Fig. 5

An illustration of the contrast between hard (a) and soft (b) selection using hypothetical values for model parameters. The fitness of both the altruistic A-type and selfish S-type are assumed to increase with the proportion \(\pi\) of A-types in the population. However, a whereas the fitness differential \({w}_{S}\left(\pi \right)-{w}_{A}\left(\pi \right)\) remains constant in the absence of nutrient limitation (Eq. 1), b it gets smaller as \(\pi \to 1\) when nutrients are limited