attempt, we discuss pros and cons of the quasispecies theory compared with classic population genetics models for haploid organisms to explain the evolution of RNA viruses.

Despite their great functional and structural diversity, all RNA viruses share the following properties (Domingo and Holland, 1997): (i) Cell–virus junction is mediated by means of specific membrane receptors. (ii) A viral particle penetrates the cell, loses its capsid, and releases its nucleic acids within the cell. (iii) The replication of the viral genome is regulated by the expression of viral genes (i.e., RNA replicase is encoded by the virus genome). (iv) The component parts of the viruses are assembled and released as virions out of the cell. In addition, these properties are complemented with four others that are relevant to understanding the evolution of RNA viruses. (v) The number of viral particles in a given infected organism may be as high as 10¹² (Domingo and Holland, 1997). Such population sizes, several orders of magnitude larger than any population size for DNA-based organisms, are related to viral generation time. (vi) In fact, a single infectious particle can produce, on average, 100,000 copies in 10 h (Domingo and Holland, 1997). If the replication machinery is working optimally, a new RNA genome is produced every 0.4 s. (vii) Genome sizes range between 3 and 30 kb. Accordingly, the number of genes per genome is also very small. (viii) Finally, RNA viruses show extremely high mutation rates (Drake and Holland, 1999). Because of the lack of proofreading by their replicases, RNA viruses show the highest mutation rates among living beings (Drake and Holland, 1999), on the order of one mutation per genome and replication round.

The above-mentioned properties of large population size, high replication rate, and short generation time are responsible, in general, for the extremely high genetic variability of RNA viral populations. Recombination and segmentation also may play an important role in generating new genetic variability (Domingo and Holland, 1997). In any case, the extent of genetic variability per generation time of any RNA virus is usually much higher than that corresponding to any DNA-based organism, providing an excellent opportunity for studying ongoing evolution in accessible terms for human observers.

According to population genetics, evolution is the change in the genetic properties of populations. Changes considered to be evolutionarily