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Natural selection favors cooperation when genes underlying it increase in frequency compared with their noncooperative counterparts (Hamilton, 1964a; Frank, 1998; West et al., 2007b). Evolutionary studies of cooperative interactions have focused on the selective advantages of cooperating, how cooperation is organized, whether cheating a cooperative system can occur, and how cheaters are controlled (Ratnieks, 1990; West et al., 2002c; Beekman and Ratnieks, 2003; Griffin et al., 2004; Sachs et al., 2004; Travisano and Velicer, 2004; Ratnieks et al., 2006; Wenseleers and Ratnieks, 2006b). These studies generally, but not always, focus on within-species interactions and have been behaviorally oriented. Social insects have been a major focus (Bourke and Franks, 1995; Robinson, 2002; Strassmann and Queller, 2007), with cooperative birds and mammals also getting considerable attention (Cockburn, 1998; Clutton-Brock et al., 2001; Cornwallis et al., 2010). The past few decades have seen phenomenal progress in understanding cooperation in these organisms by applying the powerful logic of kin selection (Queller, 1992a; Frank, 1998; West et al., 2007b).

Our advances in understanding the evolution of social behavior through kin selection have been very satisfying, but they have been isolated in some respects. This is because most organisms have not been seen to be particularly cooperative. They may come together briefly for mating but otherwise go about the business of securing nutrients, avoiding disease and predation, and producing progeny largely on their own.


Behavioral ecologists have begun to study a wider selection of organisms and are finding cooperative interactions to be much more pervasive than previously appreciated. This is particularly true for microbes, wherein the structured environments necessary for cooperation have been discovered to be pervasive (Kerr et al., 2002; Griffin et al., 2004; Vos and Velicer, 2009). Microbes are particularly affected by the actions of their neighbors, because many functions that are internal in multicellular organisms are external in single-celled organisms. Secreted compounds involved in processes like iron sequestration or food digestion are vulnerable to exploitation by neighboring individuals (Travisano and Velicer, 2004; Buckling et al., 2007; West et al., 2007a). Microorganisms evaluate their numbers with quorum sensing, kill nonclonemates with bacteriocins, hunt in groups, and cooperatively swarm through their environment, to name just a few examples of their social attributes (Crespi, 2001; Riley and Wertz, 2002; Diggle et al., 2007a; West et al., 2007a). Sociality in nontraditional study organisms is only beginning to be understood, however.

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