55. Genetic Drift in Viruses

1. Introduction

Genetic drift is an evolutionary process in which the frequencies of genetic variants change due to random events rather than differences in biological fitness.

In viral populations, genetic drift can occur when only a small, randomly sampled subset of viral particles successfully establishes infection in a new host, when viral populations contract sharply, or when stochastic events influence which variants are transmitted and amplified.

Genetic drift is particularly relevant to viruses because viral populations may undergo repeated transmission bottlenecks, rapid expansions, and population fluctuations. These events can substantially change the genetic composition of a viral population, even when the variants involved have similar biological fitness.

Genetic drift differs from mutation and natural selection. Mutation generates new genetic variation, natural selection changes variant frequencies according to differences in fitness, and genetic drift changes variant frequencies through chance.

Key concept: Genetic drift changes the frequency of existing genetic variants through random sampling and stochastic events. It does not necessarily produce a new mutation or confer a biological advantage on the variants that become common.

2. Definition

Genetic drift is the random change in the frequency of genetic variants within a population across generations or successive rounds of replication and transmission.

In virology, the term is used to describe stochastic changes in viral genetic composition caused by chance events, particularly during transmission, population bottlenecks, and replication in finite populations.

The magnitude of genetic drift depends on factors such as:

• The effective population size.

• The strength of transmission bottlenecks.

• The extent of population contraction.

• The number of replication and transmission events.

• The distribution of variants in the source population.

• The contribution of chance relative to selection.

Genetic drift can cause a variant to become more common, remain rare, or disappear entirely, irrespective of whether it improves viral fitness.

3. Genetic drift versus mutation and natural selection

These three processes are central to viral evolution but operate differently.

A mutation may initially occur at very low frequency. Genetic drift can then eliminate it or allow it to persist by chance. If it affects fitness, natural selection may subsequently increase or decrease its frequency.

4. Why is genetic drift important in viral populations?

Viral populations often contain numerous genetically related variants. However, the number of variants present in a sample does not necessarily reflect the number that successfully contribute to the next stage of infection.

Several processes make genetic drift important:

1. Transmission bottlenecks: Only a subset of viral particles establishes infection in a new host.

2. Population bottlenecks within hosts: Immune responses, antiviral treatment, or changes in tissue availability may reduce the effective viral population.

3. Founder effects: A new infection may begin with a small subset of variants from the source population.

4. Stochastic replication: Differences in which viral genomes successfully replicate can alter variant frequencies.

5. Spatial compartmentalization: Viral populations in different tissues may evolve partly independently because only some variants move between compartments.

The consequences depend on the virus, the host, the transmission route, and the particular stage of infection.

5. Transmission bottlenecks

A transmission bottleneck occurs when only a small subset of the viral population in one host successfully establishes infection in another host.

For example, a source host may contain many genetically distinct viral variants. However, only a few may reach the relevant site in a recipient and initiate productive infection.

The recipient's initial viral population may therefore differ substantially from that of the source host.

Mechanism

1. A source host contains a genetically diverse viral population.

2. Only some viral particles are released or reach the transmission interface.

3. A subset survives environmental, anatomical, or immune barriers.

4. Only some particles establish productive infection in the recipient.

5. The recipient's viral population expands from this restricted sample.

The variants that establish infection are not necessarily the most biologically fit variants in every environment. Their success may partly reflect chance.

Clinical significance

Transmission bottlenecks can:

• Reduce genetic diversity in newly infected hosts.

• Cause variants to be lost during transmission.

• Produce genetic differences between linked infections.

• Influence the early composition of a recipient's viral population.

• Affect the interpretation of viral sequencing data.

A bottleneck can also interact with natural selection. A variant may be favoured by selection but fail to transmit because it was absent from the small subset of viral particles that established infection.

6. Founder effect in viral evolution

The founder effect occurs when a new population is established by a small subset of individuals or genetic variants from a larger source population.

In virology, a founding viral population may contain only a fraction of the variants present in the source host or community.

Once the founding population expands, its genetic composition can differ from that of the original population.

Example

Imagine a source population containing three viral variants:

• Variant A: 50%

• Variant B: 30%

• Variant C: 20%

If a new infection is established by a very small number of viral particles, the recipient might initially contain only variants A and C, or perhaps only variant B.

After replication, the recipient's population may become dominated by a variant that was relatively uncommon in the source population.

This does not automatically mean that the dominant variant has a fitness advantage. The observed difference may reflect the initial sampling process.

Important distinction: The founder effect is a particular consequence of establishing a population from a small sample. Genetic drift is the broader evolutionary process through which random sampling changes variant frequencies.

7. Genetic drift during within-host infection

Genetic drift can occur after a virus enters a host.

Although some infections contain large numbers of viral genomes, the effective population size may be much smaller than the total number of viral particles or RNA copies measured in a sample.

Effective population size describes the number of independently contributing genomes or lineages that meaningfully participate in the process under study.

Factors that may influence within-host drift

• Immune-mediated elimination of infected cells.

• Localized replication in specific tissues.

• Movement of viruses between anatomical compartments.

• Fluctuations in viral load.

• Antiviral treatment.

• Stochastic loss of infected cell lineages.

• Limited migration between viral populations.

For example, viral populations in different anatomical compartments may become genetically differentiated if only a small number of viral lineages move between them.

However, a change in variant frequency during infection cannot be attributed to genetic drift alone without considering selection, treatment effects, mutation, and sampling limitations.

8. Genetic drift and the effective population size

Genetic drift generally becomes stronger as the effective population size decreases.

When only a small number of genomes contribute to the next generation or transmission event, random sampling can cause substantial fluctuations in variant frequencies.

When many independent genomes contribute, random changes in frequency are generally smaller relative to the total population.

This relationship is particularly important in viral transmission because the total number of viral particles in a source sample may be large, while the number that successfully establish infection may be small.

Conceptual example

Suppose two equally fit viral variants are present in a population.

• In a very large effective population, random fluctuations may produce only small changes in their relative frequencies.

• In a small effective population, one variant may become dominant or disappear simply because of chance.

Genetic drift is therefore not determined solely by viral load. A high viral load does not guarantee a large effective population size.

9. Genetic drift and viral diversity

Genetic drift can increase or decrease measured diversity depending on the population and time scale being examined.

9.1 Loss of diversity

A strong bottleneck may eliminate rare variants.

If only a small subset of the source population establishes infection, many variants may be absent from the recipient's initial population.

9.2 Fixation

A variant is said to be fixed when it reaches a frequency of 100% at the genetic position or variant definition being considered.

Genetic drift can lead to fixation, particularly in small effective populations, even when the variant is not advantageous.

9.3 Divergence between populations

Two viral populations that begin with similar genetic compositions may diverge if they experience independent bottlenecks and stochastic changes.

This can produce genetic differences between transmission chains or geographically separated populations.

9.4 Loss of rare lineages

Rare variants may be lost during transmission or local population contraction. Their disappearance does not necessarily indicate that they were biologically inferior.

10. Genetic drift in important human viruses

10.1 HIV

HIV populations can contain substantial genetic diversity within an infected individual.

Transmission often involves a bottleneck in which a limited number of viral lineages establish infection. The founding population may therefore be genetically less diverse than the source population.

Subsequent replication, mutation, recombination, immune selection, and treatment can reshape the recipient's viral population.

The genetic composition of the transmitted population should not be interpreted as a direct measure of which variant is universally most fit.

10.2 Influenza viruses

Influenza virus populations can undergo transmission bottlenecks during spread between hosts.

Genetic drift can contribute to stochastic changes in variant frequencies, particularly when only a subset of viral particles successfully establishes infection.

However, influenza antigenic drift has a specific conventional meaning: the gradual accumulation of mutations in antigenic proteins, especially haemagglutinin and neuraminidase, leading to antigenic change.

It is not synonymous with population-genetic drift.

This distinction is essential for clinical microbiology and examination purposes.

10.3 SARS-CoV-2

SARS-CoV-2 transmission can involve bottlenecks that influence which variants establish infection.

Genetic drift may contribute to changes in variant frequencies, especially in small transmission chains or after population contraction.

Nevertheless, the emergence and expansion of major SARS-CoV-2 lineages can also reflect natural selection, epidemiological advantages, founder effects, and changes in contact patterns.

A lineage becoming common does not, by itself, prove that it has an intrinsic biological advantage.

10.4 Hepatitis C virus

Hepatitis C virus exhibits genetic diversity within and between infected individuals.

Population bottlenecks, compartmentalization, mutation, and selection can all influence the distribution of variants.

The relative contribution of drift depends on the stage of infection, the anatomical compartment, and the viral population being studied.

11. Genetic drift versus antigenic drift

The word drift is used in two related but distinct contexts.

Population-genetic drift refers to random changes in genetic variant frequencies.

Antigenic drift refers to antigenic change caused primarily by the accumulation of mutations in viral antigen genes over time.

The distinction is especially important in influenza virology.

Antigenic drift may involve natural selection, while population-genetic drift is specifically associated with chance.

12. Genetic drift and natural selection can act together

Genetic drift and natural selection are not mutually exclusive.

A viral population may experience random changes in variant frequency while selection simultaneously favours variants with particular biological properties.

For example:

1. A viral variant arises through mutation.

2. It enters a small transmission bottleneck.

3. It becomes common in the recipient partly by chance.

4. If it also has a fitness advantage in the new host, selection may further increase its frequency.

Conversely, a variant with a potential fitness advantage may be lost during a severe bottleneck if it fails to establish infection.

This interaction is important when interpreting the emergence of new viral lineages.

13. Detecting genetic drift in viral populations

Genetic drift is inferred from patterns of genetic variation and population history rather than detected by a single laboratory test.

13.1 Viral sequencing

Sequencing can identify genetic variants in source hosts, recipients, and populations sampled over time.

Comparing sequences can reveal changes in variant frequencies, loss of lineages, and divergence between populations.

13.2 Deep sequencing

Deep sequencing can detect low-frequency variants, depending on assay design, depth, error rates, and analytical thresholds.

It is useful for investigating within-host diversity and transmission bottlenecks.

13.3 Phylogenetic analysis

Phylogenetic trees help reconstruct relationships among viral sequences.

They can identify clusters of related viruses and support investigation of transmission patterns, although phylogenetic proximity alone does not prove direct transmission.

13.4 Epidemiological sampling

Sampling multiple individuals across a transmission chain or outbreak helps distinguish stochastic population changes from broader epidemiological trends.

13.5 Limitations

Observed changes in variant frequency may result from several processes:

• Genetic drift.

• Natural selection.

• New mutations.

• Recombination or reassortment.

• Migration between populations.

• Sampling bias.

• Technical sequencing error.

Reliable interpretation requires combining molecular data with epidemiological and biological evidence.

14. Common misconceptions

Misconception 1: Genetic drift produces new mutations.

Incorrect. Mutation generates new nucleotide changes. Genetic drift changes the frequency of variants through random events.

Misconception 2: The most common viral variant must be the fittest.

Incorrect. A variant may become common through chance, founder effects, or epidemiological circumstances.

Misconception 3: Genetic drift occurs only in small viral populations.

Incorrect. It can occur in any finite population, but its relative effects are generally stronger when the effective population size is small.

Misconception 4: Genetic drift and antigenic drift are identical.

Incorrect. Population-genetic drift is a stochastic evolutionary process. Influenza antigenic drift refers to gradual antigenic change associated with mutations in viral antigen genes.

Misconception 5: Every transmission bottleneck selects the most transmissible variant.

Incorrect. A bottleneck may transmit only a small random subset of the source population. Selection may influence transmission, but random sampling also plays a role.

Misconception 6: Genetic drift always reduces diversity.

Incorrect. Drift can eliminate variants, cause fixation, or increase differences between populations. Its effect depends on the population structure and the time scale considered.

15. High-yield examination points

• Genetic drift is a random change in genetic variant frequencies.

• It does not require a new mutation.

• It is stronger, in relative terms, when effective population size is small.

• Transmission bottlenecks are important drivers of genetic drift in viruses.

• Founder effects occur when a new population is established by a small subset of a source population.

• Genetic drift can cause fixation or loss of variants irrespective of fitness.

• A variant becoming common does not automatically indicate increased virulence or transmissibility.

• HIV, influenza, and SARS-CoV-2 populations can experience transmission bottlenecks.

• Population-genetic drift is distinct from influenza antigenic drift.

• Genetic drift and natural selection can act simultaneously.

• Sequencing and phylogenetic analysis can support investigation of drift, but frequency changes alone do not establish its cause.

16. Take-home message

Genetic drift is a fundamental evolutionary process that changes viral population composition through chance.

In virology, its effects are particularly visible during transmission bottlenecks, population contractions, and the establishment of new infections from small founding populations.

The central distinction is that mutation creates genetic changes, drift changes their frequencies randomly, and selection changes their frequencies according to differences in fitness.

Understanding genetic drift is essential for interpreting viral sequencing data, reconstructing transmission chains, investigating within-host diversity, and avoiding the mistaken assumption that every expanding viral lineage possesses a biological advantage.

17. Standard references

1. Knipe DM, Howley PM, editors. Fields Virology. Wolters Kluwer. Consult sections on viral evolution, population diversity, and transmission.

2. Flint SJ, Racaniello VR, Rall GF, Skalka AM, Enquist LW. Principles of Virology. ASM Press. Consult sections on viral genetics and evolution.

3. Sanjuán R, Domingo-Calap P. Mechanisms of viral mutation. Cellular and Molecular Life Sciences. 2016;73:4433–4448.

4. Domingo E, Perales C. Viral quasispecies. PLoS Genetics. 2019;15(10).

5. McCrone JT, Lauring AS. Genetic bottlenecks in intraspecies virus transmission. Current Opinion in Virology. 2018;28:20–25.

6. Murray PR, Rosenthal KS, Pfaller MA. Medical Microbiology. Elsevier. Consult the chapters on viral genetics and pathogenesis.