Figure 1. Schematic of how trait group selection operates. Imagine an initial unstructured population, with a frequency (p_Global) of altruists equal to 0.46. This population is then broken up into three trait groups. The three trait groups vary in the frequency of altruists, ranging from p = 0.0 (in group 3) to p = 0.9 (in group 1). Natural selection now operates in each group. Because altruists pay a cost as compared with their selfish groupmates, the frequency of altruists in each group decreases (p₁ = 0.9, p¢₁ = 0.85; p₂ = 0.5, p¢₂ = 0.43) or remains the same (p₃ = 0.0, p¢₃ = 0.0). However, the more altruists in a group, the more total group members after selection operates. As such, selection within groups favors selfishness, and selection between groups favors altruism. Here, between-group selection is stronger than within-group selection and so, when we place our three groups back together (into an unstructured population, to restart the process of trait group selection), the global frequency of altruism has risen (from 0.46 to 0.50) despite the fact that the frequency of altruism in each of the three trait groups decreased or remained the same.