Editing Virus classification (section)

=== 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 />