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Abstract
The evolution of cooperation is a fundamental problem in biology, especially for non-relatives, where indirect fitness benefits cannot counter within-group inequalities. Multilevel selection models show how cooperation can evolve if it generates a group-level advantage, even when cooperators are disadvantaged within their group. This allows the possibility of group selection, but few examples have been described in nature. Here we show that group selection can explain the evolution of cooperative nest founding in the harvester ant Pogonomyrmex californicus. Through most of this species’ range, colonies are founded by single queens, but in some populations nests are instead founded by cooperative groups of unrelated queens. In mixed groups of cooperative and single-founding queens, we found that aggressive individuals had a survival advantage within their nest, but foundress groups with such non-cooperators died out more often than those with only cooperative members. An agent-based model shows that the between-group advantage of the cooperative phenotype drives it to fixation, despite its within-group disadvantage, but only when population density is high enough to make between-group competition intense. Field data show higher nest density in a population where cooperative founding is common, consistent with greater density driving the evolution of cooperative foundation through group selection.
Highlights
The evolution of cooperation is a fundamental problem in biology, especially for non-relatives, where indirect fitness benefits cannot counter within-group inequalities
Social evolution in these contexts fits well with inclusive fitness models built around genetic relatedness
Survival analysis showed significantly lower individual mortality in groups compared to solitary foundresses, consistent with a survival advantage to cooperation (Supplementary Fig. 1, Supplementary Table 1)
Summary
Zachary Shaffer[1], Takao Sasaki[2], Brian Haney[1], Marco Janssen[3], Stephen C. To better understand the forces driving cooperation, we studied the harvester ant Pogonomyrmex californicus, which is haplometrotic over most of its range, but has some populations where groups of two to fifty or more unrelated queens found nests together[36] These pleometrotic groups mature into colonies with two or more reproductive queens, marking one of the few cases of primary polygyny in ants[36,37] (Fig. 1b). Colony foundation can be readily observed in laboratory nests, a technique that has shed light on the development of a colony’s social organization and division of labor[38,39,40] This species has served as a model to investigate both proximate and ultimate questions in the evolution of cooperative behavior[41]. We made daily records of queen survival, brood presence, and any aggressive behavior
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