In the system with strategies ALLD and WSLS, there is a critical threshold for the benefit-to-cost ratio, b/c, that determines whether ALLD or WSLS is selected in large populations. WSLS is selected if and only if b/c exceeds the threshold. The figure shows how the critical value depends on the intensity of selection. The value is about 3.6 for strong selection, decreases with increasing background fitness, and approaches 3 in the limit of weak selection. In particular, if b/c > 3, then WSLS is selected if the intensity of selection is sufficiently weak.