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Viral evolution is a subfield of evolutionary biology and virology that is specifically concerned with the evolution of viruses. Many viruses, in particular RNA viruses, have short generation times and relatively high mutation rates (on the order of one point mutation or more per genome per round of replication for RNA viruses). This elevated mutation rate, when combined with natural selection, allows viruses to quickly adapt to changes in their host environment. Viral evolution is an important aspect of the epidemiology of viral diseases such as influenza (influenza virus), AIDS (HIV), and hepatitis (e.g. HCV). The rapidity of viral mutation also causes problems in the development of successful vaccines and antiviral drugs, as resistant mutations often appear within weeks or months after the beginning of the treatment. One of the main theoretical models to study viral evolution is the quasispecies model, as the viral quasispecies. ==Origins== Viruses are ancient. Studies at the molecular level have revealed relationships between viruses infecting organisms from each of the three domains of life and viral proteins that pre-date the divergence of life and thus the last universal common ancestor.〔Mahy, p. 25〕 This indicates that some viruses emerged early in the evolution of life,〔Mahy, p. 26〕 and that viruses have probably arisen multiple times. There are three classical hypotheses on the origins of viruses: Viruses may have once been small cells that parasitised larger cells (the ''degeneracy hypothesis''〔Leppard, p. 16〕〔Sussman, p. 11〕 or ''reduction hypothesis''〔Mahy, p. 24〕); some viruses may have evolved from bits of DNA or RNA that "escaped" from the genes of a larger organism (the ''vagrancy hypothesis''〔Sussman, pp. 11–12〕 or ''escape hypothesis''); or viruses could have evolved from complex molecules of protein and nucleic acid at the same time as cells first appeared on earth (the ''virus-first hypothesis'').〔〔Villarreal, L.P. Viruses and the Evolution of Life. ASM Press, 2005. ISBN 978-1555813093.〕 None of these hypotheses was fully accepted: the regressive hypothesis did not explain why even the smallest of cellular parasites do not resemble viruses in any way. The escape hypothesis did not explain the complex capsids and other structures on virus particles. The virus-first hypothesis was quickly dismissed because it contravened the definition of viruses, in that they require host cells.〔 Virologists are, however, beginning to reconsider and re-evaluate all three hypotheses.〔Mahy, pp. 362–78〕 One of the problems for those studying viral origins and evolution is their high rate of mutation, particularly the case in RNA retroviruses like HIV/AIDS. A recent study based on comparisons of viral protein folding structures, however, is offering some new evidence. Fold Super Families (FSF's) are proteins that show similar folding structures independent of the actual sequence of amino acids, and have been found to show evidence of viral phylogeny. Thus viruses have been found to be capable of being divided into 4 FSFs; based upon the three realms of bacterioviruses, archaeoviruses, and eukaryoviruses, together with a fourth FSF that seems to indicate that it predated the separation of the three realms. Thus "viral proteomes retain traces of ancient evolutionary history that can be recovered using advanced bioinformatics approaches." Anshan Nasir and Gustavo Caetano-Anollés, "This implies the existence of ancient cellular lineages common to both cells and viruses before the appearance of the “last universal cellular ancestor” that gave rise to modern cells. According to our data, the prolonged pressure of genome and particle size reduction eventually reduced virocells into modern viruses (identified by the complete loss of cellular makeup), whereas other coexisting cellular lineages diversified into modern cells. "〔Anshan Nasir and Gustavo Caetano-Anollés, "A phylogenomic data-driven exploration of viral origins and evolution" (Science Advances, Vol 1, No. 8, 04 September 2015)〕 Furthermore, the long genetic distance between RNA and DNA FSF's suggests that the RNA world hypothesis may have new experimental evidence, with a long intermediary period in the evolution of cellular life. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Viral evolution」の詳細全文を読む スポンサード リンク
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