Evolving cooperation.

tion of cooperation’ published by Biology to mark its 50th anniversa This article is a brief introduction to the special issue ‘Evoluthe Journal of Theoretical ry. Cooperation is a favorite topic among evolutionary biologists because it seems to be at variance with natural selection. Why should one individual help another who is a potential competitor in the struggle for survival? Yet cooperation is abundant in nature and appears to be involved in all great constructive steps of life on earth. Here I propose a definition for games that are cooperative dilemmas. The Prisoner’s Dilemma represents the most stringent situation, where natural selection opposes cooperation unless a mechanism for the evolution of cooperation is at work. In relaxed social dilemmas some cooperation can evolve even without a mechanism, but a mechanism would tend to augment the level of cooperation or facilitate its emergence. I discuss five mechanisms: direct reciprocity, indirect reciprocity, spatial selection, multi-level selection and kin selection. These mechanisms can work separately and together to promote evolution of cooperation. I give a short overview of the papers in this issue and provide an outlook of some of the goals that might lie ahead. Biologists are interested in cooperation, because it seems to be the antithesis to the competition that is fundamental to natural selection. Why should one individual help another who is a competitor in the struggle for survival? Yet cooperation is abundant in nature. It can be seen among viruses, bacteria, other microorganisms, animals, plants and humans. The evolutionary processes which have led to the eukaryotic cell, to multi-cellular organisms, to social insects and to human society are all based to some extent on cooperation. Cooperation is a key aspect of social evolution, where interactions among individuals affect reproductive success. The standard mathematical approaches for studying social behavior are offered by game theory (Harsanyi and Selten, 1988; Fudenberg and Tirole, 1991; Osborne and Rubinstein, 1994; Samuelson, 1997; Camerer, 2003) and evolutionary game theory (Maynard Smith, 1982; Hofbauer and Sigmund, 1988, 1998, 2003; Weibull, 1995; Skyrms, 1996; Dugatkin and Reeve, 1998; Cressman, 2003; Nowak and Sigmund, 2004; Nowak et al., 2004; Taylor et al.,