Fitness affects only death or reproduction. Assortment at different cost-benefit ratios and different frequencies of cooperators in the baseline scenario (n
= 10, m = 0.02). (a) No selection (s = 0). (b) Fitness affects birth and death (s = 0.5). (c) Fitness affects only death (s = 0.5). (d) Fitness affects only reproduction (s = 0.5).