Virus Evolution: How Does an Enveloped Virus Make a Regular Structure?

The evolution of viruses has been an exciting area of study, albeit an area that is fraught with difficulties be- cause of the lack of a fossil record and because of the rapid sequence divergence exhibited by viruses. All viruses in collections available for study in the laboratory have been isolated within the last 70 years. Studies of the rate of sequence divergence in viruses over this period of time, all of which have focused on RNA viruses, have given estimates of 10^2 to 10^4 changes per nucleotide per year (Takeda et al., 1994; Weaver et al., 1997). Although these rates for the fixation of mutations of necessity assay changes in only the most variable positions in the viral genome, and there are clearly positions that change much more slowly, it is nonetheless clear that it is difficult to establish relationships between two viruses that last had a common ancestor, for example, a million years ago, based solely on sequence relation- ships. Furthermore, it has become increasingly clear in the last two decades that extensive recombination over the ages has complicated the evolutionary relationships among viruses belonging to different families (Strauss et al., 1996). To ascertain distant relationships among viruses, structural studies are of increasing importance, because the structure of a protein changes much less rapidly than does the amino acid sequence that forms the structure (Rossmann et al., 1974)