Evidence for Evolution, Molecular and Biogeographical
You'll be able to interpret DNA similarity data to infer evolutionary relationships.
Comparisons of DNA sequences and genes can provide evidence of common ancestry. What a percentage means depends on which sequences are compared and how similarity is measured.
Printable Worksheets
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Q1 · How can comparisons of DNA sequences and specific genes provide evidence about evolutionary relationships? Why should scientists state which sequences they compared?
Think about why closely related species would have more similar DNA.
Q2 · Australia has marsupials like kangaroos and koalas, while other continents have mostly placental mammals. How might the movement of continents explain this pattern?
Consider what would happen if a landmass became isolated before certain groups evolved.
● Know
- That DNA and protein similarities across species indicate relatedness
- The concept of a molecular clock for estimating divergence times
- That biogeography and antibiotic resistance provide evidence for evolution
● Understand
- How molecular evidence independently supports the theory of common ancestry
- Why species distribution across continents matches evolutionary predictions
- How antibiotic resistance demonstrates evolution by natural selection in real time
● Can do
- Interpret DNA similarity data to infer evolutionary relationships
- Use biogeographical patterns to explain evolutionary history
- Explain antibiotic resistance using the principles of natural selection
Swap the DNA sequence of a human cytochrome c protein with that of a chimpanzee, and it works perfectly, the proteins differ by just 0 amino acids out of 104. Compare the same protein in a yeast cell and 44 positions differ. That measurable distance is the molecular clock of evolution. Molecular evidence for evolution comes from comparing the DNA, RNA and proteins of different species. The principle is simple: closely related species have more similar molecules because they inherited them from a more recent common ancestor. For this lesson’s chart-reading exercise, illustrative comparison figures are shown for chimpanzees, bonobos, gorillas, mice and chickens. They represent selected sequence comparisons, not the fraction of each whole genome that matches. Actual values depend on which comparable sequences and method are used.
Biogeography the study of where species live, provides complementary evidence. Related species tend to be found near each other, not randomly distributed across the globe. Australia has marsupials because marsupials evolved when Australia was connected to South America via Antarctica, and then Australia drifted northward in isolation. Placental mammals outcompeted marsupials on most other continents, but in Australia, marsupials diversified to fill ecological niches occupied by placentals elsewhere.
Text description
- Cytochrome c amino-acid differences from human: chimpanzee 0, dog 11, yeast 45.
- Phylogenetic tree: human and chimp join first, then dog, then yeast. Closer on the tree = more similar DNA.
- Biogeography: Australia (marsupials) was isolated about 50 million years ago after the Gondwana split, so its marsupials evolved uniquely.
The extinct thylacine, or Tasmanian tiger, looked and behaved so much like a dog that European settlers called it a tiger. But DNA extracted from preserved thylacine pups shows it was a marsupial, more closely related to kangaroos than to any dog or wolf. Its dog-like form evolved independently through convergent evolution, because the niche of a medium-sized predator favours similar body plans regardless of ancestry.
Australian ancient DNA: The Australian Centre for Ancient DNA (ACAD) at the University of Adelaide extracts and sequences DNA from fossils up to 50,000 years old. Their work has shown that the extinct thylacine and Tasmanian devil shared a common ancestor relatively recently, and that both lineages diversified during Australia's drying climate. Ancient DNA is rewriting what we know about Australian evolution.
Some students think molecular evidence contradicts the fossil record. In fact, the two usually agree beautifully. When they disagree, scientists do not throw out evolution, they investigate the discrepancy. Sometimes the fossils were misdated; sometimes molecular clocks need recalibration; sometimes both. The convergence of independent lines of evidence is what makes evolutionary theory so robust.
Australia separated from other continents around 50 million years ago and has been isolated ever since. Predict: would you expect Australian mammals to be more genetically similar to mammals on other continents, or to each other?
Australian mammals such as kangaroos, koalas, wombats and Tasmanian devils are all marsupials that share a more recent common ancestor with each other than with placental mammals on other continents. DNA studies confirm that Australian marsupials form a distinct evolutionary branch that diverged after the separation of Australia.
Use these terms in your explanation: isolation · divergence · marsupial · gene flow
Humans and bananas share some homologous genes involved in core cellular functions such as DNA replication and protein synthesis. These genes reflect ancient common ancestry. This does not mean that half of the DNA sequences in the two whole genomes match: a percentage of sequence similarity requires a defined set of sequences and a stated comparison method.
Convergent evolution occurs when unrelated species independently evolve similar traits because they face similar environmental challenges. The classic example is the marsupial-placental comparison. Australia has marsupial 'moles', 'mice', 'cats', 'wolves' and 'flying squirrels' that look and behave like their placental counterparts on other continents, but evolved entirely separately. These similarities are analogous (same function, different origin), not homologous (same origin, different function).
Convergent evolution is strong evidence for natural selection because it shows that similar environments produce similar solutions, even starting from different ancestral stock. It also explains why unrelated species on different continents can look confusingly alike. The key to telling homology from analogy is detailed anatomical and molecular comparison: analogous structures may look similar on the outside, but their underlying anatomy and genetics differ.
The eye has evolved independently at least 40 times in different animal lineages. Octopus eyes and human eyes look remarkably similar, both have a lens, iris, retina and optic nerve, but they evolved completely separately. In fact, octopus eyes are arguably better designed: their retinas are oriented correctly, with nerves behind the photoreceptors, so they have no blind spot. Human eyes are built backwards, a quirk inherited from our fish ancestors.
Australian biogeography: Alfred Russel Wallace, who independently conceived the theory of natural selection alongside Darwin, spent years in the Malay Archipelago and defined the Wallace Line, a sharp biogeographical boundary between Asian and Australian fauna. The line marks the edge of the former Australian continental shelf, and species distributions on either side provide dramatic evidence of how geography shapes evolution.
- Thylacine (Tasmanian tiger)
- Numbat
- Marsupial mole
- Sugar glider
- Quoll
- Anteater
- Flying squirrel
- Golden mole
- Cat
- Wolf / dog
Islands are natural laboratories for evolution. When a population becomes isolated, on an island, behind a mountain range, or on a separated continent, it can no longer exchange genes with the main population. Over time, mutations and different selection pressures cause the isolated population to diverge. If the divergence is great enough, the populations may become separate species that cannot interbreed even if they meet again.
DNA barcoding is a modern tool that uses a short, standardised segment of DNA to identify species. By comparing barcode sequences, scientists can distinguish closely related species, detect invasive species at borders, and monitor biodiversity. In Australia, DNA barcoding has revealed cryptic species, organisms that look identical but are genetically distinct, among frogs, insects and plants. This hidden diversity is crucial for conservation.
New Zealand has no native land mammals except bats. When Maori and then European settlers arrived, they introduced rats, cats and possums. These placental predators devastated native bird populations that had evolved without mammalian predators and lacked defensive behaviours. The isolation that made New Zealand's birds unique also made them vulnerable, a tragic illustration of how evolution shapes both resilience and fragility.
Australian conservation genomics: The Australian Museum's Australian Museum Research Institute uses DNA barcoding to catalogue Australia's immense biodiversity. Their work has discovered hundreds of new species and is building a reference library that helps biosecurity officers identify pests at ports and airports, protecting Australian agriculture and ecosystems from invasive species.
Marsupial evolution in Australia is one of the great biogeographical stories. When Australia separated from Gondwana around 50 million years ago, it carried marsupial mammals with it. Isolated from placental mammals that dominated other continents, Australian marsupials diversified into an astonishing array of forms: carnivorous Tasmanian devils, burrowing wombats, gliding possums, hopping kangaroos and koalas that climb eucalypts. This adaptive radiation occurred because marsupials filled ecological niches that placental mammals filled elsewhere, a pattern predicted by evolution and explained by Australia's long isolation.
Antibiotic Resistance in Australian Hospitals
The Australian Commission on Safety and Quality in Health Care tracks antibiotic resistance nationwide. In 2023, MRSA and resistant strains of E. coli and Klebsiella caused thousands of serious infections. The World Health Organization has declared antimicrobial resistance one of the top ten global public health threats. Combating it requires not just new drugs, but also public understanding of evolution: finishing prescribed courses, avoiding unnecessary antibiotic use, and recognising that bacteria evolve in response to our medical practices.
Text description
- Illustrative selected-sequence comparison values: chimpanzee ~98.8%, bonobo ~98.7%, gorilla ~98.4%, mouse ~85%, and chicken ~60%. They are classroom figures for graph reading, not percentages of whole genomes.
- For comparable sequences, greater similarity can indicate closer relatedness and more recent ancestry. Some human and banana genes are homologous, but no whole-genome percentage is assigned.
Analyse the Molecular Evidence
1 Based on DNA similarity, which species is most closely related to humans? Explain your reasoning.
2 What do homologous genes with shared cellular functions in humans and bananas suggest about common ancestry?
3 A scientist sequences cytochrome c from an unknown mammal and finds it differs from human cytochrome c by 12 amino acids. From chimpanzee cytochrome c, it differs by 11 amino acids. What can you conclude?
4 Explain how molecular evidence supports the theory of common ancestry independently from fossil evidence.
5 If molecular clock estimates suggest humans and mice diverged 90 million years ago, but a new fossil discovery pushes that date back to 100 million years, which estimate is likely more reliable? Why?
Biogeography and Australian Marsupials
1 Explain why Australia has so many marsupials while other continents have mostly placental mammals.
2 Describe how natural selection and Australia's unique environments contributed to the evolution of kangaroos, koalas and Tasmanian devils from a common marsupial ancestor.
3 Explain how antibiotic resistance in bacteria demonstrates all five principles of natural selection. Use MRSA as your example.
Copy Into Your Book
▼Molecular Evidence
- DNA/protein similarities = relatedness
- Human-chimp: ~98.8%
- Human-banana: some homologous genes with shared core cellular functions; no whole-genome percentage is specified
- Cytochrome c differences correlate with divergence time
Molecular Clocks
- Mutations accumulate at roughly steady rates
- More differences = longer since divergence
- Conceptual tool, cross-checked with fossils
Biogeography
- Species distribution matches continental drift
- Australian marsupials = Gondwana heritage + isolation
- Wallace Line separates Asian and Australian fauna
Antibiotic Resistance
- Evolution by natural selection in real time
- Variation + antibiotic pressure = resistant survivors
- MRSA is a major health threat in Australia
At the start of this lesson you were asked how DNA and gene comparisons can provide evidence of shared ancestry. This comparison was chosen to show how molecular evidence can complement fossil and anatomical evidence.
Now that you have worked through molecular similarities, DNA comparisons and biogeographical evidence, including why Australia's marsupials ended up so isolated, explain in your own words why two independent lines of evidence (molecules + geography) are more convincing together than either would be alone.
Q1. Explain how DNA similarities between species provide evidence for common ancestry. 3 MARKS
Q2. What is a molecular clock, and why is it useful for understanding evolutionary relationships? 4 MARKS
Q3. Using antibiotic resistance as an example, explain how evolution by natural selection can be observed directly. Refer to variation, selection pressure and heritability in your answer. 5 MARKS
Revisit Your Initial Thinking
Go back to your Think First responses at the top of the lesson.
- Did you correctly identify that DNA similarity indicates shared ancestry and recency of divergence?
- Did you recognise that some homologous human and banana genes support shared ancestry, while gene matches and whole-genome sequence percentages are different measures?
- Write one sentence summarising the most important new concept you learned about molecular evidence for evolution.
Model answers (click to reveal)
Comprehensive Answers
▼Activity 1, Analyse the Molecular Evidence
1. Most closely related: Chimpanzees share ~98.8% of DNA with humans, the highest similarity shown [1 mark]. This indicates the most recent common ancestor [0.5 mark].
2. Humans and bananas: Some homologous genes support common ancestry and are involved in basic cellular functions such as DNA replication and protein synthesis [1 mark]. A shared gene or function is not a percentage of whole-genome DNA sequence identity [1 mark].
3. Unknown mammal conclusion: The unknown mammal is very closely related to both humans and chimpanzees, likely another great ape [1 mark]. The small difference (11–12 amino acids) suggests divergence very recently in evolutionary terms [0.5 mark].
4. Independent support: Molecular evidence is independent of fossils because it comes from living organisms, not rocks [1 mark]. When DNA trees match fossil timelines, the conclusion is much stronger [0.5 mark].
5. More reliable estimate: Both estimates should be considered together. Molecular clocks provide relative timing, but fossils anchor estimates to absolute geological time [1 mark]. A conflict suggests the molecular rate estimate may need revision [0.5 mark].
Activity 2, Biogeography and Australian Marsupials
2. Marsupial radiation: After Gondwana separation, Australian marsupials were isolated from placental competitors [1 mark]. Different environments (arid plains, forests, grasslands) created different selection pressures [1 mark]. Variation in the ancestral population meant some individuals were better suited to each environment [1 mark]. Natural selection favoured different traits in different niches, leading to adaptive radiation [1 mark].
3. MRSA and natural selection: Variation exists, some bacteria have random mutations conferring resistance [1 mark]. Antibiotics create selection pressure, killing susceptible bacteria [1 mark]. Resistant bacteria survive and reproduce more [1 mark]. Resistance is heritable (encoded in DNA), so the trait spreads [1 mark]. The result is a population shift from susceptible to resistant, evolution observed in real time [1 mark].
Multiple Choice
1. B Closely related species share more DNA because they diverged more recently from a common ancestor. Option A confuses ecology with genetics. Option C reverses cause and effect. Option D is false.
2. C A molecular clock uses mutation rates to estimate divergence times. Options A, B and D describe laboratory equipment, not the conceptual tool.
3. A Species distribution matches continental drift and fossils. Options B, C and D are false.
4. C Antibiotic resistance is evolution by natural selection in real time. Option A describes Lamarckism. Option B describes a social issue, not a biological mechanism. Option D anthropomorphises bacteria.
5. D Multiple independent lines of evidence converge on common ancestry. Options A, B and C are false or present false conflicts.
Short Answer Model Answers
Q1 (3 marks): DNA similarities indicate common ancestry because living things inherited genetic material from shared ancestors [1 mark]. Similarity in comparable sequences can help infer how recently two species shared an ancestor; for example, the lesson’s chimpanzee comparison shows greater similarity than its mouse or chicken comparisons [1 mark]. Humans and bananas also share some homologous genes for core cellular functions, but that gene-level evidence is not a percentage of whole-genome sequence identity [1 mark].
Q2 (4 marks): A molecular clock is a conceptual tool that uses the roughly steady rate of DNA mutations to estimate when two species diverged from a common ancestor [1 mark]. Scientists count the number of DNA differences between species and use known mutation rates to calculate time [1 mark]. It is useful because it provides independent estimates of divergence times that can be compared with fossil and geological evidence [1 mark]. While not exact, molecular clocks consistently support the evolutionary relationships predicted by anatomy and fossils, strengthening the overall evidence [1 mark].
Q3 (5 marks): Antibiotic resistance is a direct observation of evolution by natural selection. Variation exists within bacterial populations, some bacteria carry random mutations that make them less susceptible to antibiotics [1 mark]. When antibiotics are used, they create a strong selection pressure that kills susceptible bacteria while resistant ones survive [1 mark]. This is differential survival the resistant bacteria reproduce more because they are alive [1 mark]. Resistance is heritable because it is encoded in bacterial DNA and passed to offspring during reproduction [1 mark]. Over many generations, the bacterial population shifts from mostly susceptible to mostly resistant, the population has evolved [1 mark]. MRSA in Australian hospitals is a direct result of this process.