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A2.3.5 (HL)—Several origins of viruses

Viruses are probably polyphyletic, with different lineages arising through escaped genetic elements, reduction of cellular parasites, or ancient pre-cellular replicators.

Syllabus
First assessment 2025
Objective
A2.3.5
Level
HL

Concept essentials

  • Viral diversity is consistent with multiple evolutionary origins rather than one universal ancestor.
  • The progressive hypothesis derives viruses from escaped cellular genetic elements.
  • The regressive hypothesis derives viruses from cellular parasites that lost independence.
  • The virus-first hypothesis derives some viruses from ancient self-replicating systems.
  • Similar host dependence may have evolved convergently in separate viral lineages.
ConceptIB Biology HL