Biology • Year 11 • Module 3 • Lesson 15
Phylogenetic Trees
Apply tree-reading skills, interpret evidence types used to build trees, evaluate claims about relatedness, and apply the principle of parsimony to competing tree arrangements.
1. Interpret a phylogenetic tree
The tree below shows relationships among five vertebrate taxa (A–E) with an outgroup (F). Use the tree to answer the questions. 9 marks
1.1 Identify the sister group pair in this tree. Explain how you determined this. 2 marks
1.2 Which taxon is the outgroup? Explain what being an outgroup tells you about its relationship to the other taxa. 2 marks
1.3 Identify a clade that includes exactly three taxa from this tree. Justify your identification. 2 marks
1.4 Which taxa in this tree are most closely related to Taxon E? Use branching points to justify your answer. 3 marks
2. Cause-and-effect chain — molecular evidence revises a phylogenetic tree
Complete the chain below. 5 marks
Cause 1: Two aquatic organisms (species X and species Y) have very similar streamlined bodies and fin shapes, leading a morphologist to place them in the same taxonomic group.
Effect 1 / Cause 2:
Cause 3: A molecular biologist sequences the DNA of both species and finds only 30% sequence similarity between them but >90% similarity between species Y and a land mammal.
Effect 3 / Cause 4:
Cause 5: Scientists apply the principle of parsimony to decide which tree arrangement best explains the molecular data.
Effect 5 (Overall outcome):
3. Case study — molecular evidence places whales with mammals
Read the scenario, then answer the questions. 5 marks
Scenario. Early classification based on body shape placed whales in a separate group from land mammals. Whales have streamlined, fish-like bodies and live entirely in water — very different from any land mammal. However, molecular analysis comparing DNA sequences showed that whales share approximately 99% DNA similarity with hippopotamuses, and are more closely related to hippos than hippos are to other even-toed ungulates (hoofed mammals) such as cattle. This molecular finding is now supported by transitional fossils showing ancestors of modern whales with reduced hind limbs adapted to shallow water.
3.1 Explain what the DNA evidence reveals about the evolutionary relationship between whales and hippos, using lesson vocabulary. 2 marks
3.2 Explain why this scenario illustrates the advantage of molecular evidence over morphological evidence in certain classification problems. 2 marks
3.3 If the molecular phylogeny places whales as the sister group of hippos, how should the phylogenetic tree be drawn to reflect this? Describe the arrangement in words (no drawing required). 1 mark
Q1.1 — Sister groups (2 marks)
Taxon A and Taxon B are sister groups [1]. They share the most recent common ancestor, represented by the upper node (the branching point that connects A and B most directly, at the right side of their shared branch). This is determined by tracing each taxon back to the nearest shared branching point [1].
Q1.2 — Outgroup (2 marks)
Taxon F is the outgroup [1]. It diverged earliest from the common ancestor shown (from the root node) and is placed outside the ingroup (A–E). This means Taxon F is most distantly related to all the other taxa on the tree and helps identify which characteristics in A–E are derived (evolved after the split from F) versus ancestral (shared with F) [1].
Q1.3 — Clade with exactly three taxa (2 marks)
A valid clade with exactly three taxa is the one containing Taxon C, Taxon D and Taxon E [1]. This group includes their most recent common ancestor (the node where C branches from D+E) and all descendants of that ancestor. It is a monophyletic group — it includes an ancestor and all of its descendants and nothing else [1]. (Accept also any larger clade correctly identified with justification.)
Q1.4 — Most closely related to Taxon E (3 marks)
Taxon D is most closely related to Taxon E [1], because D and E share the most recent common ancestor (the node directly connecting D and E on the rightmost part of the C–D–E branch) [1]. Taxon C is the next most closely related to E (they share the node connecting C with D+E, which is slightly further back in time). Taxon A and B are more distantly related; they share an ancestor with E only further back at the node connecting the A+B branch to the C+D+E branch [1].
Q2 — Cause-and-effect chain (sample answers)
Effect 1 / Cause 2: The morphological classification groups species X and Y as closely related because of their convergent appearance. If only morphology is used, the phylogenetic tree places X and Y together, suggesting recent common ancestry.
Effect 3 / Cause 4: The molecular evidence contradicts the morphological classification. Species X and Y differ strongly in DNA (~30% similarity), showing they diverged long ago. Species Y is closely related to a land mammal (>90% similarity) — their physical similarity is the result of convergent evolution in a shared aquatic habitat, not shared ancestry.
Effect 5 (Overall outcome): Applying parsimony, the tree that explains the molecular data requires fewer total evolutionary changes if Y is grouped with the land mammal rather than with X. The morphological tree is revised: X and Y are placed in separate lineages, and Y is correctly placed as a close relative of the land mammal. Molecular evidence resolved the classification ambiguity that morphology could not.
Q3.1 — What DNA reveals about whales and hippos (2 marks)
The DNA evidence shows that whales and hippos share approximately 99% DNA similarity, indicating they share a very recent common ancestor — much more recent than either shares with other even-toed ungulates [1]. Using lesson vocabulary: whales and hippos are sister groups on the mammalian phylogeny — they share the most recent common ancestor of any two lineages in that comparison [1].
Q3.2 — Why molecular is advantageous here (2 marks)
Morphological evidence (body shape) placed whales away from land mammals because their streamlined aquatic body plan looks so different from any terrestrial mammal [1]. However, this physical difference reflects convergent adaptation to aquatic life — not true divergence from mammalian ancestry. Molecular evidence reveals the actual genetic relatedness beneath the misleading physical difference, showing that whales are deeply embedded within the mammal lineage rather than being a separate group [1].
Q3.3 — How to draw the tree (1 mark)
Whales and hippos should be drawn as two branches diverging from the same node (their most recent common ancestor), with that node connected to the rest of the even-toed ungulate lineage further back on the tree. Whales are a branch within the hippo/mammal group, not an outgroup to all other mammals [1].