Fig. 1.

Hypothetical selection surface showing correlational selection on two traits: corolla length (X₁) and stamen length (X₁). The surface is based on eqn (1): w = α + β₁X₁ + β₂X₂ + 0·5γ₁₁X₁² + 0·5γ₂₂X₂² + γ₁₂X₁X₂ + ε, with α, β₁ and β₂ set equal to zero, corresponding to the absence of directional selection, γ₁₁ = –2 and γ₂₂ = –1, corresponding to stabilizing selection on both traits, and γ₁₂ = 1·41 corresponding to correlational selection.