Phylogenetic Trees
In 2020, Trevor Bedford and colleagues at the Nextstrain project analysed more than 300,000 SARS-CoV-2 genome sequences from labs worldwide. By mapping mutations onto a phylogenetic tree updated in near real-time, they tracked the Delta variant from its first detection in India in October 2020 to constituting over 60% of global sequences just four months later — demonstrating that phylogenetic trees are not just historical tools but live instruments for tracking evolution as it happens.
Practise this lesson
Four printable worksheets that build from the foundations up to exam-style questions — start at whatever level suits you.
1. If two organisms look very similar, does that always mean they are the most closely related?
2. What does a branching point on an evolutionary tree actually represent: a living species, a split in a lineage, or just a visual divider?
Know
- The key features of a phylogenetic tree: root, nodes, branches, tips.
- The difference between morphological and molecular evidence.
- What common ancestry, clades and outgroups mean.
Understand
- Why phylogenetic trees show relatedness, not just similarity.
- How molecular evidence can resolve classification conflicts.
- Why parsimony matters when comparing possible trees.
Can Do
- Read nodes, branches, roots and tips correctly.
- Identify sister groups and clades on a cladogram.
- Explain why one phylogenetic arrangement is more likely than another using parsimony.
Core Content
Relatedness through common ancestry
In October 2020, Trevor Bedford's Nextstrain team noticed a new cluster appearing on their live SARS-CoV-2 phylogenetic tree — a branching node in India with an unusually long branch and distinctive spike protein mutations. That cluster was what we now call Delta. By February 2021 it had branched into 60% of all global sequences. A phylogenetic tree made this tracking possible because it does not simply rank organisms by how alike they look — it maps which lineages share more recent common ancestors, and where the tree branches tells you exactly when and where a new lineage diverged.
Key features of a phylogenetic tree: the root is the oldest common ancestral lineage shown; a node is a branching point where one lineage diverges into two (it represents an ancestral population, not a living species); a tip is a current taxon at the end of a branch; sister groups are the two lineages that share the most recent common ancestor.
A clade (or monophyletic group) contains a common ancestor and all of its descendants. An outgroup is a species placed outside the main group of interest — its traits help scientists determine which characteristics are derived (new) versus ancestral (old). A clade includes an ancestor and all its descendants. The outgroup helps root the tree and identify which traits evolved after the lineage split from the outgroup.
Root
Oldest common ancestral lineage shown.
Node
Point where one lineage diverges into two — an ancestor, not a modern species.
Tip
Current taxon at the end of a branch.
Nodes represent ancestral divergence points, not living species. Tips are the current taxa shown. Taxon C is the outgroup — it diverged earliest from the common ancestor.
Pause — copy the highlighted definitions of root, node, clade and outgroup into your book.
A phylogenetic tree shows:
Why DNA often resolves the hardest cases
We just saw how to read a phylogenetic tree. That raises a question: what evidence is used to build them, and what happens when morphological and molecular evidence produce different trees? This card answers it — molecular evidence often resolves cases that morphology cannot.
Morphological evidence compares structures and body plans; molecular evidence compares DNA, proteins or other sequence data. Both are useful, but molecular evidence can reveal relatedness that morphology misses.
Morphological evidence includes homologous structures and shared body patterns. Molecular evidence includes DNA sequence alignment, protein comparison and mitochondrial DNA divergence. When morphological evidence is ambiguous, molecular evidence can show whether similarities came from common ancestry or convergent evolution.
| Evidence Type | What It Compares | Strength |
|---|---|---|
| Morphological | Body structure, anatomy, homologous features | Useful when DNA is unavailable and for visible structural patterns |
| Molecular | DNA sequences, protein sequences, mtDNA | Can resolve hidden relatedness and distinguish convergence from ancestry |
Add the morphological vs molecular comparison to your notes before the check below.
What is the main advantage of molecular evidence in phylogenetic studies?
Choosing the simplest likely tree and interpreting what it means
We just saw that morphological and molecular evidence can produce competing trees. That raises a question: when there are multiple possible arrangements, how do scientists decide which tree to prefer? This card answers it — the principle of parsimony.
The principle of parsimony says that the most likely phylogenetic tree is usually the one requiring the fewest evolutionary changes.
Parsimony: when comparing competing tree arrangements built from the same evidence, prefer the arrangement that explains observed similarities with the fewest separate evolutionary changes. This does not mean evolution is always simple — it means unnecessary complexity should not be assumed without evidence.
Phylogenetic trees also depend on the idea of common ancestry. Every lineage on the tree traces back through older shared ancestors, and at the deepest level all life shares a last universal common ancestor (LUCA). Clades on a tree represent groups that share a common ancestor and all its descendants — interpreting which clade something belongs to tells you about its evolutionary history, not just its appearance.
Pause — copy the parsimony definition and the LUCA/clade points into your book.
The principle of parsimony favours the phylogenetic tree that requires the fewest evolutionary changes.
A node on a phylogenetic tree represents a living species that exists between the two diverging lineages.
Sister groups are the two lineages that share the most recent common ancestor on the tree.
Activities
Label the Tree
Using the annotated tree in this lesson, identify the root, one node, one branch, one tip, the outgroup, and a pair of sister groups. Then explain what each of those labels means in terms of evolution.
Choose the Better Evidence
Two organisms have very similar streamlined bodies, but their DNA sequences differ strongly. Explain why molecular evidence may be more useful than morphology in this case, and identify what evolutionary process could have produced the misleading similarity.
Which pair are sister groups on a phylogenetic tree?
What Trees Show
- Phylogenetic trees show evolutionary relationships through common ancestry.
- Nodes represent divergence points from ancestral populations — not living species.
- Tips are the current taxa shown.
Key Features
- Root = oldest common lineage.
- Sister groups share the most recent common ancestor.
- Clade = ancestor + all descendants. Outgroup = comparison species outside the ingroup.
Evidence Types
- Morphological evidence compares structures and body plans.
- Molecular evidence compares DNA or protein sequences.
- Molecular evidence often resolves misleading visual similarity (convergent evolution).
Parsimony
- The most likely tree usually requires the fewest evolutionary changes.
- Common ancestry underpins all phylogenetic reasoning.
- All life traces back to LUCA — last universal common ancestor.
A fresh set drawn from this lesson's question bank — feedback shown immediately. +5 XP per correct · +25 XP all correct
Pick your answer, then rate your confidence — that tells the system what to drill next.
UnderstandBand 3(3 marks) 1. Define the following terms: root, node and sister groups.
1 mark each: root (oldest common ancestral lineage shown) · node (divergence point from a common ancestor — not a living species) · sister groups (two lineages sharing the most recent common ancestor)
AnalyseBand 3–4(3 marks) 2. Distinguish between morphological and molecular evidence used in constructing phylogenetic trees.
1 mark: morphological defined · 1 mark: molecular defined · 1 mark: key difference — molecular can resolve convergent evolution that misleads morphology
EvaluateBand 4–5(4 marks) 3. Assess whether visual similarity alone is enough to determine close evolutionary relationships. In your answer, refer to one case where molecular evidence changed the interpretation.
1 mark: visual similarity alone is insufficient · 1 mark: convergent evolution explained · 1 mark: example using molecular evidence (whales/fish, platypus, dolphins/sharks) · 1 mark: judgement — molecular evidence can force reclassification
Show all answers
Multiple choice
MC answers and full explanations are shown inline as you complete each question. Use the retry button to attempt a fresh set from the lesson bank.
Activity 1 — Label the Tree
Root: The oldest common ancestral lineage shown — the leftmost horizontal line before the first branching point.
Node: A branching point where one lineage diverges into two — a filled circle on the tree. It represents an ancestral population, not a living species.
Branch: The line connecting a node to a tip or another node — represents a lineage through time.
Tip: The endpoint of a branch — represents a current taxon (Taxon A, B or C on the diagram).
Outgroup: Taxon C — it diverged earliest from the common ancestor and is placed outside the main ingroup (A and B). It helps identify which traits in A and B are derived.
Sister groups: Taxon A and Taxon B — they share the most recent common ancestor (the rightmost node connecting their branches).
Activity 2 — Choose the Better Evidence
Molecular evidence is more useful here because DNA sequences reveal genetic relatedness independent of physical form. When two organisms have strongly different DNA but similar body shapes, the similarity in appearance is likely the result of convergent evolution — where separate lineages independently evolved similar forms under similar selection pressures (e.g. streamlined bodies in aquatic environments). Molecular evidence can distinguish convergent similarity from true shared ancestry, which morphology alone cannot reliably do in this case.
Short Answer Model Responses
SA1 (3 marks): The root is the oldest common ancestral lineage shown on the tree [1]. A node is a branching point representing divergence from a common ancestor — not a living species [1]. Sister groups are two lineages sharing the most recent common ancestor [1].
SA2 (3 marks): Morphological evidence uses physical structure and anatomical features such as homologous body parts or body plans [1]. Molecular evidence uses DNA sequence comparison, protein comparison or mitochondrial DNA divergence [1]. Molecular evidence is often especially useful when morphology may be misleading because of convergent evolution — it can reveal true ancestry that surface similarity conceals [1].
SA3 (4 marks): Visual similarity alone is not enough to determine close evolutionary relationships because unrelated organisms can evolve similar features under similar selection pressures — this is called convergent evolution [1]. Appearance may therefore reflect environment rather than common ancestry [1]. For example, whales may look superficially similar to fish in body shape, but molecular evidence shows they share closer ancestry with mammals such as hippopotamuses [1]. This demonstrates that common ancestry must be inferred from stronger evidence than appearance alone, and that molecular evidence can force a reclassification when morphology is misleading [1].
Trees show ancestry
Phylogenetic trees map relatedness through common ancestors — not appearance. Nodes are ancestors, not modern species.
Clades, outgroups, sister groups
A clade includes an ancestor and all descendants. Outgroup identifies derived traits. Sister groups share the most recent common ancestor.
Molecular resolves conflict
When morphology misleads (convergent evolution), DNA and protein data reveal true relationships.
Most common exam trap
Calling the organisms closest on the page "sister groups" — it is about most recent common ancestor, not visual proximity.
Rapid-fire questions on phylogenetic trees, parsimony, sister groups, clades, outgroups and evidence types. Beat the boss to bank a tier — gold (perfect + fast), silver (80%+), or bronze (cleared).
⚔ Enter the arenaYou were asked how phylogenetic trees answer the dolphin-shark relatedness question differently from comparing appearances — and whether surface similarity equals close ancestry.
Trevor Bedford's Nextstrain team demonstrated the answer in 2020: when they analysed over 300,000 SARS-CoV-2 genomes and mapped mutations onto a phylogenetic tree, Delta emerged as a distinct clade — a cluster sharing a common branching node — not because it looked like other variants, but because its sequences were inherited from a single common ancestral lineage first detected in India in October 2020. That same logic separates dolphins from sharks: dolphins share a common mammalian ancestor with horses and humans (visible at a node on the tree), while sharks share a different, far more ancient lineage. Similarity of shape is convergence; proximity of nodes is relatedness. Trees show ancestry; appearances can deceive.