Why Do Viruses Evolve So Quickly?
- Viruses evolve extraordinarily fast, accumulating genetic changes even within a single infection.
- Their short generation times, high mutation rates, and strong selective pressures drive rapid adaptation, allowing them to evade immune systems, resist drugs, and cross species barriers.
1. Short Generation Times
- Viruses replicate within minutes to hours, compared to decades for multicellular organisms.
- Each replication cycle offers new opportunities for mutations to arise and spread.
Fast replication = more replication events = faster evolution.
2. High Mutation Rates
- Mutations introduce the variation necessary for evolution.
- RNA viruses mutate faster than DNA viruses because of differences in replication enzymes:
| Virus Type | Replication Enzyme | Proofreading? | Mutation Rate |
|---|---|---|---|
| DNA viruses | DNA polymerase | Yes | Low |
| RNA viruses | RNA polymerase / reverse transcriptase | No | High |
Without proofreading, RNA viruses accumulate frequent errors, generating genetically diverse populations within a single host.
HintHigh mutation rates are risky for stability but beneficial for adaptability.
3. Intense Natural Selection
- Viruses face constant selective pressures from their hosts:
- Immune defenses (e.g., antibodies targeting viral antigens)
- Antiviral drugs
- Transmission barriers between hosts
- Variants with mutations that help them evade immunity or resist drugs survive and proliferate.
Mutations that slightly alter a surface protein can prevent antibodies from recognizing the virus.
Exam technique- Always mention variation + selection together when explaining rapid viral evolution.
- Variation without selection would not produce adaptation.
Example 1: Influenza Virus
- Influenza is an RNA virus with eight separate genome segments.
- Its RNA polymerase lacks proofreading, causing frequent replication errors.
- These changes alter the virus’s surface proteins, which are key immune targets.
Antigenic Drift: Gradual Change
- Mutations accumulate slowly in the genes coding for Haemagglutinin (H) and Neuraminidase (N).
- These small structural changes slightly modify the viral surface.
