Bulletin of the American Physical Society
APS March Meeting 2021
Volume 66, Number 1
Monday–Friday, March 15–19, 2021; Virtual; Time Zone: Central Daylight Time, USA
Session J14: Evolutionary and Ecological Dynamics IFocus Live
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Sponsoring Units: DBIO GSNP Chair: Ajay Gopinathan, University of California, Merced; Peter Yunker, Georgia Inst of Tech |
Tuesday, March 16, 2021 3:00PM - 3:12PM Live |
J14.00001: Dynamics of growth, death, and resource competition in sessile organisms Edward Lee, Christopher P Kempes, Geoffrey B West Population-level scaling in ecological systems arises from individual growth and death with competitive constraints. We build on a minimal dynamical model of metabolic growth where the tension between individual growth and mortality determines population size distribution. We include resource competition based on shared capture area separately. By varying relative rates of growth, death, and competitive attrition, we connect regular and random spatial patterns across sessile organisms from forests to ants, termites, and fairy circles. Then, we consider transient temporal dynamics in the context of asymmetric competition that primarily weakens the smaller of two competitors such as canopy shading or large colony dominance. When such competition couples slow timescales of growth with fast competitive death, it generates population shock waves similar to those observed in forest demographic data. Our minimal quantitative theory unifies spatiotemporal patterns across sessile organisms through local competition mediated by the laws of metabolic growth which in turn result from long-term evolutionary dynamics. |
Tuesday, March 16, 2021 3:12PM - 3:24PM Live |
J14.00002: Emerging spatio-temporal patterns in cyclic predator-prey systems with habitats Hana Mir, James Stidham, Michel Pleimling Cyclic predator prey systems are known to establish spiral waves that allow species to coexist. In this study, we analyze a structured heterogeneous system which gives one species an advantage to escape predation in an area, we refer to as a habitat, and study the effect on species coexistence and spatio-temporal patterns. We analyze the density of each species after giving one species a higher escape rate in a rock-paper-scissors game and find the species given the advantage carries the largest population as long as the escape rate assigned is below a threshold. After this threshold is exceeded, the species with the largest population is, instead, the predator of the species with the advantage. Numerical analysis of the spatial density of each species, as well as the analysis of a two-point correlation function for both inside and outside the habitats point to the same results. We also extend the analysis to a six species game that exhibits spontaneous spiral waves. |
Tuesday, March 16, 2021 3:24PM - 3:36PM Live |
J14.00003: Universality classes for fixation-time distributions in stochastic evolutionary games David Hathcock, Steven H Strogatz Stochastic models in evolutionary biology and ecology are often described by birth-death dynamics where absorption times are the key quantity of interest: how long does it take for a mutation to become fixed or for a fluctuating population to go extinct? We characterize two universality classes of absorption-time distributions for birth-death Markov chains. Based on generic features of the transition rates, the asymptotic distribution is either Gaussian, Gumbel, or a convolution of Gumbel distributions. We provide simple analytical criteria and intuitive heuristics for predicting the absorption-time distribution. We use our results to characterize the fixation-time distributions in two-strategy evolutionary games, which often fall into one of these classes depending on population network structure and the game parameters. More broadly, our results apply to many simple stochastic models of evolution, ecology, epidemiology, and chemical reactions. |
Tuesday, March 16, 2021 3:36PM - 3:48PM Live |
J14.00004: Differences in bacteria killing rates are mitigated by finite size effects Raymond Copeland, Peter Yunker Bacteria often live in densely packed, surface attached communities called biofilms in which different species and strains compete for space and resources. Many bacteria have evolved methods of killing competitors, and a wide range of killing abilities have been empirically observed both in type and efficiency. Typically, biofilm competition is studied on agar plates; in these experiments, there are effectively no spatial limitations on biofilm growth, and biofilms may contain millions of cells or more. However, in nature biofilms frequently grow in confinement; geometric constrains in these environments limit the size of biofilms. To determine the impact of finite size effects on deadly competitions within biofilms, we perform individual based simulations of bacteria confined to different sized environments. We find that, if both strains can kill each other, the fitness advantage of the superior killing strain decreases as the biofilm becomes more spatially limited. These results suggest that bacteria that primarily live in small groups may have little incentive to evolve to be better killers, which may partially explain the wide range of observed killing efficiencies. |
Tuesday, March 16, 2021 3:48PM - 4:00PM Live |
J14.00005: Improve it or lose it: evolvability costs of competition for expression Jacob Moran, Devon Finlay, Mikhail Tikhonov Expression level is known to be a strong determinant of a protein's rate of evolution. But the converse can also be true: evolutionary dynamics can affect expression levels of proteins. Having implications in both directions fosters the possibility of a feedback loop, where systems that are expressed higher are more likely to improve and be expressed even higher, while those that are expressed less are eventually lost to drift. Using a minimal model to study this in the context of a changing environment, we demonstrate that one unexpected consequence of such a feedback loop is that a slow switch to a new environment can allow genomes to reach higher fitness sooner than direct exposure. |
Tuesday, March 16, 2021 4:00PM - 4:12PM Live |
J14.00006: Population boundary across an environmental gradient: Effects of quenched disorder Istvan Kovacs, Robert Juhasz Population boundary is a classic indicator of climatic response in ecology. The spatial and dynamical characteristics of the boundary are not only affected by spatial gradients in the environmental factors, but also by local heterogeneities in the regional characteristics. Here, we capture the effects of quenched heterogeneities on the ecological boundary with the disordered contact process in one and two dimensions with a linear spatial trend in the local control parameter. We show that under a quasistatic change of the global environment, mimicking climate change, the front advances intermittently: long quiescent periods are interrupted by rare but long jumps. The characteristics of this intermittent dynamics are found to obey universal scaling laws in terms of the gradient, conjectured to be related to the correlation-length exponent of the model. Our results suggest that current observations might misleadingly show little to no climate response for an extended period of time, concealing the long-term effects of climate change. |
Tuesday, March 16, 2021 4:12PM - 4:24PM Live |
J14.00007: Fluctuations in population density change topology of genealogical trees in range expansions Gabriel Birzu, Oskar Hallatschek, Kirill S Korolev Spatial expansions shape patterns of genetic diversity in a wide range of biological contexts, from the growth of bacterial colonies to the spread of epidemics. Such diversity patterns are encoded in the shape of genealogical trees, which trace the ancestry of individuals in the population. However, it is not known which factors determine the shape of these genealogies in expanding populations. Here, we show that different growth dynamics lead to qualitative changes in the topology of genealogies at the front. In particular, we find that highly cooperative growth leads to genealogies in which only pairwise mergers between branches occur. In contrast, moderate and low growth cooperativity lead to genealogies in which multiple branches merge simultaneously. Importantly, we find that the transition between these regimes is universal and results from the coupling between growth dynamics and population fluctuations at the front. While previous studies have relied on approximating expansions by a series of bottlenecks of fixed size, we show that such deterministic approximations lead to qualitatively different topologies, even when front fluctuations are small. Our results thus demonstrate that density fluctuations play a key role in shaping genetic diversity in expansions. |
Tuesday, March 16, 2021 4:24PM - 5:00PM Live |
J14.00008: Self-Organization of Lifelike Behaviors Invited Speaker: Jeremy England Life is a multifarious bundle of distinct physical phenomena that are distinctive, but not unique to, living things. Self-replication, energy harvesting, and predictive sensing are three such phenomena, and each can be given a clear physical definition. In this talk, we will report recent progress in understanding what physical conditions are required for the spontaneous emergence of these various lifelike behaviors from assemblages of simple, interacting components. |
Tuesday, March 16, 2021 5:00PM - 5:12PM Live |
J14.00009: Modeling the ecology of parasitic plasmids Jaime Lopez, Ned S Wingreen, Mohamed S Donia
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Tuesday, March 16, 2021 5:12PM - 5:24PM Live |
J14.00010: Inducing stable spiral structures and population diversity in the asymmetric May--Leonard model Shannon Serrao, Uwe Tauber We study the spiral induction on a two-dimensional lattice using Monte Carlo simulations of the three-species May-Leonard model with asymmetric predation rates. Strongly asymmetric predation rates lead to rapid extinction of two species from three-species coexistence. For weakly asymmetric predation rates, only a fraction of ensembles lead to three-species coexistence. However, when spatially coupled to a symmetric May-Leonard patch, the stable spiral patterns from this region induce quasi-stationary spiral patterns in the asymmetric region. We qualitatively describe the spiral stabilization in the asymmetric patch down to the injection of periodic wavefronts from the adjacent symmetric region. The increase in robustness of stable spiral formation at extreme values of the asymmetric predation rates in the coupled system is compared to the asymmetric May-Leonard model in isolation. We delineate the quasi-stationary nature of coexistence induced in the asymmetric subsystem. In addition to engendering spiral pattern formation, we explore the spiral stability in the asymmetric region and propose a criterion for the quasi-stationary spiral stability of the asymmetric subsystem. |
Tuesday, March 16, 2021 5:24PM - 5:36PM Live |
J14.00011: Transition states of two-cycle ecological oscillators: Blume-Capel representation Vahini Reddy Nareddy, Jonathan Machta, Karen Abbott, Shadi Sadat Esmaeili-Wellman, Alan Hastings Many spatially-extended systems of ecological oscillators exhibit spatial synchrony with periodic oscillations in time. If the individual oscillators have two-cycle behavior, the transition to synchrony as a function of noise and coupling strength is in the Ising universality class. It was shown that a dynamical Ising model with memory does a good job in representing such ecological systems and predicting their future states[1]. In the Ising representation, the two phases of oscillations (high at odd times or high at even times) of an individual oscillator are represented by spin-up and spin-down. However, the behavior of an individual ecological oscillator suggests the existence of a transition state along with the two phases of oscillations. The oscillations at this transition state have amplitude very close to zero. To study such systems, we use Blume-Capel representation where the spin can take three values S={+1,-1,0} with S=0 as the transition state and S={-1,+1} as the two phases of oscillations. We model the spatially-extended ecological systems with coupled lattice maps in two-cycle regime and represent them with the Blume-Capel model. We also discuss maximum likelihood methods to infer the Blume-Capel representation. |
Tuesday, March 16, 2021 5:36PM - 5:48PM Live |
J14.00012: Time-Dependent Effective Sampling Bias in Populations with Broad Offspring Number Distributions Takashi Okada, Oskar Hallatschek It is increasingly recognized that natural populations exhibit broad family size distributions, for example, due to high fecundities and high early mortality, or due to founder effects associated with range expansion. Despite recent progress in the neutral dynamics induced by broad offspring numbers, our knowledge of interactions between broad offspring numbers and natural selection remains limited. Here, we establish several new scaling relations about the fixation probability, the extinction time, the allele-frequency fluctuations, and the site frequency spectrum, when offspring numbers are distributed according to a fat-tailed distribution with a divergent variance (1/uα+1 with 1<α<2). We validate the new findings in a class of models of range expansions, which effectively produce broad offspring number distributions with 1<α<2. |
Tuesday, March 16, 2021 5:48PM - 6:00PM Live |
J14.00013: Playing it safe: information constraints bias living systems towards less risky strategies Philipp Fleig, Vijay Balasubramanian To survive in complex, changing environments, biological organisms must employ an appropriately adapted strategy. For example, by making the right decisions, expressing the appropriate phenotype or implementing the most efficient foraging behaviour, etc. an organism can attain higher physical fitness in a given environment. However, in any realistic setting, an organism has only limited information about its environment, making it hard for it to `know’ the right strategy. We argue that strategies employed by organisms are fundamentally probabilistic, and define a probability of survival in terms of a distance measure between an organism's strategy and the statistics of the environment. Our measure for the probability of survival serves as a proxy for physical fitness. We then show that given constraints on an organism’s information gathering, the optimal strategy will have a natural bias towards lower risk, i.e., towards playing it safe. We quantify these considerations, and discuss how our prediction of inherent bias towards less risky strategies can be tested experimentally in biological systems. |
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