RNA viruses consist of populations of extremely high genetic heterogen
eity called quasispecies. Based on theoretical considerations, it has
been suggested that the unit of selection in such complex genetic popu
lations is not the single viral particle but a set of genetically rela
ted particles which form the quasispecies. In the present study we car
ried out a set of experiments with the vesicular stomatitis virus (VSV
) dealing with the evolution of life-history characters under selectio
n acting at two factors either in the same or in opposite directions.
The two factors at which selective pressure is applied are the individ
ual and the group. We show evidence that group selection modulates the
virulence of VSV populations, in opposition to an unlimited increase
in virulence by competitive optimization promoted by individual select
ion. The results are of relevance for understanding the evolution of p
arasite virulence.