Molecular Evidence

Q1 — Sample Band 6 response (7 marks)

Molecular evidence uses DNA and protein sequences to compare organisms and infer evolutionary relationships. Named molecular tools include cytochrome c amino acid sequencing — comparing amino acid order in this universally shared respiratory protein across species — and DNA barcoding, which uses a short standardised DNA sequence (commonly the CO1 mitochondrial gene) to identify and compare species. Other valid tools include BLAST (online sequence alignment and comparison against databases) and mtDNA lineage tracing. [1 — defines molecular evidence + two named tools]

Morphological evidence uses physical features, anatomical organisation, and homologous and analogous structures to infer relatedness. Its strength is that it is field-applicable and accessible without specialised laboratory equipment. Its key limitation is that it can be misled by convergent evolution, in which unrelated lineages independently evolve similar-looking structures in response to similar selective pressures. [1 — morphological evidence with strength and limitation]

Convergent evolution is the primary context in which molecular evidence is more reliable. When two species have evolved similar appearances independently, morphological classification groups them incorrectly. DNA sequence comparison reveals the actual divergence time and degree of relatedness beneath the surface similarity — convergent species will have markedly different sequences even if they look alike. [1 — convergent evolution + when molecular is more reliable]

The strongest conclusions draw on both types. Molecular data is powerful but also has limitations: molecular clocks can be unreliable over very long timescales, reference databases may be incomplete, and sequence data alone cannot reveal behavioural, ecological or anatomical information. Fossil and morphological evidence add information molecular data cannot provide. When multiple independent lines converge, confidence is highest. [1 — why combining both is strongest]

Overall, neither type is universally superior. Molecular evidence resolves ambiguities caused by convergent evolution, while morphological evidence provides accessible, contextualised information. The most robust classifications use both, with molecular evidence refining hypotheses from morphology. [1 — evaluative judgement avoiding “one winner”]

[1 quality mark]

Q2 — Sample Band 6 response (8 marks)

The platypus posed a classification problem because its morphology combined features from several very different groups — bill like a duck, tail like a beaver, fur and warm-bloodedness like a mammal, but egg-laying like a reptile. Early naturalists relying on physical appearance had no clear framework for placing an organism mixing features they had previously treated as separate. [1 — mixed morphology explained]

This demonstrates a key limitation: when strong, unique selective pressures drive unusual adaptive morphology (here, adaptations for a semi-aquatic lifestyle), physical features may not reflect common ancestry and can mislead classification. [1 — limitation identified]

Molecular evidence resolved the question by comparing mitochondrial DNA and nuclear gene sequences between the platypus and other vertebrate groups, showing that the platypus shares sequences consistent with mammalian common ancestry, branching off from the therian lineage approximately 166 million years ago. [1 — molecular evidence and result]

This was more reliable because DNA sequences reflect actual genealogical descent — the sequence relationships encode the actual branching history of lineages in a way that unusual adaptive morphology cannot. [1 — why molecular was more reliable here]

This example shows that morphological and molecular evidence are complementary, not opponents. Morphology gave a detailed picture of the platypus’s adaptations; molecular evidence clarified the phylogenetic position that morphology could not. [1 — complementary nature]

When molecular and morphological evidence conflict, molecular evidence should generally be given greater weight for inferring genealogical relationships, because sequence similarity at the level compared reflects shared ancestry rather than adaptive responses to selective pressures. However, neither is infallible. [1 — conflict resolution with nuance]

The justified conclusion is that when morphology is misleading (as in the platypus), molecular evidence is the stronger tool for phylogenetic placement, but the overall reconstruction benefits from both lines of evidence together. [1 — justified conclusion]

[1 quality mark]

Q3 — Evaluate the DNA claim (6 marks)

Overall judgement: Partly defensible but largely flawed. [1]

Defensible: DNA data does provide a more direct measure of genealogical relatedness than morphology in many cases; in cases where morphology is misleading, molecular sequence data provides a more objective basis for determining ancestry. [1]

Flawed — “no interpretation”: DNA data requires considerable interpretation — researchers must choose which gene or region to sequence, how to align sequences, which evolutionary model to use for the molecular clock, and how to weight conflicting evidence. [1]

Flawed — “makes fossils and anatomy irrelevant”: Fossil evidence reveals transitional forms, timing, and ecological context that DNA cannot provide. Morphological data captures information about structure and function that sequence data does not encode. The strongest evolutionary reconstructions draw on multiple independent lines: molecular, fossil, anatomical and biogeographic. [1]

Flawed — “removes all subjectivity”: Subjectivity shifts to methodological choices (gene selection, alignment algorithms, model assumptions, weighting of conflicting evidence) rather than disappearing. [1]

Defensible reformulation: “Molecular evidence, including DNA and protein sequence comparisons, provides powerful quantitative data about evolutionary relatedness and can resolve ambiguities that morphology alone cannot — particularly in cases of convergent evolution or incomplete specimens. However, it requires expert interpretation, has its own limitations (such as unreliable molecular clocks and incomplete reference databases), and is most reliable when used alongside morphological, fossil and biogeographic evidence. Phylogenies are strongest when multiple independent types of evidence converge on the same conclusion.” [1]