Individuals sometimes give up their own resources to benefit their neighbors. Such altruistic traits posed a difficulty for the original Darwinian formulation of natural selection, which emphasized the spread of individually advantageous characters. So how have altruistic traits become common in some populations? This is an important question because a purely individualistic world would look very different from the one that we see. With no altruism, there would be no multicellularity with specialized nonreproductive tissues, no social insects with specialized worker castes, and nothing at all like complex human societies. In this chapter, I discuss three processes that can promote altruism and the evolution of social cooperation. I start with kin selection, in which an individual may give up some of its own reproduction to aid relatives, or an individual may coordinate its behavior with phenotypically similar neighbors to promote the good of the group. Altruism toward kin helps to explain patterns of parasite virulence, sex ratios, and complex sociality with division of labor between different individuals. Some groups build up a high level of social cohesion in spite of little relatedness and low opportunity for kin selection. In the second section, I discuss how repression of competition can be a powerful force integrating the interests of individuals. With no opportunity to compete against neighbors, an individual can only increase its own success by increasing the success of the whole group. Meiosis provides the classic example, in which the strict control of chromosomal segregation into gametes prevents competition between different chromosomes. It is only through such repression of competition between chromosomes that the genome developed into a highly integrated and cohesive unit. In the third section, I turn to another key theme in the history of life---the evolutionary innovations of cooperative symbioses between different species or different kinds of genomes. The first genomes near the origin of life probably evolved by biochemical synergism between different replicating molecules; eukaryotic cells arose by symbioses between different species; and lichens, mycorrhizal-plant systems, and many other symbioses have contributed greatly to the complexity of modern life. The evolution of symbioses concerns the same social tensions between conflict and cooperation as the more familiar problems from kin groups and animal societies. Cooperative symbioses may evolve by positive feedback between partners. In such synergistic relations, one party gives up some of its resources to enhance the success of its partner, and the partner does the same. The vast majority of cooperative symbioses arose as biochemical synergisms between organisms without complex behavioral flexibility. By contrast, the exchange of benefits between partners with the capacity for memory and the potential for strategy leads to issues of cheating, detection of cheaters, and strategic assessment of partner behavior and quality. Such problems of reciprocal altruism (Trivers 1971) pervade many aspects of vertebrate sociality, in which individuals remember particular partners and their past behaviors, and individuals can assess the complex strategies of others and form their own strategies in response. I do not cover behavioral reciprocity in this chapter, in order to focus on the more genetically relevant aspects of social selection that fit the themes of this volume. My three topics of kin selection, repression of competition, and synergistic symbiosis all play fundamental roles in the evolution of genetic systems.
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