3,703 research outputs found
Prisoner's Dilemma cellular automata revisited: evolution of cooperation under environmental pressure
We propose an extension of the evolutionary Prisoner's Dilemma cellular
automata, introduced by Nowak and May \cite{nm92}, in which the pressure of the
environment is taken into account. This is implemented by requiring that
individuals need to collect a minimum score , representing
indispensable resources (nutrients, energy, money, etc.) to prosper in this
environment. So the agents, instead of evolving just by adopting the behaviour
of the most successful neighbour (who got ), also take into account if
is above or below the threshold . If an
individual has a probability of adopting the opposite behaviour from the one
used by its most successful neighbour. This modification allows the evolution
of cooperation for payoffs for which defection was the rule (as it happens, for
example, when the sucker's payoff is much worse than the punishment for mutual
defection). We also analyse a more sophisticated version of this model in which
the selective rule is supplemented with a "win-stay, lose-shift" criterion. The
cluster structure is analyzed and, for this more complex version we found
power-law scaling for a restricted region in the parameter space.Comment: 15 pages, 8 figures; added figures and revised tex
Strategy bifurcation and spatial inhomogeneity in a simple model of competing sellers
We present a simple one-parameter model for spatially localised evolving
agents competing for spatially localised resources. The model considers selling
agents able to evolve their pricing strategy in competition for a fixed market.
Despite its simplicity, the model displays extraordinarily rich behavior. In
addition to ``cheap'' sellers pricing to cover their costs, ``expensive''
sellers spontaneously appear to exploit short-term favorable situations. These
expensive sellers ``speciate'' into discrete price bands. As well as variety in
pricing strategy, the ``cheap'' sellers evolve a strongly correlated spatial
structure, which in turn creates niches for their expensive competitors. Thus
an entire ecosystem of coexisting, discrete, symmetry-breaking strategies
arises.Comment: 6 pages, 6 figures, epl2; 1 new figure, include nash equilibrium
analysis, typo fixe
Invariants from classical field theory
We introduce a method that generates invariant functions from perturbative
classical field theories depending on external parameters. Applying our methods
to several field theories such as abelian BF, Chern-Simons and 2-dimensional
Yang-Mills theory, we obtain, respectively, the linking number for embedded
submanifolds in compact varieties, the Gauss' and the second Milnor's invariant
for links in S^3, and invariants under area-preserving diffeomorphisms for
configurations of immersed planar curves.Comment: 20 pages, 1 figure, to appear in J. Math. Phy
Adaptation and enslavement in endosymbiont-host associations
The evolutionary persistence of symbiotic associations is a puzzle.
Adaptation should eliminate cooperative traits if it is possible to enjoy the
advantages of cooperation without reciprocating - a facet of cooperation known
in game theory as the Prisoner's Dilemma. Despite this barrier, symbioses are
widespread, and may have been necessary for the evolution of complex life. The
discovery of strategies such as tit-for-tat has been presented as a general
solution to the problem of cooperation. However, this only holds for
within-species cooperation, where a single strategy will come to dominate the
population. In a symbiotic association each species may have a different
strategy, and the theoretical analysis of the single species problem is no
guide to the outcome. We present basic analysis of two-species cooperation and
show that a species with a fast adaptation rate is enslaved by a slowly
evolving one. Paradoxically, the rapidly evolving species becomes highly
cooperative, whereas the slowly evolving one gives little in return. This helps
understand the occurrence of endosymbioses where the host benefits, but the
symbionts appear to gain little from the association.Comment: v2: Correction made to equations 5 & 6 v3: Revised version accepted
in Phys. Rev. E; New figure adde
Hawks and Doves on Small-World Networks
We explore the Hawk-Dove game on networks with topologies ranging from
regular lattices to random graphs with small-world networks in between. This is
done by means of computer simulations using several update rules for the
population evolutionary dynamics. We find the overall result that cooperation
is sometimes inhibited and sometimes enhanced in those network structures, with
respect to the mixing population case. The differences are due to different
update rules and depend on the gain-to-cost ratio. We analyse and qualitatively
explain this behavior by using local topological arguments.Comment: 12 pages, 8 figure
Robustness of Cooperation in the Evolutionary Prisoner's Dilemma on Complex Networks
Recent studies on the evolutionary dynamics of the Prisoner's Dilemma game in
scale-free networks have demonstrated that the heterogeneity of the network
interconnections enhances the evolutionary success of cooperation. In this
paper we address the issue of how the characterization of the asymptotic states
of the evolutionary dynamics depends on the initial concentration of
cooperators. We find that the measure and the connectedness properties of the
set of nodes where cooperation reaches fixation is largely independent of
initial conditions, in contrast with the behavior of both the set of nodes
where defection is fixed, and the fluctuating nodes. We also check for the
robustness of these results when varying the degree heterogeneity along a
one-parametric family of networks interpolating between the class of
Erdos-Renyi graphs and the Barabasi-Albert networks.Comment: 18 pages, 6 figures, revised version accepted for publication in New
Journal of Physics (2007
Ordering in spatial evolutionary games for pairwise collective strategy updates
Evolutionary games are studied with players located on a square
lattice. During the evolution the randomly chosen neighboring players try to
maximize their collective income by adopting a random strategy pair with a
probability dependent on the difference of their summed payoffs between the
final and initial state assuming quenched strategies in their neighborhood. In
the case of the anti-coordination game this system behaves alike an
anti-ferromagnetic kinetic Ising model. Within a wide region of social dilemmas
this dynamical rule supports the formation of similar spatial arrangement of
the cooperators and defectors ensuring the optimum total payoff if the
temptation to choose defection exceeds a threshold value dependent on the
sucker's payoff. The comparison of the results with those achieved for pairwise
imitation and myopic strategy updates has indicated the relevant advantage of
pairwise collective strategy update in the maintenance of cooperation.Comment: 9 pages, 6 figures; accepted for publication in Physical Review
Evolutionary Prisoner's Dilemma on heterogeneous Newman-Watts small-world network
We focus on the heterogeneity of social networks and its role to the
emergence of prevailing cooperation and sustaining cooperators. The social
networks are representative of the interaction relationships between players
and their encounters in each round of games. We study an evolutionary
Prisoner's Dilemma game on a variant of Watts-Strogatz small-world network,
whose heterogeneity can be tuned by a parameter. It is found that optimal
cooperation level exists at some intermediate topological heterogeneity for
different temptations to defect. Moreover, neither the most heterogeneous case
nor the most homogeneous one would favor the cooperators. At intermediate
heterogeneity in degree sequences, cooperators could resist the invasion of
defectors for large temptation to defect.Comment: Updated versio
Hereditary sensory and autonomic neuropathies: types II, III, and IV
The hereditary sensory and autonomic neuropathies (HSAN) encompass a number of inherited disorders that are associated with sensory dysfunction (depressed reflexes, altered pain and temperature perception) and varying degrees of autonomic dysfunction (gastroesophageal reflux, postural hypotention, excessive sweating). Subsequent to the numerical classification of four distinct forms of HSAN that was proposed by Dyck and Ohta, additional entities continue to be described, so that identification and classification are ongoing. As a group, the HSAN are rare diseases that affect both sexes. HSAN III is almost exclusive to individuals of Eastern European Jewish extraction, with incidence of 1 per 3600 live births. Several hundred cases with HSAN IV have been reported. The worldwide prevalence of HSAN type II is very low. This review focuses on the description of three of the disorders, HSAN II through IV, that are characterized by autosomal recessive inheritance and onset at birth. These three forms of HSAN have been the most intensively studied, especially familial dysautonomia (Riley-Day syndrome or HSAN III), which is often used as a prototype for comparison to the other HSAN. Each HSAN disorder is likely caused by different genetic errors that affect specific aspects of small fiber neurodevelopment, which result in variable phenotypic expression. As genetic tests are routinely used for diagnostic confirmation of HSAN III only, other means of differentiating between the disorders is necessary. Diagnosis is based on the clinical features, the degree of both sensory and autonomic dysfunction, and biochemical evaluations, with pathologic examinations serving to further confirm differences. Treatments for all these disorders are supportive
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