How Mutations Arise: Mutation, Recombination and Selection

Headlines about new variants can make it sound as though viruses decide to change. They do not. Viral evolution is the result of a few well-understood processes acting on enormous numbers of virus copies. Understanding those processes makes it easier to see why variants keep appearing and why most mutations never matter.

Copying Errors: Where Mutations Come From

When a virus infects a cell, it uses the cell's resources to make many copies of its genome. For RNA viruses, that copying is done by an enzyme called an RNA-dependent RNA polymerase. These enzymes are generally less accurate than the machinery that copies DNA in human cells, so occasional errors creep in: one letter swapped for another, a small piece deleted, or a few letters inserted.

Coronaviruses are somewhat unusual among RNA viruses because they carry a proofreading enzyme that corrects some copying errors. This helps them maintain their relatively large genomes. Errors still slip through, though, and when a virus replicates in millions of people, even a modest error rate produces a constant supply of new variations.

Host cells can also contribute. Some human enzymes that are part of the innate antiviral defense can edit viral RNA, and their characteristic signatures appear in the mutation patterns scientists observe.

Most Mutations Do Not Help the Virus

A mutation is a random change, not a purposeful improvement. Many mutations damage an important protein and leave the virus less able to replicate; those copies tend to disappear. Many others are neutral, changing a letter without altering the protein, or altering it in a way that makes little practical difference. Only a small minority provide some advantage in a given environment.

Recombination: Mixing Two Genomes

If a single cell is infected by two different viral lineages at the same time, the polymerase can switch templates partway through copying. The result is a recombinant genome that combines a section of one lineage with a section of another. Coronaviruses are known for frequent recombination.

Recombination matters because it can combine useful features from two lineages in a single step, rather than waiting for mutations to accumulate one by one. In the Pango naming system, recombinant lineages receive names beginning with the letter X, a convention described in our article on how variants are named and tracked.

Natural Selection: Filtering Variation

Selection determines which variations spread. A mutation that helps a virus enter cells more efficiently, replicate faster, or evade existing antibodies can give its carriers an edge, so they make up a growing share of infections over time.

The environment the virus faces changes as well. Early in the pandemic, most people had no prior immunity, so selection favored traits linked to transmission. As immunity from infection and vaccination built up, mutations that helped the virus slip past antibodies became more advantageous. This shifting pressure is a key reason variant waves have looked different over time.

Genetic Drift and Bottlenecks

Not every change that spreads is beneficial. Chance plays a role too. When only a few virus particles start a new infection, a transmission bottleneck occurs, and whichever variants happen to be in that small group are passed on, regardless of their fitness. Similar chance effects happen at larger scales when a single introduction seeds an outbreak in a new region. This random sampling is called genetic drift.

Long Infections as Evolutionary Workshops

Most infections are short, which limits how much evolution can happen within one person. In some people with weakened immune systems, however, infections can last for an extended period. The virus then replicates under ongoing, partial immune pressure, and can accumulate many changes. This is one leading hypothesis for how highly divergent variants, including the original Omicron lineage, emerged, as discussed in what Omicron taught us about coronavirus evolution.

Epistasis: Mutations in Context

The effect of a mutation can depend on which other mutations are present. A change that is harmful on its own may become tolerable, or even helpful, once a compensating mutation exists elsewhere in the same protein. This interaction, called epistasis, helps explain why some combinations of mutations appear together and why predicting the effect of a single mutation in isolation is difficult.

Key takeaways

• Mutations arise from copying errors and host editing, at random.
• Most mutations are neutral or harmful; few are beneficial.
• Recombination can combine features of two lineages at once.
• Selection favors traits that help the virus spread in its current environment, while chance and bottlenecks also shape which variants succeed.

Educational information only, not medical advice. For questions about testing, vaccination or treatment, follow the advice of a qualified healthcare professional and your local public health authority; in an emergency call your local emergency number.