  We consider a nonlinear partial differential equation of conservation type to
describe the dynamics of vesicular stomatitis virus observed in aliquots of
fixed particle number taken from an evolving clone at periodic intervals of
time \cite{novella 95}. The changes in time behavior of fitness function
noticed in experimental data are related to a crossover exhibited by the
solutions to this equation in the transient regime for pulse-like initial
conditions. As a consequence, the average replication rate of the population is
predicted to reach a plateau as a power t^{-(1/2)}.
