Fig. 2.

Fitness function and evolutionary stable strategy for a virus population. Panel a presents the fitness function ($\varphi (\mu )$) as a function of virulence for a given strain, assuming no direct dependence from others. It should be noted that this function exhibits a global maximum, achieved at ${\mu }_k={\mu }_k^{\ast }$, indicating ${\mu }_k^{\ast }$ as the ESS for the pathogen. Panel b illustrates how the fitness function changes with the parameter $y$. The red (with open circles) and blue curves (with vertical marks) represent the fitness function when the parameter $y$ takes values $y_2$ and $y_1$, respectively. Notably, the global maximum of each of these curves is achieved at ${\mu }_k^{\ast }(y_2)$ (for the red curve, open circles) and ${\mu }_k^{\ast }(y_1)$ (for the blue curve, vertical marks), indicating the ESS for the respective parameter values. This observation illustrates the change in ESS with a given parameter $y$ (Color figure online)