Editing Virus classification (section)

==Definitions==
{{see also|Viral quasispecies}}

=== Virus definition ===
The currently accepted and formal definition of a 'virus' was accepted by the ICTV Executive Committee in November 2020 and ratified in March 2021, and is as follows:<ref>{{cite journal|last1=Koonin|first1=Eugene V.|last2=Dolja|first2=Valerian V.|last3=Krupovic|first3=Mart|last4=Kuhn|first4=Jens H.|date=2021-09-01|title=Viruses Defined by the Position of the Virosphere within the Replicator Space|journal=Microbiology and Molecular Biology Reviews|volume=85|issue=4|pages=e0019320|language=EN|doi=10.1128/MMBR.00193-20|pmc=8483706|pmid=34468181}}</ref>

{{blockquote|Viruses ''[[sensu stricto]]'' are defined operationally by the ICTV as a type of [[Mobile genetic elements|MGE]] that encodes at least one protein that is a major component of the virion encasing the nucleic acid of the respective MGE and therefore the gene encoding the major virion protein itself or MGEs that are clearly demonstrable to be members of a line of evolutionary descent of such major virion protein-encoding entities. Any monophyletic group of MGEs that originates from a virion protein-encoding ancestor should be classified as a group of viruses.}}

=== Species definition ===
Species form the basis for any biological classification system. Before 1982, it was thought that viruses could not be made to fit [[Ernst Mayr]]'s reproductive concept of species, and so were not amenable to such treatment. In 1982, the ICTV started to define a species as "a cluster of strains" with unique identifying qualities. In 1991, the more specific principle that a virus species is a [[wikt:polythetic|polythetic]] class of viruses that constitutes a replicating lineage and occupies a particular ecological niche was adopted.<ref name=alimpiev>{{cite book |last1=Alimpiev |first1=Egor |title=Rethinking the Virus Species Concept |date=March 15, 2019 |url=https://stanford.edu/~alimpiev/thnk_ppr.pdf |archive-url=https://web.archive.org/web/20200922152304/http://stanford.edu/~alimpiev/thnk_ppr.pdf |archive-date=2020-09-22 |url-status=live}}</ref>

As at 2021 (the latest edition of the ICVCN), the ICTV definition of species states: "A species is the lowest taxonomic level in the hierarchy approved by the ICTV. A species is a [[monophyletic]] group of MGEs ([[mobile genetic element]]s) whose properties can be distinguished from those of other species by multiple criteria", with the comment "The criteria by which different species within a genus are distinguished shall be established by the appropriate Study Group. These criteria may include, but are not limited to, natural and experimental host range, cell and tissue tropism, pathogenicity, vector specificity, antigenicity, and the degree of relatedness of their genomes or genes. The criteria used should be published in the relevant section of the ICTV Report and reviewed periodically by the appropriate Study Group."<ref name=ictvcode />

=== Below species rank (named viruses/virus strains/isolates) ===
Many individually named ''viruses'' (sometimes referred to as "virus strains") exist at below the rank of virus ''species''. The ICVCN gives the examples of blackeye cowpea mosaic virus and peanut stripe virus, which are both classified in the species ''[[Bean common mosaic virus]]'', the latter a member of the genus ''[[Potyvirus]]'' that will in due course receive a binomial name as ''Potyvirus [species...]''. As another example, the virus [[SARS-CoV-1]], that causes severe acute respiratory syndrome ([[SARS]]) is different from the virus [[SARS-CoV-2]], the cause of the [[COVID-19]] pandemic, but both are classified within the same virus species, a member of the genus ''[[Betacoronavirus]]'' that is currently known as ''[[Severe acute respiratory syndrome-related coronavirus]]'' which, per the 2021 mandate from the ICTV, will also receive a binomial name in due course. As set out in the ICVCN, section 3.4, the names [and definitions] of taxa below the rank of species are not governed by the ICTV; "Naming of such entities is not the responsibility of the ICTV but of international specialty groups. It is the responsibility of ICTV Study Groups to consider how these entities may best be classified into species."<ref name=ictvcode /> Using the example given above, the virus causing the COVID-19 pandemic was given the designation "SARS-CoV-2" by the ''Coronaviridae'' Study Group (CSG) of the International Committee on Taxonomy of Viruses in 2020; in the same publication, this Study Group recommended a naming convention for particular isolates of this virus "resembl[ing] the formats used for isolates of avian coronaviruses, filoviruses and influenza virus" in the format virus/host/location/isolate/date, with a cited example as "SARS-CoV-2/human/Wuhan/X1/2019".<ref>{{cite journal |author=Coronaviridae Study Group of the International Committee on Taxonomy of Viruses |title=The species Severe acute respiratory syndrome-related coronavirus: classifying 2019-nCoV and naming it SARS-CoV-2 |journal=Nature Microbiology |volume=5 |issue=4 |pages=536–544 |date=April 2020 |pmid=32123347 |pmc=7095448 |doi=10.1038/s41564-020-0695-z}}</ref>