3,010 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
A solvable model of the evolutionary loop
A model for the evolution of a finite population in a rugged fitness
landscape is introduced and solved. The population is trapped in an
evolutionary loop, alternating periods of stasis to periods in which it
performs adaptive walks. The dependence of the average rarity of the population
(a quantity related to the fitness of the most adapted individual) and of the
duration of stases on population size and mutation rate is calculated.Comment: 6 pages, EuroLaTeX, 1 figur
Robust ecological pattern formation induced by demographic noise
We demonstrate that demographic noise can induce persistent spatial pattern
formation and temporal oscillations in the Levin-Segel predator-prey model for
plankton-herbivore population dynamics. Although the model exhibits a Turing
instability in mean field theory, demographic noise greatly enlarges the region
of parameter space where pattern formation occurs. To distinguish between
patterns generated by fluctuations and those present at the mean field level in
real ecosystems, we calculate the power spectrum in the noise-driven case and
predict the presence of fat tails not present in the mean field case. These
results may account for the prevalence of large-scale ecological patterns,
beyond that expected from traditional non-stochastic approaches.Comment: Revised version. Supporting simulation at:
http://guava.physics.uiuc.edu/~tom/Netlogo
Exposure to males, but not receipt of sex peptide, accelerates functional aging in female fruit flies
1. Increased exposure to males can affect females negatively, reducing female life span and fitness. These costs could derive from increased mating rate and also harassment by males. Additionally, early investment in reproduction can increase the onset or rate of senescence in reproductive traits. Hence, there is a tight link between reproduction and ageing.
2. Here, we assess how mating and encounter rate with males impacts declines in female functional traits that are not directly involved in reproduction. In Drosophila melanogaster fruit flies, exposure to males and mating reduces female life span through harassment and receipt of seminal proteins, including sex peptide (SP). We manipulated the intensity of female exposure to males and regularly assessed female stress responses and recorded physiological traits over her lifetime.
3. Both mating itself and increased exposure to males accelerate declines in female climbing ability and starvation resistance. However, this is not related to changes in female body mass or fat storage. Moreover, these declines are not driven by the receipt of SP.
4. Our results suggest some synchrony in senescence across traits in response to female exposure to males; however, this is not universal, as we did not find this for physiological traits. Synchrony in senescence has been theorized but little supported in the literature. It is clear that ageing is a multifaceted trait; to understand environmental impacts on ageing rates, we must measure more than life span and indeed measure senescence in multiple traits. Specifically, our work shows that we must identify which female traits are sensitive to elevated mating activity to understand the impact of antagonistic interactions between the sexes on female ageing patterns
Defensive alliances in spatial models of cyclical population interactions
As a generalization of the 3-strategy Rock-Scissors-Paper game dynamics in
space, cyclical interaction models of six mutating species are studied on a
square lattice, in which each species is supposed to have two dominant, two
subordinated and a neutral interacting partner. Depending on their interaction
topologies, these systems can be classified into four (isomorphic) groups
exhibiting significantly different behaviors as a function of mutation rate. On
three out of four cases three (or four) species form defensive alliances which
maintain themselves in a self-organizing polydomain structure via cyclic
invasions. Varying the mutation rate this mechanism results in an ordering
phenomenon analogous to that of magnetic Ising model.Comment: 4 pages, 3 figure
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.Comment: 4 pages, 5 figure
Naming Game on Adaptive Weighted Networks
We examine a naming game on an adaptive weighted network. A weight of
connection for a given pair of agents depends on their communication success
rate and determines the probability with which the agents communicate. In some
cases, depending on the parameters of the model, the preference toward
successfully communicating agents is basically negligible and the model behaves
similarly to the naming game on a complete graph. In particular, it quickly
reaches a single-language state, albeit some details of the dynamics are
different from the complete-graph version. In some other cases, the preference
toward successfully communicating agents becomes much more relevant and the
model gets trapped in a multi-language regime. In this case gradual coarsening
and extinction of languages lead to the emergence of a dominant language,
albeit with some other languages still being present. A comparison of
distribution of languages in our model and in the human population is
discussed.Comment: 22 pages, accepted in Artificial Lif
Prevalence of significant liver disease in human immunodeficiency virus-infected patients exposed to Didanosine: A cross sectional study
AIM: To identify significant liver disease [including nodular regenerative hyperplasia (NRH)] in asymptomatic Didanosine (DDI) exposed human immunodeficiency virus (HIV) positive patients. METHODS: Patients without known liver disease and with > 6 mo previous DDI use had liver stiffness assessed by transient elastography (TE). Those with alanine transaminase (ALT) above upper limit normal and/or TE > 7.65 kPa underwent ultrasound scan (U/S). Patients with: (1) abnormal U/S; or (2) elevated ALT plus TE > 7.65 kPa; or (3) TE > 9.4 kPa were offered trans-jugular liver biopsy (TJLB) with hepatic venous pressure gradient (HVPG) assessment. RESULTS: Ninety-nine patients were recruited, median age 50 years (range 31-70), 81% male and 70% men who have sex with men. Ninety-five percent with VL 9.4 kPa. Seventeen (17%) met criteria for TJLB, of whom 12 accepted. All had HVPG < 6 mmHg. Commonest histological findings were steatosis (n = 6), normal architecture (n = 4) and NRH (n = 2), giving a prevalence of previously undiagnosed NRH of 2% (95%CI: 0.55%, 7.0%). CONCLUSION: A screening strategy based on TE, liver enzymes and U/S scan found a low prevalence of previously undiagnosed NRH in DDI exposed, asymptomatic HIV positive patients. Patients were more likely to have steatosis highlighting the increased risk of multifactorial liver disease in this population
Bacteria and the evolution of honest signals. The case of ornamental throat feathers in spotless starlings
1. Mechanisms guaranteeing reliability of messages are essential in understanding the underlying
information and evolution of signals. Micro-organisms may degrade signalling traits and
therefore influence the transmitted information and evolution of these characters. The role of
micro-organisms in animal signalling has, however, rarely been investigated.
2. Here, we explore a possible role for feather-degrading bacteria driving the design of ornamental
throat feathers in male spotless starlings (Sturnus unicolor). We estimated length, bacterial
load, degradation status and susceptibility to degradation by keratinolytic bacteria in
those feathers, compared with non-ornamental adjacent feathers in males, as well as to throat
feathers in females. In addition, the volume of the uropygial gland and its secretion was measured
and the secretion extracted. We also experimentally evaluated the capacity of each secretion
to inhibit growth of a keratinolytic bacterium.
3. The apical part of male ornamental throat feathers harboured more bacteria and degraded
more quickly than the basal part; these patterns were not detected in female throat feathers or
in non-ornamental male feathers. Moreover, degradation status of male and female throat
feathers did not differ, but was positively associated with feather bacterial density. Finally, the
size of the uropygial gland in both males and females predicted volume and the inhibitory
capacity of secretion against feather-degrading bacteria. Only in males was uropygial gland
size negatively associated with the level of feather degradation.
4. All results indicate differential susceptibility of different parts of throat feathers to keratinolytic
bacterial attack, which supports the possibility that throat feathers in starlings reflect individual
ability to combat feather-degrading bacteria honestly. This is further supported by the
relationship detected between antimicrobial properties of uropygial secretion and the level of
feather degradation.
5. Our results suggest that selection pressures exerted by feather-degrading bacteria on hosts
may promote evolution of particular morphologies of secondary sexual traits with different
susceptibility to bacterial degradation that reliably inform of their bacterial load. Those results
will help to understand the evolution of ornamental signals.This work was financed by Spanish Ministerio de Ciencia e Innovaci on, European
funds (FEDER) (CGL2010-19233-C03-01, CGL2013-48193-C3-1-P). MRR and DMG received a postdoc from the program âJAE-Docâ, GT from the âJuan de la Ciervaâ, and CRC had a predoctoral fellowship, all from the Spanish Government.Peer reviewe
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
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