Viruses can remain evolutionarily frozen until they enter a new community

A new study by researchers from FSU Jena, Utrecht University and the Max Planck Institute for Evolutionary Biology in Plön shows that viruses do not always evolve rapidly, even during massive outbreaks. Instead, their evolution can depend strongly on the microbial community they encounter.

June 01, 2026

Schematic representation of evolutionary changes in compost communities over one year, including viral outbreaks in virus-containing communities and rapid viral evolution following the introduction of the virus into virus-free communities. — Microbial communities grown from compost samples showed that despite their reputation for rapid change, viruses can remain genetically frozen even during massive outbreaks (brown communties). Introducing the virus into new microbial communities (in green) triggered rapid evolutionary change— including mutations in host-recognition genes. The findings highlight how the microbial environment a virus encounters, rather than its abundance, determines whether it evolves or remains unchanged.

© Jeroen Meijer

Microbial communities grown from compost samples showed that despite their reputation for rapid change, viruses can remain genetically frozen even during massive outbreaks (brown communties). Introducing the virus into new microbial communities (in green) triggered rapid evolutionary change— including mutations in host-recognition genes. The findings highlight how the microbial environment a virus encounters, rather than its abundance, determines whether it evolves or remains unchanged.

© Jeroen Meijer

To the Point

Evolutionary stasis: Viruses can remain genetically stable for many months, even during very large outbreaks.
Community context: Viral evolution depends not only on abundance, but also on the microbial community in which a virus occurs.
Rapid change after migration: When a virus enters a new community, it can begin to evolve quickly through recombination and mutations in host-recognition genes.

The study was carried out by researchers from Friedrich Schiller University Jena, Utrecht University and the Max Planck Institute for Evolutionary Biology in Plön. The team followed viruses that infect bacteria, known as bacteriophages, in experimentally grown compost microbial communities over the course of one year. These communities were highly complex, containing hundreds of bacterial genera, and were grown under controlled laboratory conditions on cellulose.

During the experiment, the researchers discovered a previously undescribed bacteriophage, which they named Theomophage. In some microbial communities, this virus caused enormous outbreaks. At its peak, Theomophage accounted for up to 74 per cent of all genetic material recovered from a community. This makes it the largest bacteriophage outbreak documented to date.

Yet despite this extraordinary level of replication, Theomophage showed striking genetic stability. In communities where it was already established, the virus remained essentially unchanged for many months. This was unexpected, because large viral populations are often assumed to provide many opportunities for new mutations and rapid evolutionary change.

The picture changed when viruses were allowed to migrate between different microbial communities. Once Theomophage entered another type of community, it began to evolve rapidly. This evolution involved genetic recombination and mutations concentrated in genes that are likely involved in recognising and infecting bacterial hosts.

“This study shows that viral evolution is not simply a matter of how many virus particles are produced,” says Paul B. Rainey, Professor at the Max Planck Institute for Evolutionary Biology in Plön. “The ecological context matters. A virus can remain almost frozen in one community, but begin to change rapidly when it encounters a new set of potential hosts.”

The findings suggest that viral evolution in natural microbial communities may depend less on viral abundance alone than on the surrounding community. Migration into new ecological environments can expose viruses to different hosts and new selective pressures, triggering rapid evolutionary change after long periods of stasis.

The study helps explain why viruses in nature can sometimes appear surprisingly stable, despite their reputation for rapid evolution. It also highlights the importance of community context in shaping how viruses adapt, diversify and persist in complex microbial ecosystems.