..., and in the geminiviruses biologically very distinct entities have much genome homology and are closely related serologically. The biological species concept seems inappropriate or impractical for many plant viruses, and a more flexible and pragmatic approach to assigning virus isolates to nameable categories is advocated...

...R.G. Milne The species concept seems to be inescapably based on the occurrence, in a population, of regular genetic interchange which leads to the formation of common gene pools and on the presence of barriers to interchange which distinguish one gene pool from another. Present knowledge indicates...

... virologists deny the possibility of applying the species concept to asexual organisms, such as viruses. Those persons are influenced by an obsolete definition of biological species which rests on observed or inferred barriers to sexual reproduction, (iii) New taxonomic tools, such as mathematical (numerical...