The Species Problem in Virology.

Virus classification deals with conceptual species classes that have viruses as their members. A virus species cannot be described but can only be defined by listing certain species-defining properties of its member. However, it is not possible to define a virus species by using a single species-defining property. The new 2013 official definition of virus species is not appropriate because it applies equally to virus genera. A nucleotide motif is a chemical part of a viral genome and is not a species-defining property that could be used for establishing new virus species. A virus classification based solely on nucleotide sequences is a classification of viral genomes and not of viruses. The variable distribution of species-defining properties of a polythetic species class is not itself a single common property of all the members of the class, since this would lead to the paradox that every polythetic class is also a monothetic one.
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