Quick answer: A phylogenetic tree is a branching diagram that shows how species, genes or sequences are related through evolution. Branch points (nodes) represent common ancestors, and the length and pattern of branches encode the evolutionary distance between the organisms being compared.
One of the most powerful and visual ideas in biology is that all living things are connected by a family tree. Phylogenetic trees summarize those connections at a glance, and they appear in almost every molecular biology paper. This easy guide teaches you how to read, interpret and build one.
Parts of a phylogenetic tree
- Leaves (tips): the present-day sequences, species or individuals being compared, drawn at the ends of branches.
- Nodes: the fork points where a branch splits, representing a common ancestor.
- Branches: the lines connecting nodes, which can be drawn to scale to show how much change happened.
- Root: the oldest common ancestor at the base of a rooted tree.
Two organisms that share a recent node are more closely related than two whose shared node is further down the tree.
Rooted vs unrooted trees
A rooted tree has a single root that represents the common ancestor of everything in the tree, so it shows a clear direction of evolution. An unrooted tree does not specify where evolution began; it only shows the relationships among the sequences. You can root a tree using an outgroup, a sequence known to be distantly related to all the others.
What do branch lengths mean?
In many trees the branch lengths are drawn in proportion to the number of substitutions, so long branches mean a lot of evolutionary change. If the branches are the same length everywhere (a cladogram), only the branching pattern matters, not the distances. Always check the legend or scale before interpreting length.
Understanding clades and monophyletic groups
A clade is a group made of an ancestor and every one of its descendants. In a valid tree, each clade forms a single branch that you could cut off cleanly. Groups like “fish” are often not true clades because they exclude some descendants of the same ancestor.
Bootstrap values: how confident is the tree?
Trees are hypotheses, so they usually come with confidence numbers called bootstrap values. You will see them on the branches, often as percentages. A bootstrap value of 95 means that the same branch appeared in 95% of resampled datasets, so it is well supported. Values above 70 are generally considered reliable, while low values mean the branching there is uncertain.
How trees are built from data
The first step is always a multiple sequence alignment such as we describe in our multiple sequence alignment guide. From the alignment you can build a tree using different methods:
- Distance methods (Neighbor-Joining, UPGMA): fast, convert the alignment into a matrix of distances and cluster the closest pairs.
- Maximum Parsimony: finds the tree that explains the sequences with the fewest evolutionary changes.
- Maximum Likelihood & Bayesian methods: slower but statistically rigorous, widely used in modern papers (IQ-TREE, MrBayes, RAxML).
Beginner tools to build your first tree
- MEGA: a free desktop program with a friendly interface for alignment, tree building and visualization.
- Clustal Omega or MUSCLE: produce an alignment you can then feed to a tree program.
- iTOL or FigTree: excellent visualization tools to style and export published-quality trees.
Before building trees, make sure your sequences are in the right format; our FASTA and FASTQ guide shows you how. And if a specific sequence needs identifying first, try BLAST.
Frequently asked questions
What is the difference between a phylogeny and a phylogenetic tree?
They are the same idea. Phylogeny is the evolutionary history of a group, and a phylogenetic tree is the diagram that visualizes it.
How long are branches in a phylogenetic tree?
Branch length either shows the amount of evolutionary change (substitutions) or, in a cladogram, is equal everywhere and carries no time meaning.
Does an unrooted tree show evolution over time?
No. An unrooted tree shows relationships only; rooting with an outgroup is required to know where evolution started.
Which method gives the most accurate tree?
Maximum likelihood and Bayesian methods are the most accurate for large datasets, but distance methods are much faster for quick results.
What is a good bootstrap value?
Values above 70 are commonly treated as reliable, and 95 or higher is considered very strong support for a branch.