Emergent multilevel selection in a simple spatial model of the evolution of altruism
Fig 4
Quantification of multilevel selection: two approaches.
Two approaches to quantify multilevel selection are applied to the simulation of Fig 3A. S4 Fig shows the results of two more replicate simulations. (A) Population mean level of altruism versus time. (B) The first approach (MLS 1) is to mathematically split the natural selection measured acting on the organisms into two parts: selection within and selection among colonies [42]. (See S1 Text Section 4.) For the simulation shown in Fig 3A, this calculation was done for each subsequent interval of 80 generations (see section Materials and methods). Plotted is the change in the mean level of altruism (black), and the within-colony (red) and among-colony (blue) components of selection. The rotated histograms on the right-hand side show distributions based on second half of the simulation, indicated with the gray background. In this part of the simulation, the population mean level of altruism no longer changed systematically. However, within-colony selection is nearly always negative, compensated by positive among-colony selection. (C) The second approach (MLS 2) describes evolution at the level of the colonies. (See S1 Text Section 5.) Evolution taking place within colonies then appears as a transmission bias: a bias in the change between ancestor and offspring colonies in the colony mean level of altruism. This transmission bias (red) tends to be negative, but is compensated by positive selection at the colony level.