The concept of virus species
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The concept of virus species The species is the basic taxonomic group in biological systematics. The species concept can be extended to virology because viruses are biological entities, not simply chemicals. Viruses have genomes, replicate, evolve, and occupy particular ecological niches. As for all other biological entities, viruses have an intrinsic genetic variability due to the error-prone process of nucleic acid duplication. It is this built-in variability which allows biological systems to become adapted through selection and which in the end guarantees their survival. Whereas the molecules of a chemical compound are all identical, the virus particles isolated from an infected host have an inherent variability. Hence, unlike chemicals, virus species cannot be grouped into universal classes defined by a single property or set of properties that are both necessary and sufficient for class membership. Properties and classes are related abstract entities. Whatever is said about a thing is seen as ascribing a property to it, or equivalently, to assign the thing to a universal class [5]. If a virus has a positive strand RNA genome, it is automatically a member of the class of positive strand RNA viruses. In addition, the same virus could also be a member of other universal classes defined for instance by icosahedral shape or a certain genome structure. It is important to realize, however, that although viruses can be members of a variety of universal classes where each such class is defined by a single property both necessary and sufficient for class membership, the grouping known as a virus species is not a universal class definable by a single property. Instead, members of a virus species can be grouped into a different type of class known as a polythetic class, defined by a combination of properties I-8, 9]. In a polythetic class, no character or property is necessary or sufficient to define the class. The concept of polythetic class was introduced by Beckner [I] and is illustrated in the following example taken from Sattler [6]. Suppose a group of individuals is defined by a set of five properties, fl to f5. If these properties are distributed as shown in Table 1, a typical polythetic class is obtained. In this potythetic class, each individual possesses a large number but not all of the properties of the set, each property in the set is possessed by most but not all individuals, and no property is possessed by all individuals.
Table 1. The concept of polythetic class Individual
Properties of individual
1
fl fl fl ft
2 3 4 5
f2 f2 f2 f2
f3 f3 tB t3
f4 f4 f4 f4
f5 f5 f5 f5
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Virology Division News
The definition of a virus species as a polythetic class means that all members of the species do not have a single defining property in common that is necessary and sufficient for class membership. It is not appropriate, therefore, to search for an elusive, single property that would define a virus species. It should be stressed that such a hypothetical prope
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