Phylogenetic Trees

Q1 — Sample Band 6 response (7 marks)

Morphological evidence compares physical structures, body plans and anatomical features to infer shared ancestry. For example, homologous structures such as the vertebrate pentadactyl limb suggest common ancestry because the underlying bone arrangement is shared despite different functions. [1 — morphological evidence + example]

Morphological evidence can be misleading when convergent evolution produces analogous structures — similar-looking features that arose independently in unrelated lineages due to similar selective pressures. Grouping organisms by appearance when their similarity is convergent rather than homologous produces incorrect phylogenies. [1 — when morphology misleads + reason]

Molecular evidence compares DNA sequences, protein sequences (such as cytochrome c) or mitochondrial DNA divergence to reveal hidden relatedness. For example, DNA comparison showed that whales are more closely related to hippos than hippos are to other hoofed mammals — a relationship that morphology alone had not revealed. Molecular evidence is most reliable in cases where convergent evolution has produced misleading physical similarity, because DNA sequences reflect actual genealogical descent rather than adaptive responses to the environment. [1 — molecular evidence + when more reliable + example]

One limitation of molecular evidence is that the molecular clock can be unreliable over very long timescales, as mutation rates vary between lineages. Reference databases may also be incomplete, and interpreting sequence alignment requires methodological choices that introduce subjectivity. [1 — limitation of molecular evidence]

Parsimony is used when two competing tree arrangements are possible: the preferred tree is the one requiring the fewest evolutionary changes to explain the observed shared traits. This does not mean evolution is always simple — it means unnecessary complexity should not be assumed without evidence. [1 — parsimony explained]

Overall, the most reliable phylogenetic trees are constructed when both morphological and molecular evidence agree. When they disagree, molecular evidence is usually preferred for determining genealogical relationships because it reflects actual DNA descent rather than adaptive form. Morphological evidence remains valuable for contextual, ecological and transitional information that sequence data cannot provide. [1 — evaluative judgement]

[1 quality mark]

Q2 — Sample Band 6 response (8 marks)

Applying parsimony: Tree 1 requires 4 independent evolutionary changes to explain the shared traits, while Tree 2 requires 7. By the principle of parsimony, Tree 1 should be preferred because it requires the fewest evolutionary events to explain the same data — unnecessary complexity should not be assumed without evidence. [1 — parsimony applied correctly]

The molecular sequence data also supports Tree 1. P and Q share 94% DNA similarity, indicating they share a very recent common ancestor — consistent with them being sister groups. P and R share only 61% DNA similarity, indicating they diverged from a common ancestor much longer ago — consistent with them being less closely related. [1 — molecular data interpreted correctly]

The morphological and molecular evidence are in agreement in this case: both support Tree 1 (P+Q as sister groups). When two independent lines of evidence converge on the same arrangement, confidence in that arrangement is significantly higher than when only one line of evidence is available. [1 — evidence agreement identified]

The overall reliability of Tree 1 is good precisely because both parsimony (from morphological data) and molecular sequence analysis support the same arrangement. This convergence of independent evidence reduces the possibility that either line of evidence alone is misleading. [1 — reliability assessed with reasoning]

Tree 1 should be accepted. It requires fewer evolutionary changes (parsimony criterion) and is supported by molecular sequence data showing high similarity between P and Q. [1 — justified conclusion]

Note: if the molecular and morphological evidence had pointed to different arrangements, scientists would investigate whether morphological similarity in one arrangement might reflect convergent evolution, and would typically give greater weight to molecular evidence for genealogical relationships. [1 — conflict-resolution principle stated]

[1 quality mark]
[1 extra mark available for depth of analysis beyond 6 content marks]

Q3 — Evaluate the claim (6 marks)

Overall judgement: The claim contains two distinct errors about how to read phylogenetic trees. [1]

Defensible element: A phylogenetic tree does represent evolutionary relationships and shows how lineages are connected through time — so there is an element of showing “who is related to whom” across time. [1]

Flawed — “one organism evolved directly from another”: Phylogenetic trees show evolutionary relationships through common ancestors, not direct descent. A tip (modern species) did not evolve directly from another tip. Both tips evolved from a shared ancestor represented by the node where their branches diverge. [1]

Flawed — “tips represent both ancient ancestors and modern descendants”: Tips represent only current taxa (the end points of the branches). Nodes represent ancestral populations or divergence points — not living species. Confusing nodes with tips leads to misreading the tree as if one current organism descended from another current organism. [1]

Flawed — additional consequence: Sister groups are determined by which node connects them most recently, not by visual proximity on the page. Simply placing organisms close together or reading one as “above” another does not indicate ancestry. [1]

Accurate reformulation: “A phylogenetic tree shows evolutionary relationships among organisms through their common ancestors. Branch tips represent current taxa only; nodes represent ancestral divergence points where one lineage split into two. No tip organism evolved directly from another tip organism — rather, each pair of related taxa evolved from a shared ancestor represented by their shared node. Sister groups are the two lineages sharing the most recent common ancestor, identified by branching structure, not by visual position on the page.” [1]