The Evolution of Cooperation between Unrelated Individuals in Small Groups



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Altruism is widespread across the tree of life, with examples ranging from vertebrates to lichens to multispecies biofilms. The initial evolution of such altruism is likely to involve interactions that produce non-additive fitness effects among small groups of unrelated individuals in local populations. However, most models for the evolution of altruism have focused on genealogically related individuals, assume that the factors influencing individual fitness are deterministic, that populations are very large, and that the benefits of altruism increases linearly with the number of altruistic interactions. Here, we show that stochasticity and non-additive interactions can facilitate the evolution of altruism in small local groups. We derive a generalized model for the evolution of altruism and highlight the importance of separating the discussion of evolution on the trait and its fitness effects. We also show that if altruism reduces the variance in individual fitness (separate from its effect on average fitness), this can aid the evolution of altruism through directional stochastic effects. In addition, we show that the potential for the evolution of altruism is influenced by non-additivity in benefits with altruism being more likely to evolve when the marginal benefit of an altruistic act increases with the number of such acts. This work culminates by modeling the initial evolution of division of labor and the contribution of uncorrelated altruistic traits to the evolution of expression plasticity. Our model compliments traditional altruism models (kin selection, Hamilton’s rule, reciprocal altruism, green beard effects, etc.) and applies to a broad range of altruistic interactions seen in nature.



Cooperation, Altruism, stochastic fitness, non-additivity, division of labor, multicellularity, evolution, axiomatic, directional stochastic effects, environmental inheritance, group structure, relative fitness, biorthogonal polynomials