Biology • Year 12 • Module 6 • Lesson 7
Gene Pools — Mutation, Gene Flow and Genetic Drift
Apply gene-pool reasoning to real founder-effect and bottleneck case studies, interpret allele-frequency-versus-generation data, and detect Hardy–Weinberg deviation in a population.
1. Case study — founder effect on a Pacific atoll
An isolated Pacific atoll was repopulated in the early 1800s after a typhoon reduced the surviving population to about 20 individuals. Today, the descendant population of roughly 700 shows an unusually high frequency of a recessive allele m (associated with reduced colour vision) compared with neighbouring source populations. 7 marks
| Population | Size | Frequency of allele m | Notes |
|---|---|---|---|
| Source mainland | ≈ 60 000 | 0.02 | Long-established |
| Atoll, pre-typhoon (est.) | ≈ 1 000 | 0.02 | Comparable to mainland |
| Atoll, post-typhoon survivors | ≈ 20 | 0.20 | One survivor heterozygous for m by chance |
| Atoll, present day | ≈ 700 | 0.21 | Population has recovered, frequency persists |
1.1 Identify the process that best explains the rise of allele m on the atoll, and name the specific sub-type of that process. 2 marks
1.2 Using the data, explain why the post-typhoon frequency of m jumped from 0.02 to 0.20 even though the allele was not advantageous. 3 marks
1.3 Predict the long-term effect on m if (a) regular boat traffic to the mainland began, and (b) population size remained around 700 with no migration. Briefly justify each prediction. 2 marks
2. Interpret graph — bottleneck and loss of allelic diversity
The figure below shows the number of distinct alleles at a sample of 20 polymorphic loci in a large mammal species across time. The species experienced a sharp population bottleneck around 12 000 years ago. 6 marks
Stylised allelic-diversity record — illustrative of the bottleneck pattern described in Card 3 of the lesson.
2.1 Describe what happens to mean alleles per locus before, during and after the bottleneck event. Quote at least one value from the graph. 2 marks
2.2 Explain why a sharp population reduction reduces the number of alleles in the gene pool, even when the bottleneck is short-lived. 2 marks
2.3 The species has now recovered numerically to ~50 000 individuals, yet mean alleles per locus have only risen from ~2 to ~2.5. Explain why population size recovery does not automatically restore allelic diversity, naming the process that could add new alleles back into the gene pool over time. 2 marks
3. Detect Hardy–Weinberg deviation in a population
The Hardy–Weinberg equilibrium model predicts that, for a single gene with two alleles A (frequency p) and a (frequency q) in a population not undergoing evolution, the genotype frequencies will be p2 + 2pq + q2 = 1. Any major deviation between observed and expected genotype frequencies is evidence that at least one of the H–W assumptions — no mutation, no gene flow, no genetic drift, no selection, random mating, large population — has been broken. 8 marks
Stimulus. An ecologist samples a small island population of 200 lizards for a single gene with two alleles, A and a. The observed genotype counts are:
| Genotype | Observed count | Observed frequency |
|---|---|---|
| AA | 98 | 0.49 |
| Aa | 44 | 0.22 |
| aa | 58 | 0.29 |
| Total | 200 | 1.00 |
From the observed counts, calculate that p (frequency of A) = 0.60 and q (frequency of a) = 0.40.
3.1 Using p = 0.60 and q = 0.40, calculate the expected Hardy–Weinberg genotype frequencies for AA, Aa and aa. Show your working. 3 marks
3.2 Compare each observed genotype frequency with the expected Hardy–Weinberg value. State whether each genotype is in excess, deficit or close to expected. 2 marks
3.3 The population is small, isolated, and ecologists report that a typhoon four generations ago killed ~80% of individuals at random. Using two of the lesson's three processes (mutation, gene flow, genetic drift), justify which is the most likely explanation for the deviation observed in 3.2. 3 marks
4. Apply — which process dominates in each population?
For each of the three populations below, decide which of mutation, gene flow or genetic drift is likely to have the strongest effect on allele frequencies over the next 10 generations. Justify in 1–2 sentences using lesson terms. 6 marks (2 marks per population)
4.1 Population P — 30 fruit-fly individuals trapped on a tiny offshore islet after a flood. No migration in or out. No unusually high mutation rate.
4.2 Population Q — 200 000 mainland deer with a neighbouring 150 000-deer population separated only by a low ridge. Tracking collars confirm ~5% of bucks cross the ridge each year and breed with the other population.
4.3 Population R — 80 million bacteria of a single species in a chemostat for 1 000 generations. No immigration. Continuous DNA-replication errors at a measurable rate.
Q1.1 — Process and sub-type (2 marks)
Genetic drift [1], specifically the founder effect [1] — a small group (~20 survivors) re-established the population, so their allele frequencies set the baseline for the recovered population.
Q1.2 — Why m jumped from 0.02 to 0.20 (3 marks)
When the typhoon reduced the population to ~20 survivors, the surviving 40 allele copies at this locus were a small chance sample of the pre-typhoon gene pool [1]. By chance, one survivor happened to be heterozygous for m, contributing 1 copy out of 40 — but a small population means that one carrier represents a much larger proportion of the gene pool (≈ 0.20 rather than ≈ 0.02) [1]. There is no need to invoke advantage or selection — the change is consistent with chance sampling in a tiny population, which is the definition of drift [1].
Q1.3 — Long-term predictions (2 marks)
(a) With regular boat traffic and inter-breeding, gene flow from the mainland would deliver alleles at near-mainland frequencies into the atoll gene pool, and the frequency of m on the atoll would gradually fall toward the source value of ~0.02 [1].
(b) Without migration and with N≈700, m would likely remain elevated relative to the mainland because no process is acting to reduce it; further drift in small subgroups could push the frequency up or down stochastically [1].
Q2.1 — Trend description (2 marks)
Before the bottleneck (15 000–13 000 years ago), mean alleles per locus is stable at about 7. During the bottleneck (~12 000 years ago) it falls sharply to about 2 (a ~70% loss of allelic diversity). After the bottleneck and through to the present day it recovers only slightly, ending at ≈ 2.5 alleles per locus. [1 trend before/during, 1 trend after with a value]
Q2.2 — Why a sharp reduction removes alleles (2 marks)
A sharp reduction in population size means the surviving individuals are a small chance sample of the original gene pool [1]. Any allele that happened not to be carried by a survivor is permanently lost from the gene pool, so allelic diversity drops; this is a form of genetic drift (the bottleneck effect) [1].
Q2.3 — Why numerical recovery does not restore diversity (2 marks)
Once an allele is lost from a gene pool, neither gene flow (no other source population exists in this case) nor population growth can bring it back — population growth only multiplies the alleles already present [1]. Only mutation can introduce genuinely new alleles, and the mutation rate per locus per generation is small, so allelic diversity recovers very slowly [1].
Q3.1 — Expected H–W genotype frequencies (3 marks)
p2 = (0.60)2 = 0.36 [1]
2pq = 2 × 0.60 × 0.40 = 0.48 [1]
q2 = (0.40)2 = 0.16 [1]
(Check: 0.36 + 0.48 + 0.16 = 1.00 ✓.)
Q3.2 — Observed vs expected (2 marks)
AA: observed 0.49 vs expected 0.36 → AA is in excess.
Aa: observed 0.22 vs expected 0.48 → Aa is in strong deficit (less than half expected).
aa: observed 0.29 vs expected 0.16 → aa is in excess.
[1 mark for AA + aa excess, 1 mark for Aa deficit. Accept any equivalent comparison wording.]
Q3.3 — Most likely explanation (3 marks)
The population is small and isolated, and a recent typhoon killed ~80% of individuals at random — the textbook conditions for a bottleneck. Genetic drift is therefore the most likely explanation: random sampling at the typhoon strongly altered allele and genotype frequencies, and ongoing drift in the small post-bottleneck population continues to push the population away from H–W expectations (which assume a large population with random mating) [1]. Gene flow cannot explain the deviation because the population is described as isolated, so no alleles are being transferred in from elsewhere [1]. Mutation on its own changes allele frequencies very slowly per generation and cannot produce a heterozygote deficit this large in only four generations, so mutation is not the dominant driver here [1]. (Selection would also explain the pattern if there were a fitness reason; the lesson is restricted to mutation / gene flow / drift, and the stimulus gives random mortality.)
Q4.1 — Population P (2 marks)
Genetic drift dominates [1]. The population is very small (N = 30) and isolated, so chance variation in who reproduces each generation has a large proportional effect on allele frequencies — exactly the conditions described in Card 3 [1].
Q4.2 — Population Q (2 marks)
Gene flow dominates [1]. The population is large (so drift is weak per generation in proportional terms), and ~5% of bucks crossing and breeding each year is a substantial migration rate — this transfers alleles between gene pools and tends to homogenise allele frequencies between the two populations [1].
Q4.3 — Population R (2 marks)
Mutation dominates as the source of new variation [1]. With 80 million bacteria reproducing for 1 000 generations and a measurable per-base replication-error rate, new alleles will arise repeatedly; gene flow is impossible (no immigration) and drift is weak in such a huge population, so mutation is the only process able to produce ongoing novelty in the gene pool [1].