Phylogenetic Trees
When scientists tracked COVID-19 variants, they used phylogenetic trees to show how viral lineages diverged from shared ancestors. The same logic helps biologists map relationships across all life: not by surface similarity alone, but by common ancestry.
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Phylogenetic Tree
Think First
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?
Commit to your starting answer before reading the tree diagrams.
Write your first answer in your book, then compare it with your end-of-lesson reasoning.
📚 Know
- Key facts and definitions for Phylogenetic Trees
- Relevant terminology and conventions
🔗 Understand
- The concepts and principles underlying Phylogenetic Trees
- How to explain the reasoning behind key ideas
✅ Can Do
- Apply concepts from Phylogenetic Trees to exam-style questions
- Justify answers using appropriate biological reasoning
Know
- The key features of a phylogenetic tree.
- The difference between morphological and molecular evidence.
- What common ancestry means in an evolutionary tree.
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 on a cladogram.
- Explain why one phylogenetic arrangement is more likely than another.
Misconceptions to Fix
Wrong: Homeostasis means the body stays exactly the same all the time.
Right: Homeostasis involves dynamic equilibrium — constant small adjustments around a set point.
Core Content
What a Phylogenetic Tree Shows
Relatedness through common ancestry
A phylogenetic tree shows which lineages share more recent common ancestors. It does not simply rank organisms by how alike they look.
This matters because organisms can appear similar for different reasons. Some similarities reflect shared ancestry, while others come from convergent evolution. A cladogram is strongest when it represents descent from branching ancestral populations, not superficial resemblance.
Morphological vs Molecular Evidence
Why DNA often resolves the hardest cases
Morphological evidence compares structures and body plans, while 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 |
Parsimony and Common Ancestry
Choosing the simplest likely tree
The principle of parsimony says that the most likely phylogenetic tree is usually the one requiring the fewest evolutionary changes.
This does not mean evolution is always simple. It means that when biologists compare alternative trees, they prefer the arrangement that explains observed similarities with the least unnecessary complexity. Parsimony is especially useful when deciding between competing tree structures built from the same 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, often abbreviated as LUCA.
What Trees Show
- Phylogenetic trees show evolutionary relationships through common ancestry.
- Nodes represent divergence points from ancestral populations.
- Tips are the current taxa shown.
Key Features
- Root = oldest common lineage.
- Branches = lineages through time.
- Sister groups share the most recent common ancestor.
Evidence Types
- Morphological evidence compares structures and body plans.
- Molecular evidence compares DNA or protein sequences.
- Molecular evidence often resolves misleading visual similarity.
Parsimony
- The most likely tree usually requires the fewest evolutionary changes.
- Common ancestry underpins all phylogenetic reasoning.
- All life traces back to a shared ancestral lineage.
Activities
Label the Tree
Using the annotated tree in this lesson, identify the root, one node, one branch, one tip, and a pair of sister groups. Then explain what each of those labels means.
Give short definitions, not just the labels.
Label the diagram in your book, then summarise the meaning of each term here.
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.
Aim for a clear judgement backed by evidence.
Write the paragraph in your book, then use this field for your final wording.
Revisit Your Thinking
Similarity does not automatically equal close relatedness. Phylogenetic trees are about shared ancestry, and nodes represent divergence events in lineages, not current species waiting between the tips.
If your original answer treated a branching point like a living taxon, this lesson should have shifted that. Trees work because they model ancestral splits over time.