Fitness is a parameter that quantitatively measures adaptation of a virus t
o a given environment. We have previously reported exponential fitness gain
s of large populations of vesicular stomatitis virus replicating in a const
ant environment (I. S. Novella et al., Proc. Natl. Acad. Sci. USA 92:5841-5
844, 1995). In this paper, we report that during long-term passage of such
large viral populations, fitness values reached a high-fitness plateau duri
ng which stochastic fitness variations were observed. This effect appears l
ikely to be due to bottleneck effects on very high fitness populations.