Populations of RNA viruses are of a quasispecies nature. The reproduction of RNA viruses requires viral gene expression to synthesize the replication machinery. Self-reproduction, the basis of evolutionary adaptation, actually serves two purposes: it ensures conservation of information, and it provides by its autocatalytic nature a mechanism of competitive growth and selection. The existence of a threshold relation for selection suggests a new interpretation of the phenomenon of natural selection, namely, as a kind of condensation phenomenon, the condensation or localization of a sequence distribution in a limited area in sequence space. Natural selection can be shown to be a direct physical consequence of reproduction, be it vegetative or sexual. The difficulties which such an interpretation will encounter — even in the evolution of self-replicating sequences of very limited length — becomes immediately obvious if we specify mutations more quantitatively. Sequence comparison of different isolates of the same virus indicates that the fitness landscape indeed is quite connective and smooth.