Figure 5.

Evolution of host susceptibility to the defensive mutualist (black) for fixed levels of conferred protection. Host susceptibility to the defensive mutualist and the level of conferred protection increase with x and y, respectively. (A) Accelerating host trade‐off $(c_H^2=2)$; (B) decelerating host trade‐off $(c_H^2=0.5)$. Green and red curves show the equilibrium proportion of hosts infected with the defensive mutualist (M) and the parasite (P), respectively. Black curves show the singular strategy $(x^{\ast })$ for the host: solid curves correspond to continuously stable strategies, dashed curves to evolutionary repellers and dotted curves to evolutionary branching points where two host types may evolve and coexist from an initially monomorphic population. Shading corresponds to simulation outputs, where the adaptive dynamics assumptions of weak selection and rare mutations are relaxed. Hosts infected by the parasite cannot recover or reproduce (see Fig. S3 for the converse). Fixed parameters: $a=1$, $b=0.5$, $c_H^1=0.02$, $c_M^1=0.1$, $f_M=1,$ $f_P=0,$ $f_{\mathit{MP}}=0$, $q=0.5$, ${\alpha }_M=0.01$, ${\alpha }_P=1$, ${\stackrel{\sim }{\beta }}_M=5$, ${\stackrel{\sim }{\beta }}_P=5$, ${\gamma }_M=0$, ${\gamma }_P=0$.