Modern Examples of Evolutionary Change
In 2019, the World Health Organization's Global Antimicrobial Resistance and Use Surveillance System (GLASS) reported that drug-resistant infections cause more than 700,000 deaths annually worldwide. In Australian hospitals, the proportion of Staphylococcus aureus infections caused by MRSA rose from under 0.2% in 1990 to around 12% by 2020 — a 60-fold increase driven entirely by natural selection in hospital environments where antibiotics are the dominant selection pressure.
A doctor prescribes antibiotics to treat a Staphylococcus aureus infection. The patient takes the full course. Two years later, the same antibiotic barely affects that strain of bacteria in the hospital.
Before reading: how do you think the bacteria became resistant? Did the antibiotic "teach" them to resist, or did something else happen?
Know
- Three categories of modern evolutionary evidence: direct observation, fossil dating, and experimental evolution
- How cane toads and their predators are both evolving
- How antibiotic resistance demonstrates natural selection in real time
- How radiometric dating confirms evolutionary timescales
Understand
- Why antibiotics select for resistance rather than causing it
- Why bacteria evolve so rapidly (short generation time)
- How different radiometric isotopes are suited to different timescales
Can Do
- Explain each modern example using heritable variation, selection pressure, and differential survival
- Select the appropriate radiometric dating method for a given sample
- Evaluate these examples as evidence for evolution by natural selection
Core Content
Why rapid evolution is observable — and why it matters
In 1990, MRSA — methicillin-resistant Staphylococcus aureus — caused fewer than 0.2% of S. aureus infections in Australian hospitals. By 2020, the WHO's GLASS surveillance system reported that figure had risen to around 12%: a 60-fold increase in 30 years, driven entirely by antibiotic selection pressure in hospital environments. No mutations were caused by the antibiotics — the resistant alleles already existed in the bacterial population at low frequency. Natural selection simply killed every susceptible bacterium, generation after generation, until MRSA dominated. When selection pressure is strong and generation time is 20 minutes, evolution produces measurable change in years, not millions of years.
Three categories of modern evidence show evolution in action:
- Direct observation of rapid evolution — measurable changes in populations within years or decades (antibiotic resistance, cane toad, peppered moth)
- Fossil dating techniques — confirming evolutionary timescales and the sequence of change
- Experimental evolution — Lenski's E. coli experiment (40,000+ generations in a lab) showing evolution of novel traits
Modern evolutionary change is observable in real time when selection pressures are strong. Rapid evolution occurs when: the generation time is short, the selection pressure is intense, and heritable variation exists in the population.
Pause — copy the highlighted conditions for rapid observable evolution into your book.
Why do bacteria evolve antibiotic resistance so quickly compared to most organisms?
Evolution in two directions simultaneously
We just saw that rapid evolution requires strong selection pressure and short generation times. That raises a question: is there a real-world example where we can see this happening right now? This card answers it → the cane toad invasion is a live, ongoing evolutionary experiment.
The cane toad invasion since 1935 has created two simultaneous evolutionary experiments: the toads themselves are changing, and the native predators they poisoned are changing too.
Cane toads were introduced to Queensland in 1935 and now number over 200 million. The cane toad invasion provides evidence of evolution in two directions: the toads themselves are evolving faster dispersal, and native predators are evolving resistance to the toxin — both changes are driven by natural selection acting on existing heritable variation.
Add both directions of cane toad evolution (toad dispersal + predator resistance) to your notes before the check below.
True or False — The cane toad invasion shows evolution only in native predator populations, not in the toads themselves.
Longer-legged toads at the invasion front have higher survival because they can disperse more quickly into unoccupied territory.
Natural selection at 20 minutes per generation
We just saw that cane toads and their predators are both evolving in response to a new selection pressure. That raises a question: is there an example with even faster, more measurable change? This card answers it → bacteria with 20-minute generations produce thousands of "generations" of evolution per year.
Bacteria reproduce every 20 minutes — meaning thousands of generations per year. When a powerful selection pressure (an antibiotic) is applied, natural selection can drive measurable change within months.
The key mechanism: resistance mutations exist in bacterial populations before any antibiotic is used — they arise randomly through DNA replication errors. Antibiotic resistance demonstrates natural selection in real time: bacteria with pre-existing resistance mutations survive treatment and reproduce. The antibiotic is the selection pressure — it does not cause mutations, it selects for mutations that already exist.
Antibiotic resistance arises through natural selection on pre-existing variation — the antibiotic is the selection pressure, not the cause of mutations
Add the mechanism of antibiotic resistance (pre-existing variation → selection → resistance frequency increases) to your notes before the check below.
Radiometric dating uses the ___ rate of radioactive isotopes to determine the ___ of fossils.
Applying the Natural Selection Framework
Pattern — Apply the Three-Condition Framework
For each scenario, identify: (a) the heritable variation, (b) the selection pressure, and (c) the likely outcome over time. Answer in your book:
- Cane toad population at the invasion front: some toads have longer legs, some have shorter legs.
- A hospital ward where MRSA has developed: some S. aureus bacteria carry a resistance gene, most do not.
- Freshwater crocodile population exposed to cane toads: some individuals have slightly lower bufotoxin sensitivity.
- Peppered moth population near an industrial city in 1870: some moths have dark wing patterns, most are pale.
Radiometric dating — matching isotopes to timescales
We just saw rapid evolution happening in years. That raises a question: how do we confirm the much longer evolutionary timescales in the fossil record? This card answers it → radiometric dating uses the known, constant decay rates of radioactive isotopes to calculate age.
Every radioactive isotope decays at a predictable rate — its half-life. By measuring how much of the parent isotope remains and how much daughter product has accumulated, we can calculate when the organism lived.
Two main types of dating:
- Relative dating — rock layer position (stratigraphy): deeper layers are older. Index fossils are used as time markers. Does not give an absolute date — only "older than" or "younger than."
- Absolute (radiometric) dating — uses radioactive decay rates to give an actual age in years.
Radiometric dating uses the known decay rate of radioactive isotopes to calculate the age of fossils and rocks. Different isotopes are used for different time scales: C-14 for recent samples (<50,000 yrs), K-Ar and U-Pb for ancient geological specimens (millions–billions of years).
Add the three radiometric methods and their timescales to your notes before the check below.
Match each radiometric isotope to its appropriate use.
A measured change in allele frequency — evolution made visible
We just saw how fossil dating confirms evolutionary timescales. That raises a question: is there an example where we can directly measure allele frequency changing in response to a specific identified selection pressure? This card answers it → the peppered moth in industrial England is one of the most carefully measured examples of natural selection.
Before 1850: pale moths dominated. After industrialisation: dark moths dominated. After the Clean Air Act: pale moths recovered. The same population responded predictably to a changing selection pressure — exactly as Darwin's theory predicts.
The peppered moth (Biston betularia) exists in light and dark forms. Both forms were present before industrialisation — the dark (melanic) form was rare (~1% frequency) because it was visible on lichen-covered pale trees and eaten by birds.
As industrial soot killed lichens and blackened trees: Industrial melanism in the peppered moth is a direct demonstration of allele frequency change driven by natural selection. As the environment changed (soot → pollution; cleanup → recovery), the selection pressure shifted and the population responded predictably — exactly as Darwin's theory predicts.
Dark allele frequency in peppered moths tracked the changing selection pressure — soot blackened trees (favouring dark moths), then clean air allowed lichen recovery (favouring pale moths again)
Add the peppered moth mechanism (pale → dark → pale again, driven by changing selection pressure) to your notes before the check below.
Name THREE modern examples that demonstrate evolutionary change occurring in real time or that provide direct evidence for evolution.
Evaluating Modern Evidence for Evolution
Pattern — Evaluate as Evidence
Choose TWO of the following examples and, for each: (a) identify the heritable variation, (b) identify the selection pressure, (c) describe the outcome, and (d) evaluate the strength of this example as evidence for evolution by natural selection. Answer in your book:
- Cane toad leg length at the invasion front
- MRSA antibiotic resistance
- Peppered moth allele frequency change
- Radiometric dating of the fossil sequence
Conditions for rapid evolution
- Short generation time; intense selection pressure; heritable variation exists.
- Observable within years when all three conditions are met.
Cane toad evolution
- Toad: invasion-front toads have longer legs → faster dispersal selected for.
- Predators: some crocodiles/snakes evolving reduced toxin sensitivity.
- Both changes = natural selection on pre-existing heritable variation.
Antibiotic resistance
- Resistance mutations pre-exist; antibiotic = selection pressure (not cause).
- Susceptible bacteria die; resistant bacteria reproduce → resistance frequency increases.
- MRSA: multi-drug resistance evolved in hospitals from heavy antibiotic use.
Radiometric dating
- C-14: up to ~50,000 years (recent fossils).
- K-Ar: millions of years (volcanic rock, hominin fossils).
- U-Pb: billions of years (oldest geological specimens).
- Confirms evolutionary sequence: simpler → complex organisms over time.
Peppered moth
- Pre-industrial: pale moths common (camouflaged on lichen); dark ~1%.
- Industrial era: soot → dark moths camouflaged; dark allele rose to ~90%.
- Post-Clean Air Act: pale moths recovered as lichen returned.
- Direct, measured allele frequency change in response to identified selection pressure.
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 2(1 mark) Q1. Industrial melanism in peppered moths shows evolution because:
Answer: B — the dark allele pre-existed; soot changed which form was better camouflaged, shifting allele frequency.
ApplyBand 3(1 mark) Q2. A new antibiotic is introduced for treating MRSA. After 2 years it is no longer effective. The most likely explanation is:
Answer: C — resistance pre-existed; the antibiotic selected for it by removing susceptible individuals.
ApplyBand 3(1 mark) Q3. Carbon-14 dating would be MOST appropriate for dating:
Answer: B — C-14's effective range is up to ~50,000 years; a 30,000-year-old mammoth bone falls within that range.
ApplyBand 3(1 mark) Q4. Cane toads at the invasion front have longer legs than those in established populations. This is best explained as:
Answer: B — natural selection acting on pre-existing variation in leg length; longer-legged toads had higher dispersal success and reproductive output at the frontier.
EvaluateBand 5(1 mark) Q5. The peppered moth data is considered particularly strong evidence for natural selection because:
Answer: B — the reversal after the Clean Air Act is particularly compelling: the same population responded predictably to both the imposition and removal of the selection pressure.
ApplyBand 4(4 marks) 1. Explain how antibiotic-resistant bacteria are an example of evolution by natural selection. In your answer, identify: the heritable variation, the selection pressure, and the outcome. Explain whether the antibiotic CAUSES the resistance mutation or SELECTS for it.
1 mark: heritable variation (resistance mutations pre-exist) · 1 mark: selection pressure (antibiotic removes susceptible bacteria) · 1 mark: outcome (resistance frequency increases) · 1 mark: antibiotic selects, does not cause — clear distinction
ApplyBand 4(4 marks) 2. Describe how radiometric dating provides evidence for evolution. In your answer, name TWO radiometric dating methods, state the timescales each is suited for, and explain what the resulting dates reveal about the sequence of evolutionary change.
1 mark per method correctly named + timescale stated (2 max); 1 mark: dating confirms simpler organisms are older; 1 mark: transitional fossils appear at predicted timepoints — consistent with evolutionary theory
EvaluateBand 5(5 marks) 3. The cane toad invasion has been described as a "real-time evolution experiment." Using examples from BOTH the cane toad population AND native predator populations, explain how this supports Darwin's Theory of Evolution by Natural Selection. In your answer, identify the selection pressures acting on each population and predict what would happen if cane toads were eradicated.
1 mark: toad evolution (longer legs; selection pressure = dispersal advantage); 1 mark: predator evolution (toxin resistance; selection pressure = toad toxin); 1 mark: both examples correctly linked to heritable variation + differential survival; 1 mark: prediction for eradication (toad → dispersal selection disappears; predator → toxin resistance no longer selected for, frequency may decline over many generations); 1 mark: clear overall explanation of how both changes demonstrate natural selection operating on existing variation
Show all answers
Multiple Choice Answers
Q1: B — allele frequency change in response to identified selection pressure, then reversal. Q2: C — pre-existing resistant mutants survived and reproduced. Q3: B — C-14 is effective up to ~50,000 years; mammoth bone at 30,000 years fits. Q4: B — natural selection on pre-existing leg-length variation at the invasion front. Q5: B — reversal of the change when the selection pressure reversed is especially strong evidence.
Short Answer Model Answers
Q1 (4 marks): The heritable variation is the presence of resistance mutations in some individual bacteria within the population — these mutations arise randomly through DNA replication errors and exist before any antibiotic is used. The selection pressure is the antibiotic: it kills bacteria that lack the resistance gene, but those with the resistance mutation survive, because the mutation disrupts the antibiotic's mechanism of action. These resistant bacteria then reproduce — passing the resistance gene to all offspring. Over many bacterial generations (bacteria reproduce every ~20 minutes, so within months), the frequency of the resistance gene in the population increases dramatically. Critically, the antibiotic does NOT cause the resistance mutation: it selects for mutations that already existed. If all bacteria in a population were susceptible (no resistance mutations), treating with the antibiotic would kill them all and resistance would not develop. The antibiotic simply removes the competition that was keeping the rare resistant mutants at low frequency.
Q2 (4 marks): Method 1 — Carbon-14 (¹⁴C): half-life ~5,730 years; used for organisms that lived up to approximately 50,000 years ago (e.g. mammoth bones, archaeological human remains, charcoal from ancient fires). Method 2 — Potassium-Argon (K-Ar): half-life ~1.3 billion years; used for volcanic rocks and the fossils embedded in them, dating material from hundreds of thousands to hundreds of millions of years old (e.g. the volcanic ash layers surrounding hominin fossils such as Lucy at ~3.2 million years). The dates produced confirm the evolutionary sequence: fossil ages show that simpler organisms (single-celled life, invertebrates, fish) appear in progressively older rock layers, while more complex organisms (amphibians, reptiles, mammals, hominins) appear more recently. Transitional fossils — organisms showing features intermediate between two groups (e.g. Tiktaalik, a fish with limb-like fins) — appear in rock layers dated to the timepoints predicted by evolutionary theory. If species had been created separately and recently, there would be no reason for this pattern; it is exactly what evolution predicts.
Q3 (5 marks): In the cane toad population, heritable variation exists in leg length. At the invasion front, toads with longer legs can travel further per day, reaching unoccupied territory with more food and fewer competitors — a strong selection pressure favouring dispersal ability. Longer-legged individuals reproduce more successfully than shorter-legged ones, so the frequency of the longer-leg alleles increases at the invasion front; studies confirm that invasion-front toads now have measurably longer legs than toads from established populations. In native predator populations (e.g. freshwater crocodiles, some snake species), heritable variation in sensitivity to bufotoxin exists. The selection pressure is the toad toxin itself: predators that eat toads and have genetic variants reducing toxin sensitivity survive and reproduce, while sensitive individuals die. Over the decades since toad arrival, the frequency of toxin-resistance alleles has increased measurably in some high-exposure predator populations. Both changes are natural selection acting on existing heritable variation — differential survival due to a new selection pressure. This supports Darwin's theory because both populations were not "designed" to respond to cane toads; they changed only because individuals with pre-existing advantageous variants survived and reproduced more. If cane toads were eradicated: the dispersal-advantage selection pressure on leg length would disappear in the toad population (though the toads would be gone). In native predator populations, the toxin-resistance selection pressure would be removed; over many generations, if toxin resistance had any cost (energy, trade-offs), its frequency might slowly decline — but it would not immediately disappear, as there is no selection pressure actively removing it.
Five timed questions on antibiotic resistance, cane toad evolution, radiometric dating, and peppered moths. Beat the boss to bank a tier — gold (perfect + fast), silver (80%+), or bronze (cleared).
Enter the arenaClimb platforms using your knowledge of antibiotic resistance, cane toads, fossil dating, and peppered moths. Pool: lessons 1–16.
You were asked how MRSA frequency rose from under 0.2% in 1990 to around 12% in Australian hospitals by 2020 — and whether the antibiotics caused the resistance mutations.
The WHO's 2019 GLASS report documents this 60-fold increase, and the mechanism is pure natural selection, not induced mutation. Resistance mutations arise randomly through DNA replication errors — they existed in the bacterial population before any antibiotic was used. In a large population of bacteria reproducing every 20 minutes, by chance a tiny fraction carry a mutation that happens to block the antibiotic's action. When hospitals used antibiotics routinely, every susceptible bacterium died in each treated patient — but the rare MRSA variants survived and reproduced. Over 30 years and billions of bacterial generations, the resistant allele rose from near-zero to 12% of all hospital S. aureus.
The antibiotic is the selection pressure, not the mutagen. The key test: if you grew bacteria in a completely antibiotic-free environment and then suddenly applied the antibiotic, resistance would still emerge — because some of those bacteria already carry the mutation.