Biology • Year 11 • Module 3 • Lesson 15
Phylogenetic Trees
Lock in the core vocabulary — root, node, branch, tip, clade, outgroup, derived characteristic, parsimony — and understand how phylogenetic trees represent evolutionary relationships rather than direct descent.
1. Term–definition match
The eight definitions below are shuffled. In the right-hand column write the matching term from this list: phylogenetic tree, cladogram, clade, outgroup, derived characteristic, parsimony, sister groups, node. 8 marks
| # | Definition (shuffled) | Matching term |
|---|---|---|
| 1.1 | A diagram showing evolutionary relationships among species based on common ancestry. | |
| 1.2 | A branching diagram showing derived characteristics used to infer evolutionary relationships. | |
| 1.3 | A group of organisms including a common ancestor and all of its descendants. | |
| 1.4 | A species placed outside the group of interest, used to determine which characteristics are derived versus ancestral. | |
| 1.5 | A trait present in descendants but absent in the ancestor; it evolved after the lineage branched from the outgroup. | |
| 1.6 | The principle that the most likely phylogenetic tree is usually the one requiring the fewest evolutionary changes. | |
| 1.7 | Two lineages that share the most recent common ancestor on a phylogenetic tree. | |
| 1.8 | A branching point on a phylogenetic tree representing an ancestral population from which two lineages diverged. |
2. True or false — with correction
For each statement, circle T or F. If the statement is false, write the corrected version. 6 marks (1 for T/F, 1 for correction where needed)
2.1 A node on a phylogenetic tree represents a living species that exists between the two diverging lineages. T / F
2.2 The principle of parsimony favours the phylogenetic tree that requires the fewest evolutionary changes to explain the observed shared traits. T / F
2.3 Sister groups are the two lineages drawn closest together visually on a page. T / F
3. Annotate the tree
The diagram below shows a simple phylogenetic tree with Taxon A, Taxon B and Taxon C (outgroup). Answer each question. 8 marks (2 each)
3.1 Which taxon is the outgroup? What does its position tell you about its relationship to Taxon A and Taxon B?
3.2 Which two taxa are sister groups? Explain how you determined this from the tree.
3.3 What does the node connecting Taxon A and Taxon B represent? Is it a living species?
3.4 Which of the three taxa is most distantly related to Taxon A? Justify using the tree.
4. Cloze — complete the paragraph
Fill each blank with the correct word from the word bank. Use each word once only. 8 marks
Word bank: ancestor | clade | convergent | derived | fewest | molecular | node | outgroup
A phylogenetic tree shows evolutionary relationships through common ancestors rather than direct descent. Each branching point called a represents an ancestral population from which two lineages diverged — it is not a living species. A includes a common and all its descendants. The is placed outside the ingroup to help identify which characteristics are (evolved after divergence from the outgroup). The principle of parsimony says the most likely tree requires the evolutionary changes to explain shared traits. When morphological evidence and evidence produce different trees, scientists investigate whether the physical similarity reflects evolution rather than true shared ancestry.
Q1 — Term–definition matches
1.1 phylogenetic tree • 1.2 cladogram • 1.3 clade • 1.4 outgroup • 1.5 derived characteristic • 1.6 parsimony • 1.7 sister groups • 1.8 node.
Q2 — True / False with correction
2.1 False. Correction: a node represents an ancestral population from which two lineages diverged — it is not a living species. It is a branching point in time, representing the last common ancestor of the two lineages.
2.2 True.
2.3 False. Correction: sister groups are the two lineages sharing the most recent common ancestor, determined by tracing branching points — not by which organisms happen to be drawn closest together on the page. Visual proximity on the tree is not a reliable guide.
Q3.1 — Outgroup
Taxon C is the outgroup. It diverged earliest from the common ancestor, before the split between Taxon A and Taxon B, and is placed outside the main ingroup (A and B). This means Taxon C is more distantly related to both Taxon A and Taxon B than A and B are to each other.
Q3.2 — Sister groups
Taxon A and Taxon B are sister groups. They share the most recent common ancestor, which is represented by the node where their two branches diverge (the upper node connecting A and B directly). This is determined by tracing back from both taxa to the first branching point they share.
Q3.3 — What the node represents
The node connecting Taxon A and Taxon B represents an ancestral population from which both lineages diverged — it is the last common ancestor of Taxon A and Taxon B. It is not a living species; it is a point in evolutionary history at which one lineage split into two.
Q3.4 — Most distantly related to Taxon A
Taxon C is most distantly related to Taxon A. Tracing back from Taxon A, the first branching point reached leads to Taxon B (the most recent shared ancestor of A and B). The next branching point further back leads to the common ancestor of A, B and C — this more distant shared ancestor indicates that C is more distantly related to A than B is.
Q4 — Cloze answers (in order of blanks)
node • clade • ancestor • outgroup • derived • fewest • molecular • convergent