Biology • Year 11 • Module 3 • Lesson 16
Modern Examples of Evolutionary Change
Apply the three-condition natural selection framework to data, choose the correct radiometric method for a sample, and reason about observed change in real populations.
1. Complete the natural selection framework table
For each modern example, complete the empty cells using the three-condition framework: heritable variation, selection pressure, and likely outcome. 9 marks, 1 per correct cell
| Example | Heritable variation | Selection pressure | Likely outcome over time |
|---|---|---|---|
| MRSA in a hospital ward | Antibiotic kills susceptible bacteria | ||
| Cane toads at the invasion front | Variation in leg length | ||
| Peppered moth after industrialisation | Dark allele frequency rises |
2. Choose the right dating method
Use the isotope data below to answer the questions. 8 marks
Isotope reference.
| Isotope | Half-life | Effective range |
|---|---|---|
| Carbon-14 (¹⁴C) | ~5,730 years | up to ~50,000 years |
| Potassium-Argon (K-Ar) | ~1.3 billion years | >100,000 years |
| Uranium-Lead (U-Pb) | billions of years | oldest geological specimens |
2.1 A researcher finds a mammoth bone estimated to be about 25,000 years old. Which method should be used, and why? 2 marks
2.2 A hominin fossil is found in a layer of volcanic ash thought to be about 3 million years old. Which method is most appropriate, and why is Carbon-14 unsuitable here? 2 marks
2.3 After one half-life of Carbon-14 (~5,730 years), what fraction of the original parent isotope remains? After two half-lives? 2 marks
2.4 Explain the difference between relative dating and absolute (radiometric) dating in one sentence each. 2 marks
3. Cause-and-effect chain, the peppered moth
Complete the cause-and-effect chain by filling in each empty box. The first box is done for you. 5 marks
Step 1: On lichen-covered pale trees, dark moths were more visible to bird predators, so the dark form was kept rare by _________________________.
Step 2: Industrial soot killed the lichens and blackened the trees, which reversed the camouflage so that now the _________________________ moths were better hidden.
Step 3: Better-camouflaged moths survived and reproduced more, so the dark allele frequency _________________________ to about _________________________.
Outcome: After the Clean Air Act reduced pollution and lichens returned, the selection pressure reversed again and the _________________________ form recovered.
4. Applied scenario, does the antibiotic cause resistance?
Read each scenario and decide whether the reasoning is biologically correct. Explain your answer. 4 marks
4.1 A student says: "The antibiotic damaged the DNA of the bacteria and made them mutate into resistant forms." Is this correct? Explain. 2 marks
4.2 A student says: "If we stopped using all antibiotics tomorrow, every resistant bacterium would instantly become susceptible again." Is this correct? Explain. 2 marks
Q1, Natural selection framework table (marking criteria)
- MRSA: Heritable variation some bacteria carry pre-existing resistance mutations, most do not. Outcome resistant bacteria survive and reproduce, so the frequency of the resistance gene increases.
- Cane toads: Selection pressure advantage of faster dispersal into unoccupied territory at the invasion front. Outcome longer-legged toads dominate the invasion front; average leg length increases.
- Peppered moth: Heritable variation variation in wing colour (pale vs dark/melanic alleles). Selection pressure bird predation on moths that are poorly camouflaged on soot-blackened trees.
Award 1 mark per correctly completed cell. Accept other valid entries supported by lesson content.
Q2.1, Mammoth bone
Carbon-14 [1] because 25,000 years is within Carbon-14's effective range of up to about 50,000 years, and the bone is recent organic material that originally contained carbon [1].
Q2.2, Hominin fossil in volcanic ash
Potassium-Argon (K-Ar) [1], because it dates ancient volcanic rock and suits ages greater than 100,000 years (3 million years is well within range). Carbon-14 is unsuitable because its effective range is only up to about 50,000 years, so at 3 million years almost all the Carbon-14 would have decayed and could not be measured reliably [1].
Q2.3, Half-life fractions
After one half-life, one half (50%) of the parent isotope remains [1]. After two half-lives, one quarter (25%) remains [1].
Q2.4, Relative vs absolute dating
Relative dating uses rock-layer position (stratigraphy) to say one fossil is older or younger than another, but gives no actual age in years [1]. Absolute (radiometric) dating uses the known decay rate of radioactive isotopes to give an actual age in years [1].
Q3, Cause-and-effect chain (marking criteria)
- Step 1: natural selection / bird (predator) predation [1]
- Step 2: dark (melanic) [1]
- Step 3: increased / rose, to about 90% [1 for direction, 1 for ~90% value]
- Outcome: pale (light) [1]
Note: total 5 marks across the four steps as marked above.
Q4.1, Antibiotic damaging DNA (incorrect)
This is incorrect [1]. The antibiotic does not cause resistance mutations. Resistance mutations arise randomly through DNA replication errors and pre-exist in the population before any antibiotic is used. The antibiotic acts only as a selection pressure that removes susceptible bacteria and allows the rare pre-existing resistant individuals to survive and reproduce [1].
Q4.2, Resistance disappearing instantly (incorrect)
This is incorrect [1]. Removing the antibiotic removes the selection pressure, but it does not delete the resistance alleles already present in surviving bacteria, those bacteria keep and pass on the gene. Resistance frequency would only decline slowly over many generations, and only if carrying the resistance gene had some fitness cost when antibiotics are absent [1].