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contrast, higher genetic diversity can create social heterosis, or the ability to exploit a wider range of resources more effectively, and thus increase total group benefits (Nonacs and Kapheim, 2007). Although more restrictive on the possible skew between reproducers and helpers, within-group outcomes become less important relative to across-group competition. The evolutionarily inescapable point is, however, that nepotism, as predicated on Hamilton’s rule, must come at the expense of genetic diversity. Similarly, selection for genetic diversity may require behavioral biases against closer genetic relatives. Relatedness and diversity have an unavoidable tradeoff between them.

The model presented here gives no advantage to genetic diversity; therefore, cooperation evolves to maximize population-level genetic similarity. Alleles go to high frequency or fixation acting either as greenbeards or to maximize genome-level inclusive fitness. Nevertheless, one might expect that the diversity/ relatedness tradeoff could be different for green-beard or Hamiltonian alleles. Any natural system where the benefits of cooperation are primarily directed to close relatives would strongly select against genetic diversity. In contrast, cooperation could evolve with considerably less of a tradeoff with a greenbeard kin recognition mechanism. The benefits of cooperation would not necessarily only flow to the closest relatives. As argued above in the case of ant unicoloniality, selection for nepotism based on greenbeard similarity could potentially affect only a limited part of the genome. Social heterosis could simultaneously select for genetic diversity at the remainder of the genome, with the result being a patchwork genome of regions of low and high genetic diversity (Nonacs and Kapheim, 2007). It is difficult to imagine how such opposing selective processes could simultaneously operate when one or more traits are being selected relative to their probabilities of being shared. Kin nepotism following Hamilton’s rule will always work to reduce genetic diversity. It is the dynamic evolutionary consequences of selection for kin vs. selection for genetic diversity that should draw the future attention of both theoreticians and experimentalists.


I thank J. Field, D. C. Queller, T. Wang, and an anonymous reviewer for helpful comments on the manuscript.

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