WHENCE TIT-FOR-TAT

In theoretical and empirical studies of the evolution of cooperation, the tit-for-tat strategy (i.e. cooperate unless your partner did not c ooperate in the previous interaction) is widely considered to be of ce ntral importance. Nevertheless, surprisingly little is known about the conditions in which tit-for-tat appears and disappears across generat ions in a population of interacting individuals. Here, we apply a newl y developed classifier-system model (EvA) in addressing this issue whe n the key features of interactions are caricatured using the iterated prisoner's dilemma game. Our simple representation of behavioural stra tegies as algorithms composed of two interacting rules allowed us to d etermine conditions in which tit-for-tat can replace noncooperative st rategies and vice versa. Using direct game-theoretic analysis and simu lations with the EvA model, we determined that no strategy is evolutio narily stable, but larger population sizes and longer sequences of int eractions between individuals can yield transient dominance by tit-for -tat. Genetic drift among behaviourally equivalent strategies is the k ey mechanism underlying this dominance. Our analysis suggests that tit -for-tat could be important in nature for cognitively simple organisms of limited memory capacity, in strongly kin-selected or group-selecte d populations, when interaction sequences between individuals are rela tively short, in moderate-sized populations of widely interacting indi viduals and when defectors appear in the population with moderate freq uency.