How to Read a Viral Family Tree: Phylogenetics Basics
Phylogenetic trees are among the most common images in variant reporting, and also among the most misread. They look like family trees, and in a sense they are, but they follow conventions that are not obvious at first glance. This guide explains the basic parts of a viral phylogeny and how to avoid common interpretation mistakes.
What a Phylogenetic Tree Shows
A phylogenetic tree is a hypothesis about how a set of genomes are related through common ancestry. It is built by comparing sequences and asking which arrangement of branches best explains the shared and differing mutations among them. Genomes that share more recent mutations are placed closer together.
Because it is an inference, a tree is an estimate rather than a direct observation. Different methods and different sets of genomes can produce slightly different trees, especially for closely related samples.
The Parts of a Tree
- Tips (or leaves) are the sampled genomes, usually one per sequenced sample.
- Internal nodes are points where branches split. Each one represents an inferred common ancestor that was not directly sampled.
- Branches connect nodes and tips. Their length often represents the amount of genetic change, or in time-scaled trees, the amount of time elapsed.
- The root is the inferred common ancestor of everything in the tree.
- A clade is a group made up of an ancestor and all of its descendants. Many variant and lineage names correspond to clades.
Branch Lengths and the Molecular Clock
In a divergence tree, a longer branch means more mutations separate a genome from its ancestor. In a time-scaled tree, the horizontal axis is calendar time, and node positions estimate when ancestors existed. Time-scaling relies on the idea of a molecular clock: mutations accumulate at a roughly steady average rate, so the number of differences can be translated into elapsed time, with uncertainty.
An unusually long branch is worth noticing. It means a lineage accumulated many changes with few sampled intermediates. This can happen when a virus evolved in a poorly sampled region or population, or within a single long-lasting infection, a pattern discussed in our article on what Omicron variants taught us about coronavirus evolution.
Common Misreadings
Reading Order Along the Tips
The vertical order of tips usually carries no meaning. Branches can be rotated around any node without changing the relationships, much like a hanging mobile. Two tips sitting next to each other on the page are not necessarily close relatives; what matters is how recently their branches join.
Assuming One Tip Descends From Another
Tips are sampled genomes, not ancestors of each other. Two sampled variants share an ancestor at the node where their branches meet. Neither "came from" the other unless the tree specifically places a sampled genome at an ancestral position.
Treating the Tree as a Transmission Chain
Closely related genomes suggest that infections may be linked, but a tree alone rarely proves who infected whom. Unsampled infections can sit between any two sampled cases, and identical genomes can arise in people who never met.
Equating Tree Size With Case Numbers
The number of tips in a clade reflects how many samples were sequenced, which depends on where and how much sequencing occurred. A large clade may simply reflect intensive sequencing in a few places.
How Trees Support Surveillance
Trees help scientists spot when a group of genomes forms a distinct, growing clade, place a newly sequenced sample in context, and estimate when a lineage likely began circulating. Combined with sampling dates and locations, they can suggest how lineages moved between regions. This analysis feeds into lineage naming systems and into the epidemiological models used to project variant spread.
Key takeaways
- A tree is an inferred hypothesis of shared ancestry, not a direct record.
- Relationships are read from where branches join, not from tip order.
- Long branches signal many changes with few sampled intermediates.
- Trees reflect sampling effort and do not by themselves prove transmission chains or case counts.
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.