Distinguishing mechanisms for the evolution of co-operation.

The existence of co-operation between species has been cast as a problem to the selfish-gene view of evolution: why does co-operation persist, when it would seem that individual selection should favor the unco-operative individual who exploits the co-operative tendencies of its partner and gives nothing in return? The recent literature has emphasized one type of model as underlying the evolution and stability of interspecific co-operation, which we term the "partner-fidelity" model, and which is typified by the game theory model known as the iterated Prisoner's Dilemma game. Under this mechanism, individuals are associated with the same partner(s) during an indefinite sequence of interactions. Individuals who at any time fail to co-operate with their partner can be penalized by those same partners in subsequent trials, hence the co-operation can be evolutionarily stable. Many examples of biological co-operation that have been offered appear to conform to this model. However, a few examples appear instead to fit a different and unrecognized mechanism, termed "partner-choice". Under partner-choice, individuals are associated for just one interaction, but an asymmetry enables one member to differentially reward co-operative vs. unco-operative partners in advance of any possible exploitation. Possible examples of co-operation maintained through partner-choice mechanisms are provided by the yucca/yucca moth system and the fig/fig wasp system.