The big idea: A virus is about as simple as biology gets: a short stretch of nucleic acid (DNA or RNA) wrapped in a protein capsid. That very simplicity makes two questions hard:
1. Where did viruses originally come from? We are not certain — and the evidence suggests viruses did not all arise once. Their origin is most likely polyphyletic: viruses probably arose several separate times, in different ways.
2. Why do viruses change so fast? Because they have enormous populations, very short generation times and high mutation rates, so natural selection acts on them at extraordinary speed — fast enough to dodge our immune systems and our drugs within a single flu season.
A virus is just a nucleic-acid genome wrapped in a protein capsid (some also have a lipid envelope studded with glycoprotein spikes). The origin hypotheses ask where that simple genome-plus-capsid package first came from.
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- Polyphyletic
- Having more than one independent evolutionary origin — not all descended from a single common ancestor. Viruses are thought to be polyphyletic.
- Capsid
- The protein coat that surrounds and protects a virus's genetic material.
- Mutation rate
- How often errors are introduced when a genome is copied. Viruses — especially RNA viruses — have very high mutation rates.
- Natural selection
- The process by which variants that survive and reproduce best become more common over generations. It acts on virus populations exactly as it does on any other population.
'Uncertain' is the correct answer: For the origin of viruses, the examiner wants you to say it is uncertain / not fully known, and that viruses are probably polyphyletic (multiple independent origins).
Confidently claiming one definite origin would actually be wrong — the honesty is part of the mark.
There are three leading hypotheses for where viruses came from. They are not rivals to choose between — because viruses are polyphyletic, different viruses may have arisen in different ways, so more than one hypothesis can be true at once.
| Hypothesis | Core idea | A way to picture it |
|---|---|---|
| Escape (progressive) | Viruses began as host genes or mobile genetic elements (e.g. transposons, plasmids) that broke free of the cell and gained a protein capsid to travel between cells. | A piece of a cell's own DNA 'escapes', wraps itself in a protein coat, and starts moving from cell to cell. |
| Reduction (regressive) | Viruses descend from once free-living cellular parasites that, living inside hosts, lost gene after gene until only a genome and a coat remained. | A small parasitic cell strips itself down over time until it can no longer live on its own — only replicate inside a host. |
| Virus-first (co-evolution) | Self-replicating molecules existed before the first cells; some of these early replicators evolved alongside cells and became viruses. | Simple replicating genetic 'pre-life' came first, and cells and viruses both descended from that early molecular world. |
Why the three together point to 'polyphyletic': Each hypothesis explains some viruses well but not all of them.
The escape route fits viruses whose genes closely resemble their hosts'. The reduction route fits unusually large, complex viruses. The virus-first route fits the idea that replication came before cells.
No single story covers every virus — which is exactly why biologists conclude viruses had several independent origins (polyphyletic), rather than one.
Now the second question: why do viruses evolve so fast? Read it as a chain of cause and effect, and notice it is just natural selection running at high speed on a virus population.
The rapid-evolution chain (cause → effect)
- A virus makes billions of copies inside a single host — a huge population.
- Each genome is copied with many errors; RNA polymerases lack proofreading, so RNA viruses mutate especially fast — this creates lots of variation.
- A new generation is produced in hours, so selection acts over many generations very quickly.
- Any variant that survives the immune system or a drug leaves more offspring — that is natural selection.
- Within weeks the population is dominated by the better-adapted variant — the virus has evolved.
| Factor | What it means | Why it speeds up evolution |
|---|---|---|
| Huge population size | A single infected person can carry billions of virus particles | More copies = more chances for a useful mutation to appear somewhere |
| Very short generation time | A new generation can be produced in hours, not years | Natural selection runs through many generations very quickly |
| High mutation rate | Genomes are copied with many errors — RNA virus polymerases lack proofreading | Lots of variation is created for selection to act on |
Influenza: antigenic drift and shift: Influenza shows two named patterns of this change, both affecting its surface proteins (the proteins your immune system recognises):
Antigenic drift — small, gradual mutations build up, so the surface proteins slowly change. This is why the flu vaccine is updated most years.
Antigenic shift — a large, sudden change when two strains swap whole genome segments in one host. The surface proteins can look almost new, existing immunity barely works, and a pandemic can result.
| Antigenic DRIFT | Antigenic SHIFT | |
|---|---|---|
| What changes | Small, gradual changes in the surface proteins | A large, sudden change in the surface proteins |
| How it happens | Point mutations build up over time as the virus copies its genome | Two different strains infect one host and swap whole genome segments (reassortment) |
| Size of change | Minor — the virus stays recognisably similar | Major — the surface proteins can look almost brand-new |
| Effect on immunity | Existing immunity is partly lost, so flu jabs are updated most years | Existing immunity may be almost useless — can trigger a pandemic |
| Influenza example | Why the seasonal flu strain drifts year to year | Why occasional new pandemic flu strains appear |
The surface proteins matter for evolution: the immune system recognises the capsid and the glycoprotein spikes, so mutations that change those surface proteins let new variants slip past existing immunity.
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SARS-CoV-2 variants: the same process, live: During the COVID-19 pandemic, SARS-CoV-2 produced a stream of variants (Alpha, Delta, Omicron and more).
Each was a population in which mutations had changed the spike protein. Variants that spread faster or escaped immunity from earlier infection or vaccination were selected for and took over — a textbook case of natural selection acting on a virus in real time, and the reason vaccines were reformulated to match new variants.
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How this is tested: Two angles come up.
Origin: state that it is uncertain and probably polyphyletic, then give the hypotheses (escape from host genes; reduction of a parasitic cell; virus-first co-evolution). Don't commit to one origin as definite.
Rapid evolution: explain it as natural selection powered by large populations + short generation times + high mutation rates (RNA polymerases lack proofreading), then give a consequence — antigenic drift/shift in flu, new SARS-CoV-2 variants, immune escape, antiviral resistance, and why vaccines must be updated.
IB-style question — why viruses evolve rapidly and what it means for vaccines
Explain why viruses such as influenza evolve rapidly, and why this means vaccines often have to be updated. [5]
How to score all five marks
- Set up natural selection. Viruses reproduce in huge numbers with a very short generation time, so natural selection acts on the population very quickly.
- Source of variation. They have a high mutation rate — RNA virus polymerases lack proofreading — so each generation throws up lots of new variants.
- Selection of escape variants. Variants whose surface proteins are changed are not recognised by existing immunity; they survive and reproduce more, so they become more common (antigenic drift; antigenic shift if whole segments are swapped).
- Consequence for the population. Over time the virus population is dominated by variants that the host's antibodies no longer bind well — the virus has evolved away from the immunity people had.
- Why the vaccine is updated. The old vaccine raises antibodies against the old surface proteins, which no longer match; so the vaccine must be reformulated to match the current circulating strain. (Award 1 mark per distinct point, up to 5.)
Final answer
Viruses have huge populations, very short generation times and high mutation rates (RNA polymerases lack proofreading), so natural selection acts fast and generates many variants. Variants with altered surface proteins escape existing immunity (antigenic drift/shift), are selected for, and come to dominate the population. Because the circulating strain's surface proteins no longer match the old vaccine, the vaccine must be updated to match the current strain.
✓ Why this scores full marks: It frames the change as natural selection (not vague 'the virus adapts'), names the three drivers (population size, generation time, mutation rate / no proofreading), explains immune escape via changed surface proteins, and closes the loop on why the vaccine no longer matches and must be updated.
A common way to lose marks is to say a virus 'tries' or 'wants' to change — selection has no intent; it simply favours variants that already happen to survive better.