11,076 research outputs found
Nonlinear deterministic equations in biological evolution
We review models of biological evolution in which the population frequency
changes deterministically with time. If the population is self-replicating,
although the equations for simple prototypes can be linearised, nonlinear
equations arise in many complex situations. For sexual populations, even in the
simplest setting, the equations are necessarily nonlinear due to the mixing of
the parental genetic material. The solutions of such nonlinear equations
display interesting features such as multiple equilibria and phase transitions.
We mainly discuss those models for which an analytical understanding of such
nonlinear equations is available.Comment: Invited review for J. Nonlin. Math. Phy
Predictions and Outcomes for the Dynamics of Rotating Galaxies
A review is given of a priori predictions made for the dynamics of rotating
galaxies. One theory - MOND - has had many predictions corroborated by
subsequent observations. While it is sometimes possible to offer post hoc
explanations for these observations in terms of dark matter, it is seldom
possible to use dark matter to predict the same phenomena.Comment: 36 pages (10 are references), 9 figures. Invited review for the
Galaxies special Issue "Debate on the Physics of Galactic Rotation and the
Existence of Dark Matter." Provides test cases for the importance of prior
predictions in the application of the scientific metho
Evolution and non-equilibrium physics. A study of the Tangled Nature Model
We argue that the stochastic dynamics of interacting agents which replicate,
mutate and die constitutes a non-equilibrium physical process akin to aging in
complex materials. Specifically, our study uses extensive computer simulations
of the Tangled Nature Model (TNM) of biological evolution to show that
punctuated equilibria successively generated by the model's dynamics have
increasing entropy and are separated by increasing entropic barriers. We
further show that these states are organized in a hierarchy and that limiting
the values of possible interactions to a finite interval leads to stationary
fluctuations within a component of the latter. A coarse-grained description
based on the temporal statistics of quakes, the events leading from one
component of the hierarchy to the next, accounts for the logarithmic growth of
the population and the decaying rate of change of macroscopic variables.
Finally, we question the role of fitness in large scale evolution models and
speculate on the possible evolutionary role of rejuvenation and memory effects.Comment: 6 pages, 6 figure
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Laser wakefield and direct acceleration in the plasma bubble regime
Laser wakefield acceleration (LWFA) and direct laser acceleration (DLA) are two different kinds of laser plasma electron acceleration mechanisms. LWFA relies on the laser-driven plasma wave to accelerate electrons. The interaction of ultra-short ultra-intensive laser pulses with underdense plasma leads the LWFA into a highly nonlinear regime (“plasma bubble regime”) that attracts particular interest nowadays. DLA accelerates electrons by laser electromagnetic wave in the ion channel or the plasma bubble through the Betatron resonance. This dissertation presents a hybrid laser plasma electron acceleration mechanism. We investigate its features through particle-in-cell (PIC) simulations and the single particle model. The hybrid laser plasma electron acceleration is the merging concept between the LWFA and the DLA, so called laser wakefield and direct acceleration (LWDA). The requirements of the initial conditions of the electron to undergo the LWDA are determined. The electron must have a large initial transverse energy. Two electron injection mechanisms that are suitable for the LWDA, density bump injection and ionization induced injection, are studied in detail. The features of electron beam phase space and electron dynamics are explored. Electron beam phase space appears several unique features such as spatially separated two groups, the correlation between the transverse energy and the relativistic factor and the double-peak spectrum. Electrons are synergistically accelerated by the wakefield as well as by the laser electromagnetic field in the laser-driven plasma bubble. LWDA are also investigated in the moderate power regime (10 TW) in regarding the effects of laser color and polarization. It is found that the frequency upshift laser pulse has better performance on avoiding time-jitter of electron energy spectra, electron final energy and electron charge yield. Some basic characters that related to the LWDA such as the effects of the subluminal laser wave, the effects of the longitudinal accelerating field, the electron beam emittance, the electron charge yield and potentially applications as radiation source are discussed.Physic
Fluctuations of fitness distributions and the rate of Muller's ratchet
The accumulation of deleterious mutations is driven by rare fluctuations
which lead to the loss of all mutation free individuals, a process known as
Muller's ratchet. Even though Muller's ratchet is a paradigmatic process in
population genetics, a quantitative understanding of its rate is still lacking.
The difficulty lies in the nontrivial nature of fluctuations in the fitness
distribution which control the rate of extinction of the fittest genotype. We
address this problem using the simple but classic model of mutation selection
balance with deleterious mutations all having the same effect on fitness. We
show analytically how fluctuations among the fittest individuals propagate to
individuals of lower fitness and have a dramatically amplified effects on the
bulk of the population at a later time. If a reduction in the size of the
fittest class reduces the mean fitness only after a delay, selection opposing
this reduction is also delayed. This delayed restoring force speeds up Muller's
ratchet. We show how the delayed response can be accounted for using a path
integral formulation of the stochastic dynamics and provide an expression for
the rate of the ratchet that is accurate across a broad range of parameters.Comment: Genetics 201
Real time unsupervised learning of visual stimuli in neuromorphic VLSI systems
Neuromorphic chips embody computational principles operating in the nervous
system, into microelectronic devices. In this domain it is important to
identify computational primitives that theory and experiments suggest as
generic and reusable cognitive elements. One such element is provided by
attractor dynamics in recurrent networks. Point attractors are equilibrium
states of the dynamics (up to fluctuations), determined by the synaptic
structure of the network; a `basin' of attraction comprises all initial states
leading to a given attractor upon relaxation, hence making attractor dynamics
suitable to implement robust associative memory. The initial network state is
dictated by the stimulus, and relaxation to the attractor state implements the
retrieval of the corresponding memorized prototypical pattern. In a previous
work we demonstrated that a neuromorphic recurrent network of spiking neurons
and suitably chosen, fixed synapses supports attractor dynamics. Here we focus
on learning: activating on-chip synaptic plasticity and using a theory-driven
strategy for choosing network parameters, we show that autonomous learning,
following repeated presentation of simple visual stimuli, shapes a synaptic
connectivity supporting stimulus-selective attractors. Associative memory
develops on chip as the result of the coupled stimulus-driven neural activity
and ensuing synaptic dynamics, with no artificial separation between learning
and retrieval phases.Comment: submitted to Scientific Repor
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