Biology • Year 12 • Module 6 • Lesson 19
Variation and Allele Frequency
Apply allele-frequency reasoning to real data, distinguish selection from drift in scenarios, and predict how a gene pool changes under different conditions.
1. Interpret data, a resistance allele under selection
An insect population was repeatedly exposed to a pesticide. The frequency of a pesticide-resistance allele was measured in the population's gene pool over four generations. 7 marks
| Generation | Resistance allele frequency | Pesticide applied? |
|---|---|---|
| 0 | 0.04 (4%) | No (baseline) |
| 1 | 0.18 (18%) | Yes |
| 2 | 0.46 (46%) | Yes |
| 3 | 0.71 (71%) | Yes |
Hypothetical data illustrating selection for a resistance allele.
1.1 Describe the trend in resistance-allele frequency across the four generations. 2 marks
1.2 Calculate the increase in resistance-allele frequency from generation 0 to generation 3, expressed as a percentage point change. Show your working. 2 marks
1.3 Using lesson content, explain why the resistance allele increased so rapidly. Identify the mechanism and why it is non-random. 3 marks
2. Compare and contrast, natural selection vs genetic drift
Complete the table using information from the lesson. Where a cell is already filled, use it as a clue for the adjacent row. 8 marks
| Criterion | Natural selection | Genetic drift |
|---|---|---|
| Cause of change | Selection pressure favours some variants over others | |
| Random or non-random? | Random with respect to fitness | |
| Population size effect | ||
| Link to fitness | Allele frequency changes for a fitness reason | |
| Example | A storm randomly kills many individuals on a small island |
3. Scenario analysis, selection or drift?
Read each scenario and answer the questions. 6 marks
Scenario A. On a Galápagos island, a severe drought leaves only large, hard seeds. Finches with deeper beaks can crack these seeds; shallow-beaked finches starve. Over the next generation, the frequency of deep-beak alleles rises sharply.
Scenario B. A flood washes ten individuals from a beetle population onto a new, empty island. By chance, six of the ten happen to carry a rare green-colour allele, so the new population has a far higher green-allele frequency than the source population, even though green colour gives no survival advantage there.
3.1 Classify Scenario A as natural selection or genetic drift and justify your choice. 2 marks
3.2 Classify Scenario B as natural selection or genetic drift and justify your choice. 2 marks
3.3 State the single key feature that distinguishes the mechanism in A from the mechanism in B. 2 marks
4. Predict and justify, a shrinking population
A wild population of a small mammal falls from 5,000 individuals to about 60 after habitat loss, forming an isolated remnant. 5 marks
4.1 Predict how the strength of genetic drift in this population will change, and explain why. 2 marks
4.2 Predict one consequence of strong drift for the population's genetic diversity, and explain the risk this creates. 2 marks
4.3 Suggest one management action that could restore variation to this gene pool. Justify in one sentence. 1 mark
Q1.1, Trend description (2 marks)
The resistance-allele frequency increases steadily and rapidly across all four generations, from 0.04 at generation 0 to 0.71 at generation 3 [1]. The rate of increase is large in every generation while pesticide is applied, showing a strong directional change [1].
Q1.2, Increase calculation (2 marks)
Working: 0.71 − 0.04 = 0.67, i.e. 71% − 4% = 67 percentage points [1 for correct subtraction; 1 for correct expression as a percentage-point change] [1].
Q1.3, Why the allele rose rapidly (3 marks)
The pesticide is a selection pressure: susceptible insects are killed before reproducing, while resistant insects survive [1]. Because resistance is inherited, resistant survivors leave proportionally more offspring, so the resistance allele's frequency rises in the next generation, repeating each generation [1]. This is natural selection because the change is non-random with respect to fitness, the environment consistently favours the resistant variant, not chance [1].
Q2, Compare-and-contrast table (8 marks, 1 per correct cell)
| Criterion | Natural selection | Genetic drift |
|---|---|---|
| Cause of change | Given (selection pressure favours some variants) | Chance events, especially in small populations [1] |
| Random or non-random? | Non-random with respect to fitness [1] | Given (random with respect to fitness) |
| Population size effect | Operates in populations of any size where a pressure acts [1] | Strongest in small populations; weak in large ones [1] |
| Link to fitness | Given (changes for a fitness reason) | No fitness reason; change is not linked to any advantage [1] |
| Example | Pesticide resistance rises as only resistant insects survive [1] | Given (a storm randomly kills many on a small island) |
Q3.1, Scenario A classification (2 marks)
Scenario A is natural selection [1]. The drought (a selection pressure) consistently favours deep-beaked finches that can crack the available hard seeds; shallow-beaked finches starve, so the change in beak-allele frequency is non-random and tied to a fitness advantage [1].
Q3.2, Scenario B classification (2 marks)
Scenario B is genetic drift (specifically a founder effect) [1]. The ten founders were carried by chance, and the high green-allele frequency arose by random sampling, not because green colour gives any survival advantage on the new island [1].
Q3.3, Key distinguishing feature (2 marks)
The key distinction is whether the allele-frequency change is linked to fitness: in A the change is non-random and driven by differential survival/reproduction (selection), whereas in B the change is random chance unrelated to fitness (drift) [2]. (Award 2 marks for clearly contrasting non-random/fitness-linked with random/chance; 1 mark for a partial statement.)
Q4.1, Change in drift strength (2 marks)
Genetic drift will become much stronger [1], because drift has its greatest effect in small populations: with only ~60 individuals, chance events in who survives and reproduces cause large random swings in allele frequency that would be negligible in a population of 5,000 [1].
Q4.2, Consequence for diversity (2 marks)
Strong drift will randomly fix some alleles and lose others, reducing the population's genetic diversity [1]. Low diversity reduces the population's ability to adapt to future selection pressures (e.g. disease or climate change) and increases the risk of inbreeding and extinction [1].
Q4.3, Management action (1 mark)
Introduce gene flow by translocating individuals from another population, because migration brings in new alleles and restores variation to the depleted gene pool [1]. (Accept any valid action that adds genetic variation, with reasoning.)