Fig. (2) Fitness tradeoffs across hosts. (a) Expected fitness for specialist and generalist viruses if a tradeoff exists. Although both specialist genotypes perform well in their respective hosts, each one is poorly adapted in the other host. The green bars illustrate the behavior of a generalist virus that performs fairly well in both hosts but has lower fitness than either specialist in its preferred host. According with this picture, a specialist virus will always outcompete a generalist one on its host but if hosts vary in time or space, the generalist may have an overall advantage. (b) Outcome of three evolution experiments. Virus evolved in single host become specialists on their respective hosts; by contrast, a virus evolved in a fluctuating host landscape becomes generalist and improves fitness in both hosts at the same time (green bars).