Explain why viruses are not assigned to any of the three domains of life, and discuss the implications of this for their classification within a cladistic framework.

Viruses are not considered living organisms because they lack cellular structure and cannot carry out metabolic processes independently; they require a host cell for replication. The three domains of life – Bacteria, Archaea and Eukarya – are defined by cellular organisation and by similarities in ribosomal RNA sequences, features that viruses do not possess. Cladistics relies on shared derived characters (synapomorphies) that are inherited from a common ancestor; because viruses do not share a common cellular ancestor with the domains, they cannot be placed on the same phylogenetic tree of life. Consequently, viral taxonomy is based on alternative criteria such as capsid morphology, genome type (DNA or RNA, single‑ or double‑stranded) and replication strategy, rather than on cladistic ancestry.