Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session Q13: Focus Session: Stochastic Processes in Biology I 
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Sponsoring Units: GSNP DBP Chair: Ira Schwartz, Naval Research Laboratory Room: B112 
Wednesday, March 17, 2010 11:15AM  11:27AM 
Q13.00001: Epidemics in adaptive networks with community structure Leah Shaw, Ilker Tunc Models for epidemic spread on static social networks do not account for changes in individuals' social interactions. Recent studies of adaptive networks have modeled avoidance behavior, as noninfected individuals try to avoid contact with infectives. Such models have not generally included realistic social structure. Here we study epidemic spread on an adaptive network with community structure. We model the effect of heterogeneous communities on infection levels and epidemic extinction. We also show how an epidemic can alter the community structure. [Preview Abstract] 
Wednesday, March 17, 2010 11:27AM  11:39AM 
Q13.00002: Emergence of traveling waves in the spreading of dengue fever Simone Bianco, Andrea Faatz, Derek Cummings, Leah Shaw Dengue fever is a multistrain mosquitoborne subtropical disease that exhibits complex oscillatory outbreaks. Epidemiological data from Thailand displays traveling waves of infection originating in Bangkok, the largest population center (Cummings et al., Nature 427: 344, 2004). We present a multistrain metapopulation model in which traveling wave like behavior results from migration coupling between heterogeneous regions. The region with the highest effective persontoperson contact rate leads the dynamics. A stochastic version of the model will also be presented. [Preview Abstract] 
Wednesday, March 17, 2010 11:39AM  11:51AM 
Q13.00003: Stochastic amplitude scaling in time dependent population models Ira Schwartz, Eric Forgoston We consider the problem of stochastic fluctuations in timedependent populations modeling SEIRtype epidemic outbreaks. Stochastic model reduction is used to explore the fluctuations in dynamics when contacts in the population are seasonal. The scaling effects of noiseinduced outbreak amplitudes are derived in terms of biological and social parameters, and explored in meanfield models. The theory is applied directly to spatiotemporal data to: 1: Construct the dynamics of unobserved asymptomatic individuals. 2. Show the scaling effects of fluctuations on the asymptomatic exposed population. [Preview Abstract] 
Wednesday, March 17, 2010 11:51AM  12:27PM 
Q13.00004: Human mobility and epidemic invasion Invited Speaker: Vittoria Colizza The current H1N1 influenza pandemic is just the latest example of how human mobility helps drive infectious diseases. Travel has grown explosively in the last decades, contributing to an emerging complex pattern of traffic flows that unfolds at different scales, shaping the spread of epidemics. Restrictions on people's mobility are thus investigated to design possible containment measures. By considering a theoretical framework in terms of reactiondiffusion processes, it is possible to study the invasion dynamics of epidemics in a metapopulation system with heterogeneous mobility patterns. The system is found to exhibit a global invasion threshold that sets the critical mobility rate below which the epidemic is contained. The results provide a general framework for the understanding of the numerical evidence from detailed datadriven simulations that show the limited benefit provided by travel flows reduction in slowing down or containing an emerging epidemic. [Preview Abstract] 
Wednesday, March 17, 2010 12:27PM  12:39PM 
Q13.00005: Constrained Branching Random Walks as a minimal model for adaptive evolution Oskar Hallatschek Models of both sexual and asexual adaptation in wellmixed populations usually lead to solitary waves of adaptation. This means that the fitness distribution of the population assumes the form of a wave and moves to higher fitness at a certain speed of adaptation. This nonequilibrium steady state is easy to obtain in simulations but usually hard to analyze due to lack of detailed balance. Here, we introduce an analytically tractable minimal model that captures the essence of fitness waves of adaptation: i) a branching random walk of genotypes. ii) a global constraint that keeps the populations size finite. We show that for certain constraints an exact solution can be found. This exact solution, which can be summarized as a deterministic PDE with a peculiar cutoff, also turns out to approximate conventional models of adaptation in an unprecedented accuracy. [Preview Abstract] 
Wednesday, March 17, 2010 12:39PM  12:51PM 
Q13.00006: Reconstruction of bonds potentials from first passage time distributions Tom Chou, PakWing Fok We explore the reconstruction the functional form of the potential energy surface of a molecular bond from distributions of its rupture times. For a single measured first passage time distribution (FPTD)the inverse problem is illposed and only a few attributes (such as the height and width of an energy barrier) can be reconstructed. However, we find optimal temperatures and initial bond configurations that yield the most efficient reconstruction of simple potentials. Reconstruction of finer details of more complicated bond potentials can be achieved by simultaneously using two or more measured FPT distributions, obtained under different physical conditions. For example, by changing the effective potential energy surface by known amounts, through for example, externally applied forces, the additional FPT distributions render the inverse problem less illposed. We demonstrate the feasibility of reconstructing potential with multiple minima, motivate heuristic rulesofthumb for optimizing the reconstruction, and discuss further applications and extensions. [Preview Abstract] 
Wednesday, March 17, 2010 12:51PM  1:03PM 
Q13.00007: Demographic Fluctuations versus Spatial Variation in the Competition between Fast and Slow Dispersers Charles R. Doering, Jack N. Waddell, Leonard M. Sander Dispersal is an important strategy employed by populations to locate and exploit favorable habitats. Given competition in a spatially heterogeneous landscape, what is the optimal rate of dispersal? Continuous population models predict that, all other features the same, a species with a lower dispersal rate always drives a competing species to extinction in the presence of spatial variation of resources. But the introduction of intrinsic demographic fluctuations can reverse this conclusion. We present a simple model in which competition between the exploitation of resources and population birthdeath fluctuations leads to victory by either the faster or slower of two species depending on the environmental parameters. A simplified limiting case of the model, analyzed by closing the moment and correlation hierarchy, quantitatively predicts which species will win in the complete model under given parameters of spatial variation and average carrying capacity. [Preview Abstract] 
Wednesday, March 17, 2010 1:03PM  1:15PM 
Q13.00008: Evolving towards the optimal path to extinction in stochastic processes Eric Forgoston, Simone Bianco, Leah Shaw, Ira Schwartz A large, rare stochastic fluctuation can cause an epidemic or a species to become extinct. In large, finite populations, the extinction process follows an optimal path which maximizes the probability of extinction. We show theoretically that the optimal path also possesses a maximal sensitivity to initial conditions. As a result, the optimal path emerges naturally from the dynamics and may be characterized using the finitetime Lyapunov exponents. Our theory is general, and is demonstrated with several stochastic epidemiological models. [Preview Abstract] 
Wednesday, March 17, 2010 1:15PM  1:27PM 
Q13.00009: Extinction Rate Fragility in Population Dynamics Michael Khasin, Mark Dykman We study population extinction due to fluctuations in a system of coupled populations and find the logarithm Q of the extinction rate. The formulation turns out to be substantially different from that for the seemingly similar and extensively studied problem of the rate of interstate switching in nonequilibrium systems. This difference quite generally leads to the extinction rate fragility, where a very small perturbation can change the extinction rate exponentially strongly [1]. Formally, it means that the limit of Q for the perturbation going to zero differs from the value of Q calculated in the absence of the perturbation. ~The fragility is related to the discontinuity of the quasistationary extinction current. A general condition for the onset of fragility is derived. We show that one of the bestknown models of epidemiology, the susceptibleinfectioussusceptible model, is fragile to total population fluctuations. The analysis [1]is extended to incorporate external noise. The analytical results are fully confirmed by simulations.[1] M. Khasin and M. I. Dykman, Phys. Rev. Lett. 103 , 068101 (2009) [Preview Abstract] 

Q13.00010: ABSTRACT WITHDRAWN 
Wednesday, March 17, 2010 1:39PM  1:51PM 
Q13.00011: Effects of particle mobility in onedimensional rockpaperscissors games Siddharth Venkat, Michel Pleimling As the behavior of a system composed of cyclically competing species is strongly influenced by the presence of fluctuations, it is of interest to study cyclic dominance in low dimensions where these effects are the most prominent. We here discuss rockpaperscissors games on a onedimensional lattice where the interaction rates and the mobility can be species dependent. Allowing only single site occupation, we realize mobility by swapping particles of different species. When the interaction and swapping rates are species independent, a strongly enhanced swapping rate yields an increased mixing of the species, leading to a meanfield like coexistence even in onedimensional systems. This coexistence is transient when the rates are asymmetric, and eventually only one species will survive. Interestingly, in our spatial games the surviving species can differ from the species that would survive in the corresponding zerodimensional model. We identify different regimes in the parameter space and construct the corresponding dynamical phase diagram. [Preview Abstract] 
Wednesday, March 17, 2010 1:51PM  2:03PM 
Q13.00012: Combinatorial control of heterogeneous cell populations C. Piermarocchi, P. Duxbury, G. Paternostro, J. Feala, S. Tiziani, J. Axelrod, A. Chaudhury, J. Choi, A. McCulloch, J. Cortes In medicine, a recent pharmacological approach involves systematic discovery of combinatorial therapies, in which different drugs are simultaneously used to control different pathways associated with a cellular function. This control must occur with minimal response in other nontarget cells exposed to treatment, i.e. it has to be selective. We have investigated the statistics of selective control of the human apoptosis (cell death) signaling network. We have built a model for a heterogeneous population of cells, characterized by a signaling network with identical topology, but having different link strengths. The control of the life/death signal is realized by acting with external perturbations, modeling the effect of drugs, on the nodes and on the signaling flow. Concepts from statistical physics and information theory, including entropy, frustration, and nonlinearity have been used to characterize the general properties of selective control. This knowledge was used as a guide in designing algorithms for identifying selective perturbations. Some of these algorithms have been implemented \textit{in vitro }in high throughput experiments on real cell lines where a large number of combinations of different drugs can be tested. [Preview Abstract] 

Q13.00013: ABSTRACT WITHDRAWN 
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