Elena Pierre
Viruses have short generation times, and many—in particular RNA viruses—have relatively high mutation rates (on the order of one point mutation or more per genome per round of replication). This elevated mutation rate, when combined with natural selection, allows viruses to quickly adapt to changes in their host environment. In addition, most viruses provide many offspring, so any mutated genes can be passed on to many offspring quickly. Although the chance of mutations and evolution can change depending on the type of virus (double stranded DNA, double stranded RNA, single strand DNA, etc.), viruses overall have high chances for mutations. 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 a treatment. One of the main theoretical models applied to viral evolution is the quasispecies model, which defines a viral quasispecies as a group of closely related viral strains competing within an environment. Three classical hypotheses. Viruses are ancient. Studies at the molecular level have revealed relationships between viruses infecting organisms from each of the three domains of life, suggesting viral proteins that pre-date the divergence of life and thus infecting the last universal common ancestor. This indicates that some viruses emerged early in the evolution of life and that they have probably arisen multiple times. It has been suggested that new groups of viruses have repeatedly emerged at all stages of evolution, often through the displacement of ancestral structural and genome replication genes. There are three classical hypotheses on the origins of viruses and how they evolved: Virus-first hypothesis: Viruses evolved from complex molecules of protein and nucleic acid before cells first appeared on earth. By this hypothesis, viruses contributed to the rise of cellular life. This is supported by the idea that all viral genomes encode proteins that do not have cellular homolog’s. The virus-first hypothesis has been dismissed by some scientists because it violates the definition of viruses, in that they require a host cell to replicate. Reduction hypothesis (degeneracy hypothesis): Viruses were once small cells that parasitized larger cells. This is supported by the discovery of giant viruses with similar genetic material to parasitic bacteria. However, the hypothesis does not explain why even the smallest of cellular parasites do not resemble viruses in any way. Escape hypothesis (vagrancy hypothesis): Some viruses evolved from bits of DNA or RNA that "escaped" from the genes of larger organisms. This doesn't explain the structures that are unique to viruses and are not seen anywhere in cells. It also does not explain the complex capsids and other structures of virus particles.