Molecular Evidence
In 1987, Allan Wilson and colleagues at UC Berkeley published a landmark study of mitochondrial DNA from 147 people across 5 populations. Their analysis found that all modern human mtDNA traces back to a single ancestral population in Africa approximately 200,000 years ago — a finding that quantified human common ancestry using molecular divergence rates of 2–4% per million years and reshaped our understanding of Homo sapiens origins.
Practise this lesson
Four printable worksheets that build from the foundations up to exam-style questions — start at whatever level suits you.
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?
Know
- How DNA and protein similarity are used as evidence for relatedness.
- Why cytochrome c and mtDNA are useful molecular tools.
- What molecular clocks and BLAST analysis do.
Understand
- Why greater sequence similarity usually implies a more recent common ancestor.
- Why molecular evidence can resolve cases where morphology is misleading.
- How lineages can be traced using maternally inherited mtDNA.
Can Do
- Interpret simple sequence-comparison evidence in terms of relatedness.
- Explain why DNA barcoding is useful when morphology is unavailable or ambiguous.
- Evaluate molecular evidence against morphological evidence in an evolution question.
Core Content
Sequence comparison turns relatedness into measurable data
In 1987, Allan Wilson and colleagues at UC Berkeley extracted mitochondrial DNA from 147 people representing 5 populations across Africa, Asia, Europe, Australia, and the Americas. When they compared the sequences, they found that all modern human mtDNA traces back to a single ancestral African population approximately 200,000 years ago — calculated using a mutation rate of 2–4% per million years. That study demonstrated the central logic of molecular evidence: related species inherit many sequences from a shared ancestor, and mutations accumulate over time, so the more differences you count between two sequences, the more distant the common ancestor.
The central logic of molecular evidence: the more similar the DNA or protein sequence between two species, the more recent their common ancestor is likely to be. Mutations accumulate over time, so more differences usually mean a more ancient split.
This is why humans and chimpanzees show much higher coding-DNA similarity than humans and yeast. All living organisms still share some core sequences linked to universal cellular processes — a shared deep ancestry underlies all of life's molecular machinery. Human vs chimpanzee coding DNA is approximately 98.7% similar; human vs yeast is approximately 31%.
| 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 |
Molecular comparisons turn relatedness into quantitative evidence instead of relying only on appearance.
Pause — copy the highlighted DNA similarity logic and the comparison table into your book.
Molecular evidence (DNA comparison) supports evolution because more closely related species share:
Conserved molecules and lineage tracing tools
We just saw that DNA sequence similarity reveals relatedness. That raises a question: which specific molecules are most useful for comparing species, and can sequences even estimate how long ago lineages diverged? This card answers it — cytochrome c, molecular clocks, and mtDNA each play a distinct role.
Cytochrome c is useful in evolution because it is found in all aerobic organisms and performs the same essential role in cellular respiration — making broad comparison possible.
Cytochrome c is present in all aerobic organisms — the fewer amino acid differences between species, the more closely related they are. The molecular clock uses known mutation rates to estimate how long ago two lineages diverged: the more differences in a sequence, the longer the time since separation.
Mitochondrial DNA adds another layer. mtDNA is maternally inherited and mutates faster than much nuclear DNA — making it useful for tracing lineages over relatively recent evolutionary time, including patterns of human migration out of Africa and other phylogenetic relationships.
Cytochrome c
Shared across aerobic life, making broad species comparison possible.
Molecular Clock
Uses known mutation rates to estimate when lineages diverged.
mtDNA
Maternally inherited, faster mutation rate — useful for tracing recent lineages.
Add the cytochrome c, molecular clock and mtDNA points to your notes before the check below.
Why is cytochrome c useful in evolutionary comparisons?
When molecules solve problems that appearance cannot
We just saw that cytochrome c and mtDNA can trace evolutionary relationships quantitatively. That raises a question: how do scientists actually carry out these comparisons, and what tools exist when morphology is unavailable? This card answers it — BLAST analysis and DNA barcoding.
BLAST (Basic Local Alignment Search Tool) is used to compare a DNA or protein sequence against databases of known sequences to identify the closest matches — revealing relatedness and common ancestry from raw sequence data.
DNA barcoding identifies species using a short, standardised DNA sequence (commonly the CO1 gene from mitochondrial DNA in animals). It is powerful because it works even when the whole organism is not available or when morphology is misleading — damaged specimens, processed food samples, larval stages or illegal wildlife products can all be identified from a barcode sequence.
| 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 (DNA/protein) | 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 |
Evaluation: molecular evidence can resolve ambiguities caused by convergent evolution that mislead morphology. Best practice combines both — molecular evidence refines or tests conclusions from morphology. BLAST and barcoding are the tools that make molecular comparisons practical.
Add the BLAST, barcoding and evaluation points to your notes before the check below.
Greater DNA sequence similarity between two species usually indicates a more recent common ancestor.
DNA barcoding can identify species even when morphology is unavailable, such as from a damaged specimen or larval stage.
Molecular evidence is always more reliable than morphological evidence and completely replaces it in modern biology.
Activities
Read the Sequence Evidence
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.
When Molecules Beat Appearance
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.
Why can molecular evidence be stronger than morphological evidence in some cases?
Sequence Similarity
- More similar DNA or protein sequences usually indicate a more recent common ancestor.
- More differences usually indicate a longer time since divergence.
Cytochrome c and mtDNA
- Cytochrome c is found across aerobic life and can be compared across species.
- Molecular clock: uses mutation rates to estimate divergence time.
- mtDNA is maternally inherited and useful for tracing lineages.
BLAST and DNA Barcoding
- BLAST: compares sequences to databases to identify closest matches.
- DNA barcoding uses a standardised short DNA sequence to identify species.
- Useful when morphology is missing, damaged or misleading.
Evaluation
- Molecular evidence can resolve ambiguities caused by convergent evolution.
- Best practice often combines molecular and morphological evidence.
A fresh set drawn from this lesson's question bank — feedback shown immediately. +5 XP per correct · +25 XP all correct
Pick your answer, then rate your confidence — that tells the system what to drill next.
UnderstandBand 3(3 marks) 1. Explain how DNA sequence comparison provides evidence for evolution.
1 mark: more similar sequences = more recent common ancestor · 1 mark: mutations accumulate over time · 1 mark: allows relatedness to be inferred quantitatively
AnalyseBand 3–4(3 marks) 2. Describe one use of cytochrome c comparison and one use of DNA barcoding in evolutionary or ecological work.
1 mark: cytochrome c use correctly described · 1 mark: DNA barcoding use correctly described · 1 mark: clear link to identifying relatedness or species
EvaluateBand 4–5(4 marks) 3. Assess whether molecular evidence is always better than morphological evidence for classification and evolutionary study.
1 mark: molecular strengths · 1 mark: molecular limitations · 1 mark: morphology strengths · 1 mark: judgement — best practice combines both
Show all answers
Multiple choice
MC answers and full explanations are shown inline as you complete each question. Use the retry button to attempt a fresh set from the lesson bank.
Activity 1 — Read the Sequence Evidence
Species A and B are likely more closely related because they share 97% of the DNA sequence, meaning fewer mutations have accumulated since their common ancestor. Species A and C share only 64%, implying a longer time since divergence and therefore a more ancient common ancestor. The logic follows from the principle that mutations accumulate over time: the more similar the sequence, the more recent the split.
Activity 2 — When Molecules Beat Appearance
For the larval insect, DNA barcoding would be the better tool because larvae often look very different from adults and morphological features used for adult identification may not be present. For the adult insect, morphological identification is viable if the specimen is intact — but DNA barcoding can verify or supplement the identification, especially if the species is cryptic or prone to misidentification. For the damaged tissue sample, DNA barcoding is clearly the better choice because morphological identification requires intact, observable structures that are absent in a damaged sample. A strong evaluation acknowledges that combining both methods is the most reliable approach.
Short Answer Model Responses
SA1 (3 marks): DNA sequence comparison provides evidence for evolution because species that share more similar DNA sequences are usually more closely related [1]. This implies they diverged from a common ancestor more recently than species with many sequence differences [1]. Because mutations accumulate over time, sequence comparison lets scientists infer evolutionary relationships and relatedness quantitatively [1].
SA2 (3 marks): Cytochrome c can be compared across aerobic species — the fewer amino acid differences, the more recently the species diverged from a common ancestor [1]. 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 [1]. Both tools add precision that morphology alone cannot provide [1].
SA3 (4 marks): 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 [1]. However, it is not always better in every context [1]. Morphological evidence remains useful for field identification, whole-organism comparison and ecological interpretation [1]. The strongest scientific approach usually combines both, using molecular evidence to refine or test conclusions suggested by morphology [1].
More similar = more recent
Greater DNA/protein similarity means a more recent common ancestor — mutations accumulate over time.
Cytochrome c + mtDNA
Cytochrome c compares across aerobic life; mtDNA traces maternal lineages; molecular clock estimates divergence time.
BLAST and DNA barcoding
BLAST finds sequence matches in databases; barcoding identifies species from a short standardised sequence — powerful when morphology is absent or misleading.
Most common exam trap
Saying molecular evidence is always better. Both types have strengths and limits — the best science combines them.
Rapid-fire questions on DNA similarity, cytochrome c, mtDNA, molecular clocks, BLAST and comparing molecular vs morphological evidence. Beat the boss to bank a tier.
⚔ Enter the arenaYou were asked which evidence to trust more when two organisms look similar on the outside but have very different DNA sequences.
Allan Wilson and colleagues' 1987 UC Berkeley study of 147 people from 5 populations across Africa, Asia, Europe, Australia, and the Americas demonstrated why molecular evidence often outranks morphology: the mtDNA sequences produced a clear tree tracing all modern human diversity back to a single African ancestral population approximately 200,000 years ago, using a mutation rate of 2–4% per million years. Morphology alone would never have resolved that question — but sequence data, treated as a molecular clock, converted relatedness into a measurable number. When appearance and molecular data conflict, molecular evidence is typically more reliable because it directly counts inherited changes rather than interpreting convergently evolved forms.