Large cooperation in social dilemmas : effects of spatial structure and the evolutionary origin of cooperators and defectors.
Christoph Hauert (Harvard University, Cambridge, USA)

Jeudi 6 janvier 2005

Understanding the evolution of cooperation represents a major challenge in evolutionary biology and behavioral ecology. The essence of the evolutionary conundrum is captured by social dilemmas, in which groups of cooperators do better than groups of defectors, yet defecting individuals outcompete cooperators in mixed groups.

Over the years, the Prisoner’s Dilemma (PD) has established as the leading mathematical metaphor to study the evolution of cooperation. In the PD, cooperative acts incur costs to the acting individual while benefits accrue only to others. In this case, cooperators are doomed in absence of supporting mechanisms such as direct or indirect reciprocity, spatially extended population structures or voluntary interactions.

In contrast, the social dilemma is relaxed if the benefits of cooperative acts not only accrue to others but also to the cooperator itself. Such interactions are described by the Snowdrift game (SD, also known as the Hawk-Dove and Chicken game). The SD presents a biologically appealing scenario and promising alternative to the PD.

Because of the less stringent conditions of the SD, cooperation appears to be more easily achieved. While this holds for unstructured populations, it turns out not to be true in general. In fact and in contrast to common belief, spatial structure tends to be deleterious in the SD and may eliminate cooperation altogether.

This approach, together with the vast majority of theoretical investigations, assumes a priori the presence of two distinct behavioral traits, the cooperators and the defectors. In nature, however, continuously varying degrees of cooperative investments often seem more realistic.

A continuous variant of the SD takes this into account and displays rich evolutionary dynamics but most intriguingly, it provides a natural explanation for social diversification and the evolutionary origin of cooperators and defectors.