Calculating Evolutionary Dynamics in Structured Populations

Evolution is shaping the world around us. At the core of every evolutionary process is a population of reproducing individuals. The outcome of an evolutionary process depends on population structure. Here we provide a general formula for calculating evolutionary dynamics in a wide class of structured populations. This class includes the recently introduced “games in phenotype space” and “evolutionary set theory.” There can be local interactions for determining the relative fitness of individuals, but we require global updating, which means all individuals compete uniformly for reproduction. We study the competition of two strategies in the context of an evolutionary game and determine which strategy is favored in the limit of weak selection. We derive an intuitive formula for the structure coefficient, σ, and provide a method for efficient numerical calculation

Critical dynamics in the evolution of stochastic strategies for the iterated Prisoner's Dilemma

The observed cooperation on the level of genes, cells, tissues, and individuals has been the object of intense study by evolutionary biologists, mainly because cooperation often flourishes in biological systems in apparent contradiction to the selfish goal of survival inherent in Darwinian evolution. In order to resolve this paradox, evolutionary game theory has focused on the Prisoner's Dilemma (PD), which incorporates the essence of this conflict. Here, we encode strategies for the iterated Prisoner's Dilemma (IPD) in terms of conditional probabilities that represent the response of decision pathways given previous plays. We find that if these stochastic strategies are encoded as genes that undergo Darwinian evolution, the environmental conditions that the strategies are adapting to determine the fixed point of the evolutionary trajectory, which could be either cooperation or defection. A transition between cooperative and defective attractors occurs as a function of different parameters such a mutation rate, replacement rate, and memory, all of which affect a player's ability to predict an opponent's behavior.Comment: 27 pages, including supplementary information. 5 figures, 4 suppl. figures. Version accepted for publication in PLoS Comp. Bio

BRIEF REPORT: Factors Associated with Depression Among Homeless and Marginally Housed HIV-Infected Men in San Francisco

OBJECTIVES: To evaluate the prevalence of and factors associated with depression among HIV-infected homeless and marginally housed men. DESIGN: Cross-sectional study. PARTICIPANTS AND SETTING: Homeless and marginally housed men living with HIV in San Francisco identified from the Research on Access to Care in the Homeless (REACH) Cohort. MEASUREMENTS: The primary outcome was symptoms of depression, as measured by the Beck Depression Inventory (BDI). Multivariate logistic regression was used to identify associations of sociodemographic characteristics, drug and alcohol use, housing status, jail status, having a representative payee, health care utilization, and CD4 T lymphocyte counts. RESULTS: Among 239 men, 134 (56%) respondents screened positive for depression. Variables associated with depression in multivariate analysis included white race (adjusted odds ratio [AOR]=2.2, confidence interval [CI]=1.3 to 3.9), having a representative payee (AOR=2.4, CI=1.3 to 4.2), heavy alcohol consumption (AOR=4.7, CI=1.3 to 17.1), and recently missed medical appointments (AOR=2.6, CI=1.4 to 4.8). CONCLUSIONS: Depression is a major comorbidity among the HIV-infected urban poor. Given that missed medical appointments and alcohol use are likely indicators of depression and contributors to continued depression, alternate points of contact are necessary with many homeless individuals. Providers may consider partnering with payees to improve follow-up with individuals who are HIV-positive, homeless, and depressed