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inclusive fitness (Emlen et al., 1991; Komdeur, 1996). Once social groups are established, more elaborated forms of cooperation could evolve with enhanced benefits provided to helped individuals that are potentially in line with Hamilton’s rule.

Another trait that could result through runaway social selection is the phenomenon within ants of unicoloniality. Unicoloniality occurs when adjacent nests show atypically low or no aggression toward each other and is commonly associated with invasive ant species, where supercolonies can arise that extend over thousands of kilometers (Helanterä et al., 2009). Unicolonial associations are genetically homogeneous, with the majority of ants having low relatedness to each other. The reduction in intraspe-cific aggression appears to be attributable to a loss of genetic diversity at recognition loci (Suarez et al., 2008). Although unicolonial behavior may be enhanced by genetic bottlenecks, it is also present in situ in native habitats and apparently can evolve within large populations (Pedersen et al., 2006; Wang et al., 2010). In other words, unicoloniality can be selectively favored and is not simply always a byproduct of genetic bottlenecks from introductions. Interestingly, unicoloniality could be categorized as workers showing excessive cooperation by accepting and raising unrelated individuals, exactly as predicted by the model of greenbeard cooperation. The suggestion would be that if greenbeard alleles arise in a species at their recognition loci, it would be possible for such alleles to sweep through a population, carrying unicoloniality in their wake. This would lead to high genetic similarity at recognition loci in populations where within- and across-nest relatedness is almost identical [which is indeed observed in unicolonial ants (Brandt et al., 2009)]. Finally, it is a suggestive coincidence that one of the known greenbeard allele systems (Gp-9 in fire ants) appears to have arisen in an exotic species as that species was simultaneously evolving a unicolonial population social structure (Keller, 2007).

Genetic Diversity, Relatedness, and Social Heterosis

Controversies about kin selection often concern the best methods to model the effects of genetic relatedness (Wilson and Hölldobler, 2005; Foster et al., 2006; Lehmann et al., 2007; West et al., 2007c; Wilson and Wilson, 2007; Nowak et al., 2010). This tends to obscure the more salient point of how important the level of relatedness is for favoring cooperation (Nonacs, 2011). The dynamics of social groups can be dominated by either their genetic relatedness or their genetic diversity, because both bring evolutionarily selective advantages. High relatedness means that group benefits will tend to be exclusively shared by alleles identical by descent. It becomes relatively less important which individuals are the reproducers and which individuals are the helpers (Nonacs and Hager, 2011). In

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