad range of behaviors. Nevertheless, the agents tend to be similar to their opponents in terms of performance.

cond-mat.stat-mech↗

arXiv2002

The Tangled Nature model as an evolving quasi-species model

We show that the Tangled Nature model can be interpreted as a general formulation of the quasi-species model by Eigen et al. in a frequency dependent fitness landscape. We present a detailed theoretical derivation of the mutation threshold, consistent with the simulation results, that provides a valuable insight into how the microscopic dynamics of the model determine the observed macroscopic phenomena published previously. The dynamics of the Tangled Nature model is defined on the microevolutionary time scale via reproduction, with heredity, variation, and natural selection. Each organism reproduces with a rate that is linked to the individuals' genetic sequence and depends on the composition of the population in genotype space. Thus the microevolutionary dynamics of the fitness landscape is regulated by, and regulates, the evolution of the species by means of the mutual interactions. At low mutation rate, the macro evolutionary pattern mimics the fossil data: periods of stasis, where the population is concentrated in a network of coexisting species, is interrupted by bursts of activity. As the mutation rate increases, the duration and the frequency of bursts increases. Eventually, when the mutation rate reaches a certain threshold, the population is spread evenly throughout the genotype space showing that natural selection only leads to multiple distinct species if adaptation is allowed time to cause fixation.

cond-mat.stat-mech↗

arXiv2002

Aging and species abundance in the Tangled Nature model of biological evolution

We present an individual based model of evolutionary ecology. The reproduction rate of individuals characterized by their genome depends on the composition of the population in genotype space. Ecological features such as the taxonomy and the macro-evolutionary mode of the dynamics are emergent properties. The macro-dynamics exhibit intermittent two mode switching with a gradually decreasing extinction rate. The generated ecologies become gradually better adapted as well as more complex in a collective sense. The form of the species abundance curve compares well with observed functional forms. The models error threshold can be understood in terms of the characteristics of the two dynamical modes of the system.

cond-mat.stat-mech↗

arXiv2002

Broken scaling in the Forest Fire Model

We investigate the scaling behavior of the cluster size distribution in the Drossel-Schwabl Forest Fire model (DS-FFM) by means of large scale numerical simulations, partly on (massively) parallel machines. It turns out that simple scaling is clearly violated, as already pointed out by Grassberger [P. Grassberger, J. Phys. A: Math. Gen. 26, 2081 (1993)], but largely ignored in the literature. Most surprisingly the statistics not seems to be described by a universal scaling function, and the scale of the physically relevant region seems to be a constant. Our results strongly suggest that the DS-FFM is not critical in the sense of being free of characteristic scales.

cond-mat.stat-mech↗

arXiv2001

Aging and memory phenomena in magnetic and transport properties of vortex matter: a brief review

There is mounting experimental evidence that strong off-equilibrium phenomena, such as ``memory'' or ``aging'' effects, play a crucial role in the physics of vortices in type II superconductors. We give a short review, based on a recently introduced schematic vortex model, of current progresses in understanding out of equilibrium vortex behaviours. We develop a unified description of ``memory'' phenomena in magnetic and transport properties, such as magnetisation loops and their ``anomalous'' 2nd peak, logarithmic creep, ``anomalous'' finite creep rate in the limit of vanishing temperature, ``memory'' and ``irreversibility'' in I-V characteristics, time dependent critical currents, ``rejuvenation'' and ``aging'' of the system response.

cond-mat↗

arXiv2001

Universal fluctuations and extreme value statistics

We study the effect of long range algebraic correlations on extreme value statistics and demonstrate that correlations can produce a limit distribution which is indistinguishable from the ubiquitous Bramwell-Holdsworth-Pinton distribution. We also consider the square-width fluctuations of the avalanche signal. We find, as recently predicted by T. Antal, M. Droz G. Gyorgyi and Z. Racz for logarithmic correlated 1/f signals, that these fluctuations follow the Fisher-Tippett-Gumbel distribution from uncorrelated extreme value statistics.

cond-mat.stat-mech↗

arXiv2001

Off equilibrium properties of vortex creep in superconductors

We study a model for the dynamics of vortices in type II superconductors. In particular, we discuss glassy ``off equilibrium'' properties and ``aging'' in magnetic creep. At low temperatures a crossover point is found, Tg, where relaxation times seem to diverge a' la Vogel-Tamman-Fulcher. Magnetic creep changes by crossing Tg: above Tg power law creep is found asymptotically followed by stretched exponential saturation; below Tg the creep is logarithmic and vortex motion strongly subdiffusive. In this region violation of time translation invariance is found along with important dynamical scaling properties. A thermodynamic glassy transition point can be found at a lower temperature Tc.

cond-mat↗