1.5.3Inquiry Process
Origin and Evolution of Viruses — IB Biology HL Revision Notes & Topical Questions
5 min read 5 parts Lesson 14 of 160
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Part 1 of 5Hypotheses of Origin
Viruses leave little fossil evidence, so origin ideas rely on comparing genomes.
Escape hypothesis
- Escape hypothesis Viruses began as fragments of genetic material that escaped from cells and acquired a protein coat.
Regressive hypothesis
- Regressive hypothesis Viruses descend from small parasitic cells that lost genes and became dependent on hosts.
Virus-first hypothesis
- Virus-first hypothesis Viruses arose from self-replicating molecules before cells existed and later adapted to infect them.
No single origin
- No single origin Different virus groups probably arose by different routes, so no one hypothesis explains them all.
Part 2 of 5Why Viruses Evolve Fast
Why Viruses Evolve Fast
Short generations
- Short generations A virus can produce thousands of copies in hours, giving natural selection many chances to act.
High mutation rate
- High mutation rate RNA polymerases of RNA viruses lack proofreading, so many errors appear in each genome copy.
Large populations
- Large populations Huge numbers of virus particles in one host contain a wide pool of variants for selection.
Recombination
- Recombination Mixing of genetic material from two viruses in one host cell can create new combinations.
Part 3 of 5Evolution in Action
1
Influenza accumulates small mutations in its surface proteins, called antigenic drift, so last season immunity fails.
2
If two flu strains infect one cell, segments can be swapped, giving a sudden major change called antigenic shift.
3
HIV mutates within one patient so quickly that resistant variants arise against a single drug.
4
Natural selection favours variants that spread easily or escape the immune system.
5
This is why vaccines need regular updating and drugs are used in combinations.
| Process | Scale of change | Example |
|---|---|---|
| Antigenic drift | Small, gradual | Seasonal flu |
| Antigenic shift | Large, sudden | Flu pandemic strain |
Part 4 of 5Worked Example
Question
- A flu virus mutates once per 10 000 bases copied in a genome of 13 500 bases.
- Estimate the mean number of new mutations per genome copy and explain what this means for evolution.
- Mutations per copy = genome length × mutation rate per base.
- 13 500 × (1 ÷ 10 000) = 1.35.
- On average each new virus carries more than one mutation, so a host with millions of viruses produces many different variants.
Answer About 1.35 mutations per genome copy, giving large variation on which natural selection can act, so the virus evolves rapidly.
Part 5 of 5Exam Checklist & Tips
1.5.3 – Key Points for the Exam
- Mutation A random change in the base sequence of nucleic acid.
- Antigenic drift Gradual accumulation of mutations in antigens.
- Antigenic shift Sudden major change in antigens by gene reassortment.
- Proofreading Enzymatic correction of copying errors.
Common Mistakes
- Stating a single proven origin for all viruses; the origin remains uncertain.
- Thinking viruses mutate because they need to; mutations are random and selection acts afterwards.
- Confusing drift with shift.
- Assuming DNA viruses mutate as fast as RNA viruses.
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