Emergent multilevel selection in a simple spatial model of the evolution of altruism
Fig 2
Altruism and colonies emerge in the two-dimensional habitat.
Results are shown from a representative simulation run (see S1 Fig for replicates) with default parameters (see Table 1). (A) Mean level of altruism versus time (thick colored line) as well as the cumulative contribution of natural selection (black), which is consistently positive (see S1 Text, section 3). (B) Snapshots of the simulation habitat; also see the S1–S3 Videos. In time, the population self-organizes into a hexagonal pattern of discrete colonies. A section of a hexagonal grid is superimposed in the right-most panel. (C) The hexagonal pattern is also apparent from the radial distribution function at t = 8000, the distribution of distances between pairs of individuals normalized by the random expectation. Black arrows indicate the distances occurring in an exact hexagonal grid with grid constant a = 8.4 and their relative frequency. (D) Enlargements of a small domain of the habitat, showing that the colonies behave like Darwinian entities: they disappear as a result of a within-colony tragedy of the commons [38] (red circle and cross), and reproduce by binary fission (green arrows).