Fig. 6

The effect of nutrient supply on the strength of altruism. The difference \({{w}_{A}\left({\pi }^{\mathrm{^{\prime}}}\right)-w}_{S}\left(\pi \right)\) represents the change in the fitness of a focal S-type if it were to convert to an A-type. This difference is negative under the larger nutrient supply \({R}_{in}=50\) (when \({N}_{cap}\left(\pi =1\right)=1000\) A-type cells), indicating that whole-group trait altruism is strong. But it is positive under the smaller nutrient supply \({R}_{in}=5\) (when \({N}_{cap}\left(\pi =1\right)=100\) A-type cells). The reduction in the influx of nutrient therefore causes a shift in the fitness structure that makes whole-group trait altruism effectively weak