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 the 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 into 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 addition, 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 back only to the turn of this century