![]() The matter is, for many, not yet settled. ![]() The actual criteria used vary by the taxon, and can be inconsistent (arbitrary similarity thresholds) or unrelated to lineage (geography) at times. In July 2013, the ICTV definition of species changed to state: "A species is a monophyletic group of viruses whose properties can be distinguished from those of other species by multiple criteria." These criteria include the structure of the capsid, the existence of an envelope, the gene expression program for its proteins, host range, pathogenicity, and most importantly genetic sequence similarity and phylogenetic relationship. In 1991, the more specific principle that a virus species is a polythetic class of viruses that constitutes a replicating lineage and occupies a particular ecological niche was adopted. In 1982, the ICTV started to define a species as "a cluster of strains" with unique identifying qualities. 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. ![]() Species form the basis for any biological classification system. See also: Viral quasispecies Species definition
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