57,051 research outputs found
Environmental Regulation Can Arise Under Minimal Assumptions
Models that demonstrate environmental regulation as a consequence of organism and environment coupling all require a number of core assumptions. Many previous models, such as Daisyworld, require that certain environment-altering traits have a selective advantage when those traits also contribute towards global regulation. We present a model that results in the regulation of a global environmental resource through niche construction without employing this and other common assumptions. There is no predetermined environmental optimum towards which regulation should proceed assumed or coded into the model. Nevertheless, polymorphic stable states that resist perturbation emerge from the simulated co-evolution of organisms and environment. In any single simulation a series of different stable states are realised, punctuated by rapid transitions. Regulation is achieved through two main subpopulations that are adapted to slightly different resource values, which force the environmental resource in opposing directions. This maintains the resource within a comparatively narrow band over a wide range of external perturbations. Population driven oscillations in the resource appear to be instrumental in protecting the regulation against mutations that would otherwise destroy it. Sensitivity analysis shows that the regulation is robust to mutation and to a wide range of parameter settings. Given the minimal assumptions employed, the results could reveal a mechanism capable of environmental regulation through the by-products of organisms
Clonal Interference, Multiple Mutations, and Adaptation in Large Asexual Populations
Two important problems affect the ability of asexual populations to
accumulate beneficial mutations, and hence to adapt. First, clonal interference
causes some beneficial mutations to be outcompeted by more-fit mutations which
occur in the same genetic background. Second, multiple mutations occur in some
individuals, so even mutations of large effect can be outcompeted unless they
occur in a good genetic background which contains other beneficial mutations.
In this paper, we use a Monte Carlo simulation to study how these two factors
influence the adaptation of asexual populations. We find that the results
depend qualitatively on the shape of the distribution of the effects of
possible beneficial mutations. When this distribution falls off slower than
exponentially, clonal interference alone reasonably describes which mutations
dominate the adaptation, although it gives a misleading picture of the
evolutionary dynamics. When the distribution falls off faster than
exponentially, an analysis based on multiple mutations is more appropriate.
Using our simulations, we are able to explore the limits of validity of both of
these approaches, and we explore the complex dynamics in the regimes where
neither are fully applicable.Comment: 24 pages, 5 figure
A microscopic model of evolution of recombination
We study the evolution of recombination using a microscopic model developed
within the frame of the theory of quantitative traits. Two components of
fitness are considered: a static one that describes adaptation to environmental
factors not related to the population itself, and a dynamic one that accounts
for interactions between organisms e.g. competition. We focus on the dynamics
of colonization of an empty niche. As competition is a function of the
population, selection pressure rapidly changes in time. The simulations show
that both in the case of flat and steep static fitness landscapes,
recombination provides a high velocity of movement in the phenotypic space thus
allowing recombinants to colonize the highest fitness regions earlier than non
recombinants that are often driven to extinction. The stabilizing effects of
competition and assortativity are also discussed. Finally, the analysis of
phase diagrams shows that competition is the key factor for the evolution of
recombination, while assortativity plays a significant role only in small
populations.Comment: to appear in Physica
Lethal Mutagenesis in Viruses and Bacteria
Here we study how mutations which change physical properties of cell proteins
(stability) impact population survival and growth. In our model the genotype is
presented as a set of N numbers, folding free energies of cells N proteins.
Mutations occur upon replications so that stabilities of some proteins in
daughter cells differ from those in parent cell by random amounts drawn from
experimental distribution of mutational effects on protein stability. The
genotype-phenotype relationship posits that unstable proteins confer lethal
phenotype to a cell and in addition the cells fitness (duplication rate) is
proportional to the concentration of its folded proteins. Simulations reveal
that lethal mutagenesis occurs at mutation rates close to 7 mutations per
genome per replications for RNA viruses and about half of that for DNA based
organisms, in accord with earlier predictions from analytical theory and
experiment. This number appears somewhat dependent on the number of genes in
the organisms and natural death rate. Further, our model reproduces the
distribution of stabilities of natural proteins in excellent agreement with
experiment. Our model predicts that species with high mutation rates, tend to
have less stable proteins compared to species with low mutation rate
Analysis of close encounters with Ganymede and Callisto using a genetic n-body algorithm
In this work we describe a genetic algorithm which is used in order to study
orbits of minor bodies in the frames of close encounters. We find that the
algorithm in combination with standard orbital numerical integrators can be
used as a good proxy for finding typical orbits of minor bodies in close
encounters with planets and even their moons, saving a lot of computational
time compared to long-term orbital numerical integrations. Here, we study close
encounters of Centaurs with Callisto and Ganymede in particular. We also
perform n-body numerical simulations for comparison. We find typical impact
velocities to be between and for
Ganymede and between and for
Callisto.Comment: 18 pages, 3 figure
Blood Vessel Tortuosity Selects against Evolution of Agressive Tumor Cells in Confined Tissue Environments: a Modeling Approach
Cancer is a disease of cellular regulation, often initiated by genetic
mutation within cells, and leading to a heterogeneous cell population within
tissues. In the competition for nutrients and growth space within the tumors
the phenotype of each cell determines its success. Selection in this process is
imposed by both the microenvironment (neighboring cells, extracellular matrix,
and diffusing substances), and the whole of the organism through for example
the blood supply. In this view, the development of tumor cells is in close
interaction with their increasingly changing environment: the more cells can
change, the more their environment will change. Furthermore, instabilities are
also introduced on the organism level: blood supply can be blocked by increased
tissue pressure or the tortuosity of the tumor-neovascular vessels. This
coupling between cell, microenvironment, and organism results in behavior that
is hard to predict. Here we introduce a cell-based computational model to study
the effect of blood flow obstruction on the micro-evolution of cells within a
cancerous tissue. We demonstrate that stages of tumor development emerge
naturally, without the need for sequential mutation of specific genes.
Secondly, we show that instabilities in blood supply can impact the overall
development of tumors and lead to the extinction of the dominant aggressive
phenotype, showing a clear distinction between the fitness at the cell level
and survival of the population. This provides new insights into potential side
effects of recent tumor vasculature renormalization approaches
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