Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session T44: Evolutionary and Ecological Systems |
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Sponsoring Units: DBP Chair: Ao Ping, Shanghai Jiaotong University, China Room: A309 |
Wednesday, March 23, 2011 2:30PM - 2:42PM |
T44.00001: Spatial Disorder in Cyclic Three-Species Predator-Prey Models Qian He, Mauro Mobilia, Uwe C. T\"auber By numerically studying the oscillatory dynamics of several variants of cyclic three-species predator-prey models with conserved total particle density, we investigate the effects of spatial variability of the reaction rates and site occupancy restrictions on the system's co-evolutionary dynamics. It is shown that both quenched disorder in the reaction rates and lattice site occupancy restrictions have only minor effects on the dynamics of cyclic competing systems. This result is starkly different from the finding in two-species predator-prey model where spatial disorder can greatly enhance species fitness. We also numerically compute the dependence of the mean extinction time, for small systems, on system size. \par \noindent Reference: Phys. Rev. E {\bf 82}, 051909 (2010). [Preview Abstract] |
Wednesday, March 23, 2011 2:42PM - 2:54PM |
T44.00002: A Two-Species Social Dominance Model Swapnil Jawkar, Geoffrey Adams, Uwe C. T\"auber We study the general properties of a stochastic two-species social-domination model defined on a $d$-dimensional lattice. The introduction of spatial degrees of freedom and allowance of stochastic fluctuations surprisingly does not invalidate the deterministic mean-field picture. In the active state, where the dominant and submissive species coexist, no patch formation is observed, with correlation lengths restricted to a few lattice sites. Oscillations seen in the submissive population density are strongly damped and restricted to a small section of the parameter space. Observations are explained to be a result of the two-particle reactions being restricted to the same social group. [Preview Abstract] |
Wednesday, March 23, 2011 2:54PM - 3:06PM |
T44.00003: Cooperation and cheating in microbes Jeff Gore Understanding the cooperative and competitive dynamics within and between species is a central challenge in evolutionary biology. Microbial model systems represent a unique opportunity to experimentally test fundamental theories regarding the evolution of cooperative behaviors. In this talk I will describe our experiments probing cooperation in microbes. In particular, I will compare the cooperative growth of yeast in sucrose and the cooperative inactivation of antibiotics by bacteria. In both cases we find that cheater strains---which don't contribute to the public welfare---are able to take advantage of the cooperator strains. However, this ability of cheaters to out-compete cooperators occurs only when cheaters are present at low frequency, thus leading to steady-state coexistence. These microbial experiments provide fresh insight into the evolutionary origin of cooperation. [Preview Abstract] |
Wednesday, March 23, 2011 3:06PM - 3:18PM |
T44.00004: Cooperative Bacterial Growth Dynamics Predict the Evolution of Antibiotic Resistance Tatiana Artemova, Ylaine Gerardin, Sophia Hsin-Jung Li, Jeff Gore Since the discovery of penicillin, antibiotics have been our primary weapon against bacterial infections. Unfortunately, bacteria can gain resistance to penicillin by acquiring the gene that encodes beta-lactamase, which inactivates the antibiotic. However, mutations in this gene are necessary to degrade the modern antibiotic cefotaxime. Understanding the conditions that favor the spread of these mutations is a challenge. Here we show that bacterial growth in beta-lactam antibiotics is cooperative and that the nature of this growth determines the conditions in which resistance evolves. Quantitative analysis of the growth dynamics predicts a peak in selection at very low antibiotic concentrations; competition between strains confirms this prediction. We also find significant selection at higher antibiotic concentrations, close to the minimum inhibitory concentrations of the strains. Our results argue that an understanding of the evolutionary forces that lead to antibiotic resistance requires a quantitative understanding of the evolution of cooperation in bacteria. [Preview Abstract] |
Wednesday, March 23, 2011 3:18PM - 3:30PM |
T44.00005: Slowly switching between environments facilitates reverse evolution in small populations Longzhi Tan, Jeff Gore The rate at which a physical process occurs usually changes the behavior of a system. In thermodynamics, the reversibility of a process generally increases when it occurs at an infinitely slow rate. In biological evolution, adaptations to a new environment may be reversed by evolution in the ancestral environment. Such fluctuating environments are ubiquitous in nature, although how the rate of switching affects reverse evolution is unknown. Here we use a computational approach to quantify evolutionary reversibility as a function of the rate of switching between two environments. For small population sizes, which travel on landscapes as random walkers, we find that both genotypic and phenotypic reverse evolution increase at slow switching rates. However, slow switching of environments decreases evolutionary reversibility for a greedy walker, corresponding to large populations (extensive clonal interference). We conclude that the impact of the switching rate for biological evolution is more complicated than other common physical processes, and that a quantitative approach may yield significant insight into reverse evolution. [Preview Abstract] |
Wednesday, March 23, 2011 3:30PM - 3:42PM |
T44.00006: Bacterial cheating limits the evolution of antibiotic resistance Hui Xiao Chao, Manoshi Datta, Eugene Yurtsev, Jeff Gore The widespread use of antibiotics has led to the evolution of resistance in bacteria. Bacteria can gain resistance to the antibiotic ampicillin by acquiring a plasmid carrying the gene beta-lactamase, which inactivates the antibiotic. This inactivation may represent a cooperative behavior, as the entire bacterial population benefits from removing the antibiotic. The cooperative nature of this growth suggests that a cheater strain--which does not contribute to breaking down the antibiotic--may be able to take advantage of cells cooperatively inactivating the antibiotic. Here we experimentally find that a ``sensitive'' bacterial strain lacking the plasmid conferring resistance can invade a population of resistant bacteria, even in antibiotic concentrations that should kill the sensitive strain. We observe stable coexistence between the two strains and find that a simple model successfully explains the behavior as a function of antibiotic concentration and cell density. We anticipate that our results will provide insight into the evolutionary origin of phenotypic diversity and cooperative behaviors found in nature. [Preview Abstract] |
Wednesday, March 23, 2011 3:42PM - 3:54PM |
T44.00007: A slowly evolving host moves first in symbiotic interactions James Damore, Jeff Gore Symbiotic relationships, both parasitic and mutualistic, are ubiquitous in nature. Understanding how these symbioses evolve, from bacteria and their phages to humans and our gut microflora, is crucial in understanding how life operates. Often, symbioses consist of a slowly evolving host species with each host only interacting with its own sub-population of symbionts. The Red Queen hypothesis describes coevolutionary relationships as constant arms races with each species rushing to evolve an advantage over the other, suggesting that faster evolution is favored. Here, we use a simple game theoretic model of host- symbiont coevolution that includes population structure to show that if the symbionts evolve much faster than the host, the equilibrium distribution is the same as it would be if it were a sequential game where the host moves first against its symbionts. For the slowly evolving host, this will prove to be advantageous in mutualisms and a handicap in antagonisms. The model allows for symbiont adaptation to its host, a result that is robust to changes in the parameters and generalizes to continuous and multiplayer games. Our findings provide insight into a wide range of symbiotic phenomena and help to unify the field of coevolutionary theory. [Preview Abstract] |
Wednesday, March 23, 2011 3:54PM - 4:06PM |
T44.00008: How do the effects of mutations add up? Andrea Velenich, Mingjie Dai, Jeff Gore Genetic mutations affect the fitness of any organism and provide the variability necessary for natural selection to occur. Given the fitness of a wild type organism and the fitness of mutants A and B which differ from the wild type by a single mutation, predicting the fitness of the double mutant AB is a fundamental problem with broad implications in many fields, from evolutionary theory to medicine. Analysis of millions of double gene knockouts in yeast reveals that, on average, the fitness of AB is the product of the fitness of A and the fitness of B. However, most pairs of mutations deviate from this mean behavior in a way that challenges existing theoretical models. We propose a natural generalization of the geometric Fisher's model which accommodates the experimentally observed features and allows us to characterize the fitness landscape of yeast. [Preview Abstract] |
Wednesday, March 23, 2011 4:06PM - 4:18PM |
T44.00009: Theory of cooperation in a micro-organismal snow-drift game Zhenyu Wang, Nigel Goldenfeld We present a mean field model for the phase diagram of a community of micro-organisms, interacting through their metabolism so that they are, in effect, engaging in a cooperative social game. We show that as a function of the concentration of the nutrients glucose and histidine, the community undergoes a phase transition separating a state in which one strain is dominant to a state which is characterized by coexisting populations. Our results are in good agreement with recent experimental results, correctly predicting quantitative trends and the phase diagram. [Preview Abstract] |
Wednesday, March 23, 2011 4:18PM - 4:30PM |
T44.00010: Spatial population genetics in a petri dish Kirill Korolev, Joao Xavier, Melanie Muller, Nilay Karahan, Oskar Hallatschek, Kevin Foster, Andrew Murray, David Nelson The evolution of natural populations involves more than mutations followed by natural selection: Stochasticity and spatial migrations are also important. The effects of fluctuations and spatial structure become especially pronounced when organisms expand to new territories. The fluctuations are enhanced because the number of organisms at the front of the expansion is typically small, and the spatial structure is more pronounced due to dimensional reduction from two to one spatial dimension (because colonization occurs along the quasi-one-dimensional periphery of the population). The interplay of fluctuations and space leads to spatial segregation of different genotypes, which significantly alters the evolutionary dynamics of the population. We investigate this process by combining theory, simulations, and experiments on microbial expansions on the surface of a Petri dish. In particular, I will discuss how one can use simple microbiology experiments to measure important parameters of microbial populations such as the strength of fluctuations, migration rate, and relative fitness. [Preview Abstract] |
Wednesday, March 23, 2011 4:30PM - 4:42PM |
T44.00011: The Goldilocks Principle and Rapid Evolution of Antibiotic Resistance in Bacteria Qiucen Zhang, Robert Austin Goldilocks sampled the three bear's wares for the ``just right'' combination of taste, fit and comfort. Like Goldilocks's need for the just right parameters, evolution proceeds most rapidly when there is the just right combination of a large number of mutants and rapid fixation of the mutants. We show here using a two-dimensional micro-ecology that it is possible to fix resistance to the powerful antibiotic ciprofloxacin (Cipro) in wild-type E. coli in 10 hours through a combination of extremely high population gradients, which generate rapid fixation, convolved with the just right level of antibiotic which generates a large number of mutants and the motility of the organism. Although evolution occurs in well-stirred chemostats without such Goldilocks conditions, natural environments are rarely well stirred in nature.For complex environments such as the Galapagos Islands, spatial population gradients and movement of mutants along these population gradients can be as important as genomic heterogeneity in setting the speed of evolution. The design of our micro-ecology is unique in that it provides two overlapping gradients, one an emergent and self generated bacterial population gradient due to food restriction and the other a mutagenic antibiotic gradient. Further, it exploits the motility of the bacteria moving across these gradients to drive the rate of resistance to Cipro to extraordinarily high rates. [Preview Abstract] |
Wednesday, March 23, 2011 4:42PM - 4:54PM |
T44.00012: Evolution on a Lattice under Strong Mutation Jakub Otwinowski, Stefan Boettcher The most common approach to study biological evolution in a population considers mutations to arise one at a time, and spread to the whole population. However, recent experimental work has shown that under conditions of strong mutation and strong selection, multiple mutations may arise simultaneously. Such overlapping mutations compete with each other and make the results difficult to analyse. Theorists are working on understanding the relationships between different parameters such as population size, mutation rate, and selection coefficients, in the way they affect observables such as the speed of evolution, and the probability of fixation. We have shown with simulations that under additional spatial constraints the dynamics are very different compared to well-mixed populations. A surface in fitness space evolves, akin to surface growth phenomena, with non-trivial power-law exponents. The result is that the speed of evolution is restricted and the probability of fixation is reduced. [Preview Abstract] |
Wednesday, March 23, 2011 4:54PM - 5:06PM |
T44.00013: Modeling Political Populations with Bacteria Chris Cleveland, David Liao Results from lattice-based simulations of micro-environments with heterogeneous nutrient resources reveal that competition between wild-type and GASP rpoS819 strains of E. Coli offers mutual benefit, particularly in nutrient deprived regions. Our computational model spatially maps bacteria populations and energy sources onto a set of 3D lattices that collectively resemble the topology of North America. By implementing Wright-Fishcer re- production into a probabilistic leap-frog scheme, we observe populations of wild-type and GASP rpoS819 cells compete for resources and, yet, aid each other's long term survival. The connection to how spatial political ideologies map in a similar way is discussed. [Preview Abstract] |
Wednesday, March 23, 2011 5:06PM - 5:18PM |
T44.00014: Mutualistic Interactions and Community Structure in Biological Metacommunities Per Arne Rikvold, Elise Filotas, Martin Grant, Lael Parrott The role of space in determining species coexistence and community structure is well established. However, previous studies mainly focus on simple competition and predation systems, and the role of mutualistic interspecies interactions is not well understood. Here we use a spatially explicit metacommunity model, in which new species enter by a mutation process, to study the effect of fitness-dependent dispersal on the structure of communities with interactions comprising mutualism, competition, and exploitation [1]. We find that the diversity and interaction network undergo a nonequilibrium phase transition with increasing dispersal rate. Low dispersion rate favors spontaneous emergence of many dissimilar, strongly mutualistic and species-poor local communities. Due to the local dissimilarities, the {\it global\/} diversity is high. High dispersion rate promotes local biodiversity and supports similar, species-rich local communities with a wide range of interactions. The strong similarity between neighboring local communities leads to reduced global diversity. \\[4pt] [1]. E.~Filotas, M.~Grant, L.~Parrott, P.A.\ Rikvold, J.\ Theor.\ Biol.\ {\bf 266}, 419 (2010); Ecol.\ Modell.\ {\bf 221}, 885 (2010). [Preview Abstract] |
Wednesday, March 23, 2011 5:18PM - 5:30PM |
T44.00015: Universal Description of Interactive Growth Carlos Condat, Lucas Barberis Although the existence of organism-organism interactions during ontogenesis is well documented, ontogenetic growth models usually focus exclusively on the organism-environment interaction. We develop a new formalism to describe the interactive growth of two or more organisms in a given environment. Using a vector formulation of the Phenomenological Universalities concept, we are able to characterize the joint growth of two or more interacting organisms and assess the direct mutual influences between them, as well as the indirect influences that operate through environment modifications. The resulting equations describe synergetic, antagonistic, and cooperative growth, and can be applied to biological and ecological problems. As an example, we examine the growth dynamics in a mixed-species plantation. [Preview Abstract] |
Wednesday, March 23, 2011 5:30PM - 5:42PM |
T44.00016: Large fluctuations and fixation in evolutionary games Michael Assaf, Mauro Mobilia One of the most striking effects of fluctuations in evolutionary game theory is the possibility for mutants to fixate (take over) an entire population. In this work we use a semi-classical theory to study fixation in evolutionary games under non-vanishing selection, and investigate the relation between selection intensity and demographic (random) fluctuations. This approach allows the accurate treatment of rare large fluctuations and yields the probability and mean time of fixation beyond the weak-selection limit, often considered in previous works. The power of the theory is demonstrated on prototypical models of cooperation dilemmas with multiple absorbing states, and we find excellent agreement between the theoretical predictions and numerical simulations. Furthermore, we show that our treatment is superior to the Fokker-Planck approximation for finite selection intensity. M. Assaf and M. Mobilia, J. Stat. Mech. P09009 (2010). M. Mobilia and M. Assaf, Euro. Phys. Lett. 91, 10002 (2010). [Preview Abstract] |
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