Figure 5

Periodic epidemics increase the tendency for dominant types to resist invasion by rare types. The height of each bar shows the maximum cost that a rare type can bear and still successfully invade a dominant type. The light bars show the maximum cost when there is no periodicity in epidemics; we took those values from the lower right panel of Figure 1. The lower values of the dark bars show the more stringent conditions for invasion under periodic epidemics, in which rare types can invade only when they carry relatively lower costs than under nonperiodic epidemics. The top panel shows γ = 0.1; the lower panel shows γ = 1. For the periodic cases, p = 20 and a = 2. All other parameters match the lower right panel of Figure 1.