Evolution of RNA Viruses

The evolution of RNA viruses has received a great deal of attention in the past several years. The subject is based on information very different from that considered: in other evolutionary discussions. As obligate intracellular parasites, viruses leave no fossil record; indeed the oldest historical accounts describing symptoms believed to be caused by viruses are only a millenium or two old (51, 73). Recently, however, the complete nucleotide sequences of genomic RNAs of a number of viruses belonging to different virus families have been obtained, and these make possible detailed comparisons of the families on a molecular level. These comparisons have led to new insights RNA virus evolution, including the beginning of taxonomy based on the relatedness of their genomes, a better understanding of the radiation of viruses to different hosts, and renewed speculation on the origin of viruses. In , partial or complete sequences have been obtained for many different strains of certain viruses or for different members of a given family. The comparative sequence data have led to a greater understanding of the rate of divergence of RNA viruses in nature and in cell culture. All of these studies, however, have been of viruses currently extant; the oldest virus isolates date only to the turn of this century.