We present a mean-field theory for the evolution of RNA virus populati
ons. The theory operates with a distribution of the population in a on
e-dimensional fitness space, and is valid for sufficiently smooth fitn
ess landscapes. Our approach explains naturally the recent experimenta
l observation [L. S. Novella et al., Proc. Natl. Acad. Sci. U.S.A. 92,
5841-5844 (1995)] of two distinct stages in the growth of virus fitne
ss.