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was clear that such behavior could benefit the group or the population, the species, or even other species and whole communities. However, it was not obvious how such effects would be heritable. All our mathematical models—the hard work of the modern synthesis—were about individuals with one allele out-reproducing those with an alternative. This process would favor individuals with higher reproduction but would not be expected to produce self-sacrifice. Yet, apparent cooperation was routinely attributed to the good of the group, species, or community. This situation changed in the first decade of Darwinism’s second century. William D. Hamilton (1964a,b) argued that cooperation was important in nature, and that social evolution could be understood in terms of direct gains to the actor’s own fitness or indirect benefits to the fitness of others who share the cooperation allele. There followed an intense period of exploring the indirect effects of cooperation and altruism, reinterpreting sexual selection and many other phenomena in terms of individual advantage, and understanding frequency-dependent effects via game theory, efforts that continue to the present.

The puzzle of cooperation was the dominant theme of research in the early years, whereas recent work has emphasized its importance and ubiquity. Far from being a rare trait shown by social insects and a few others, cooperation is both widespread taxonomically and essential to life. Major transitions in the hierarchy of life have often involved cooperation among lower-level units to the point where they evolve into higher-level organisms (Buss, 1987; Maynard Smith and Szathmary, 1995). Examples include the assembly of the eukaryotic cell with its symbiotic organelles, the evolution of multicellular organisms, and the organismal colonies of some social insects. Organisms are, at multiple levels, those units that have evolved to have, within their boundaries, extreme cooperation and minimal conflict (Queller and Strassmann, 2009; Strassmann and Queller, 2010). The depth of research on cooperation and conflict has increased greatly, most notably in the direction of small organisms. Microbes turn out to have highly developed cooperation (West et al., 2007a), and they, along with other model organisms, have proven instrumental in beginning to understand sociality at the genetic and molecular levels, the study of real selfish genes (Santorelli et al., 2008). The social evolution approach has given us new insights on diseases often caused by microbes (Foster, 2005). At the other end of the spectrum, we are getting a much better understanding of the cooperation and conflict that matter most to our species (Alexander, 1979). Cooperation has been central to humanity’s spectacular success and will be central to our short-term and long-term fate.

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