Figure 1.

Two-dimensional illustration of deflection (θ) and evolvability [e(β)], the measures of constraint. θ is the angle between the vector of selection (β) and the predicted response to selection ($\mathbf{\Delta }\overline{\mathbf{z}}$). Evolvability e(β) is the length of the projection of $\mathbf{\Delta }\overline{\mathbf{z}}$ onto β, as a proportion of the length of β (Equation 7), and represents the magnitude of the predicted response to selection in the direction of selection; adapted from Hansen and Houle (2008).