Chromodynamics of Cooperation in Finite Populations

Abstract

Background: The basic idea of tag-based models for cooperation is that individuals recognize each other via arbitrary signals, so-called tags. If there are tags of different colors, then cooperators can always establish new signals of recognition. The resulting “chromodynamics” is a mechanism for the evolution of cooperation. Cooperators use a secret tag until they are discovered by defectors who then destroy cooperation based on this tag. Subsequently, a fraction of the population manages to establish cooperation based on a new tag. Methodology/Principal Findings: We derive a mathematical description of stochastic evolutionary dynamics of tag-based cooperation in populations of finite size. Benefit and cost of cooperation are given by b and c. We find that cooperators are more abundant than defectors if \(b/c > 1+2u/v\), where \(u\) is the mutation rate changing only the strategy and v is the mutation rate changing strategy and tag. We study specific assumptions for \(u\) and \(v\) in two genetic models and one cultural model. Conclusions/Significance: In a genetic model, tag-based cooperation only evolves if a gene encodes both strategy and tag. In a cultural model with equal mutation rates between all possible phenotypes (tags and behaviors), the crucial condition is \(b/c > (K+1)/(K-1)\), where \(K\) is the number of tags. A larger number of tags requires a smaller benefit-to-cost ratio. In the limit of many different tags, the condition for cooperators to have a higher average abundance than defectors becomes \(b > c\).