Emergent multilevel selection in a simple spatial model of the evolution of altruism

Theories on the evolutionary origins of altruistic behavior have a long history and have become a canonical part of the theory of evolution. Nevertheless, the mechanisms that allow altruism to appear and persist are still incompletely understood. It is well known, however, that the spatial structure of populations is an important determinant. In both theoretical and experimental studies, much attention has been devoted to populations that are subdivided into discrete groups. Such studies typically imposed the structure and dynamics of the groups by hand. Here, we instead present a simple individual-based model in which altruistic organisms spontaneously self-organize into spatially separated colonies that themselves reproduce by binary fission and hence behave as Darwinian entities in their own right. Using software to automatically track the rise and fall of colonies, we are able to apply formal theory on multilevel selection and thus quantify the within- and among-group dynamics. This reveals that individual colonies inevitably succumb to defectors in a within-colony “tragedy of the commons”. Even so, altruism persists in the population because more altruistic colonies reproduce more frequently and drive less altruistic ones to extinction. Evidently, the colonies promote the selection of altruism but in turn depend on altruism for their existence; the selection of altruism hence involves a kind of evolutionary bootstrapping. The emergence of the colonies also depends crucially on the length scales of motility, altruism, and competition. This reconfirms the general relevance of these scales for social evolution, but also stresses that their impact can only be understood fully in the light of the emergent eco-evolutionary spatial patterns. The results also suggest that emergent spatial population patterns can function as a starting point for transitions of individuality.Author summary: An important topic in the theory of evolution is the evolution of altruistic traits: behaviors that are costly to oneself, but beneficial to others. Natural selection can favor altruism if, somehow, altruists disproportionately enjoy the benefits, thus making up for the costs. However, what mechanisms could bring about such conditions is still under intense study. Mathematical and computational models have been key in exploring and evaluating various hypotheses. Here, we present a new computational model of organisms that evolve an altruistic trait. The organisms are uncomplicated: they merely move, reproduce, and die randomly, while competing for resources and potentially helping their neighbors. Nevertheless populations of these organisms show complex behavior: they spontaneously organize into spatially separated altruistic colonies that themselves grow, reproduce by dividing in two, and die by extinction. This means that natural selection can be considered at two levels: of the individual and the colonies. Within colonies, cheaters outcompete altruists; but more altruistic colonies reproduce more frequently and survive longer, thus enabling the emergence of altruism. Previous models did consider such “multilevel” evolution, but included the dynamics of the groups by decree. This model shows that self-organization can spontaneously induce such behavior even under very simple conditions.