Fossils, anatomy and biogeography reveal patterns, but molecules let scientists compare life at the sequence level. DNA and proteins can show relatedness directly, expose misleading surface similarities, and trace lineages in ways morphology alone cannot.
Use the PDF for classwork, homework or revision. It includes key ideas, activities, questions, an extend task and success-criteria proof.
Make your first judgement before the molecular examples do the heavy lifting.
1. If two species have very similar DNA sequences, what might that suggest about their evolutionary relationship?
2. If two organisms look similar on the outside but have very different DNA, which kind of evidence would you trust more for relatedness, and why?
Write your first response now. We will revisit it after the DNA and protein comparisons.
Write your initial answer in your book, then return here later to compare it with your final view.
Wrong: Natural selection means organisms change because they want or need to.
Right: Natural selection acts on random genetic variations; organisms do not consciously adapt.
Core Content
Sequence comparison turns relatedness into measurable data
The central logic of molecular evidence is simple: the more similar the DNA or protein sequence between two species, the more recent their common ancestor is likely to be.
That is because related species inherit many sequences from a shared ancestor. Over time, mutations accumulate. If two species diverged recently, fewer differences have had time to build up. If they diverged long ago, more differences are expected. This is why humans and chimpanzees show much higher coding-DNA similarity than humans and yeast. It is also why all living organisms still share some core sequences linked to universal cellular processes.
| Comparison | Approximate Similarity | Interpretation |
|---|---|---|
| Human vs chimpanzee coding DNA | ~98.7% | Very close relatedness and recent common ancestry |
| Human vs yeast coding DNA | ~31% | Much more distant common ancestry |
| Core genes across all life | Widely shared | Evidence of deep common ancestry for all organisms |
Conserved molecules and lineage tracing tools
Cytochrome c is useful in evolution because it is found in all aerobic organisms and performs the same essential role in cellular respiration.
Because the protein is so widespread, scientists can compare its amino acid sequence across many species. A small number of differences suggests a closer evolutionary relationship, while a larger number of differences suggests a more distant one. Mitochondrial DNA adds another layer of evidence. Because mtDNA is maternally inherited and tends to mutate faster than much nuclear DNA, it is useful for tracing lineages over relatively recent evolutionary time and has been used in studies of human migration out of Africa and other phylogenetic relationships.
These tools do not replace other evidence. They strengthen it. Fossils and morphology suggest hypotheses about relatedness, while molecular evidence can test those hypotheses with sequence-level comparisons.
When molecules solve problems that appearance cannot
DNA barcoding identifies species using a short, standardised DNA sequence, commonly the CO1 gene from mitochondrial DNA in animals.
This method is powerful because it works even when the whole organism is not available or when morphology is misleading. A damaged specimen, a processed food sample, a larval stage or illegal wildlife product may not show the features needed for morphological identification, but a barcode sequence can still match it to a species. That is why DNA barcoding is used in ecology, food fraud detection and wildlife trade enforcement.
| Evidence Type | Strength | Limitation |
|---|---|---|
| Morphological evidence | Useful in field identification and whole-organism comparison | Can be misled by convergent evolution or missing traits |
| Molecular evidence | Can reveal relatedness and identify species from sequence data | Requires specialised tools, reference data and interpretation |
| DNA barcoding | Useful when morphology is absent, incomplete or ambiguous | Depends on a reliable barcode library and suitable DNA |
Activities
A data table shows species A and B share 97% of a DNA sequence, while species A and C share 64% of the same sequence. Explain what this suggests about relatedness and common ancestry.
State which pair is more closely related and why that follows from the data.
Annotate the data comparison in your book, then summarise your reasoning here.
A larval insect, an adult insect and a damaged tissue sample need identification. Evaluate whether morphology or DNA barcoding would be the better tool in each case, and justify your answer.
A strong answer compares the strengths and limits of both approaches rather than choosing one blindly.
Draft the comparison in your book, then write your final evaluation here.
Molecular evidence is powerful because it measures relatedness beneath visible traits. That matters most when morphology is incomplete, ambiguous or distorted by convergent evolution.
If your original answer assumed appearance should always come first, the key correction is this: appearance is useful, but sequence data can reveal ancestry more directly and can resolve cases where outward form gives the wrong impression.
Assessment
Answer first, then read the explanation
1. What does high DNA sequence similarity usually suggest?
What is NOT does high DNA sequence similarity usually suggest?
2. Why is cytochrome c useful in evolutionary comparisons?
3. Which statement best describes mitochondrial DNA?
4. What is the main advantage of DNA barcoding?
What is NOT the main advantage of DNA barcoding?
5. Why can molecular evidence be stronger than morphological evidence in some cases?
1. Explain how DNA sequence comparison provides evidence for evolution. (3 marks)
1 mark: sequence similarity reflects relatedness | 1 mark: more similar means more recent common ancestor | 1 mark: clear explanation
2. Describe one use of mitochondrial DNA and one use of DNA barcoding in evolutionary or ecological work. (3 marks)
1 mark: mtDNA use | 1 mark: DNA barcoding use | 1 mark: clear description
3. Assess whether molecular evidence is always better than morphological evidence for classification and evolutionary study. (4 marks)
1 mark: judgement | 1 mark: strength of molecular evidence | 1 mark: role/strength of morphology | 1 mark: evaluative conclusion
Answers
SA1: DNA sequence comparison provides evidence for evolution because species that share more similar DNA sequences are usually more closely related. This implies they diverged from a common ancestor more recently than species with many sequence differences. Because mutations accumulate over time, sequence comparison lets scientists infer evolutionary relationships and relatedness quantitatively.
SA2: Mitochondrial DNA can be used to trace maternal lineages and reconstruct patterns such as human migration or relationships between populations. DNA barcoding can be used to identify species from short DNA sequences when morphology is unavailable, such as in damaged samples, immature organisms, food testing or illegal wildlife trade investigations.
SA3: Molecular evidence is extremely powerful because it can compare relatedness directly through DNA or protein sequences and can resolve cases where morphology is misleading due to convergence or incomplete specimens. However, it is not always better in every context. Morphological evidence remains useful for field identification, whole-organism comparison and ecological interpretation. The strongest scientific approach usually combines both, using molecular evidence to refine or test conclusions suggested by morphology.
Say each answer aloud before moving to the next prompt