AQA A-level Biology: Populations, Hardy-Weinberg, Selection and Speciation (7402) – Practice Questions

These are original questions written for Marlbridge, for revision and practice on this content. They are not reproduced past-paper questions, and they do not replicate the exam’s exact structure, question count or mark tariffs – examination boards hold copyright in their own papers. Use these alongside the official past papers from your board or school.

These questions cover AQA A-level Biology (7402) sections 3.7.2 and 3.7.3 (A-level only), from the AQA specification for first teaching 2015. Read the study guide and revision notes first. Course pages: the AQA A-level Biology hub, the printable checklist and the free diagnostics.

Questions

1. Define the terms gene pool and allele frequency. [2]

2. State three conditions that must apply for the Hardy-Weinberg principle to hold. [3]

3. In a population of 10 000 fish, 144 lack scales. Scalelessness is caused by a recessive allele.

(a) Calculate the frequency of the recessive allele. [2] (b) Calculate the number of fish expected to be heterozygous. [2]

4. A population of 300 snails has 180 BB, 90 Bb and 30 bb individuals.

(a) Calculate the frequency of the B allele. [2] (b) Calculate the number of Bb snails expected if the population were in Hardy-Weinberg equilibrium, and suggest what the difference from the observed number shows. [2]

5. Explain why siblings from the same parents may differ in phenotype. [4]

6. A population of 2000 insects has a recessive allele r at frequency 0.4. A pesticide kills every RR insect before breeding. Rr and rr insects survive and breed.

(a) Calculate the number of RR, Rr and rr insects before spraying. [2] (b) Calculate the frequency of r among the survivors. [3] (c) Name this type of selection. [1]

7. A population of 20 lizards on a small island has allele T at a frequency of 0.30. In the next generation T is present as 7 of 40 gene copies.

(a) Calculate the change in the frequency of T. [1] (b) The same loss of 5 gene copies happens in a mainland population of 20 000 lizards. Calculate the change in frequency there. [2] (c) Explain why genetic drift is important only in small populations. [2]

8. Name the type of selection described in each case.

(a) In a plant, both very early and very late flowering individuals are eaten by seed predators, and intermediate flowering time is favoured. [1] (b) In a fish, small and large individuals both breed successfully but medium-sized fish are caught by a predator. [1] (c) In a bird, a colder climate favours larger body size, and the mean size increases over generations. [1] (d) Explain how selection in (c) leads to a change in allele frequency. [3]

9. Explain how a population of mice split by a newly formed mountain range could give rise to two species. [6]

10. Compare allopatric and sympatric speciation. [4]

11. The table shows the frequency of allele a in two populations of the same species over five generations.

Generation	1	2	3	4	5
Population X (about 5000 individuals)	0.50	0.50	0.49	0.50	0.50
Population Y (10 individuals)	0.50	0.35	0.45	0.20	0.00

(a) Describe the changes shown by Population Y. [2] (b) Explain the difference between the two populations. [3] (c) Suggest what the data show about the allele in Population Y at generation 5 and what that means for future generations. [2]

12. Explain how natural selection and isolation over a long period have produced the great diversity of species. [4]

Answers

1. Gene pool: the complete set of alleles, for every gene, in a population at a given time [1]. Allele frequency: how often an allele occurs in the gene pool (as a proportion or percentage) [1]. [2] Examiner insight: Both definitions need the population and the time reference. Writing “all the genes” instead of “all the alleles” is a common slip.

2. Three from: large population [1]; random mating [1]; no mutation [1]; no migration or gene flow; no natural selection (all genotypes equally likely to survive and reproduce). Each correct condition earns a mark, to a maximum of three. [3] Examiner insight: Give three separate conditions. “Closed population” and “no migration” are one point, not two.

3. (a) q² = 144 ÷ 10 000 = 0.0144 [1]; q = 0.12 [1]. (b) p = 1 − 0.12 = 0.88; 2pq = 2 × 0.88 × 0.12 = 0.2112 [1]; 0.2112 × 10 000 = 2112 fish [1]. [4] Examiner insight: Start from the recessive phenotype, because only it equals q². Show the square root, since a bare answer leaves nothing to credit if it is wrong.

4. (a) B alleles = (2 × 180) + 90 = 450 [1]; p = 450 ÷ 600 = 0.75 [1]. (b) q = 0.25; expected Bb = 2 × 0.75 × 0.25 × 300 = 112.5 (about 113) [1]. Observed 90 is fewer, so there is a deficit of heterozygotes; the population is not in equilibrium, which suggests a condition such as random mating is not met [1]. [4] Examiner insight: Link the numerical difference to a named condition. “It is not in equilibrium” with no reason gains less than a reason.

5. Genetic: different combinations of alleles from meiosis through crossing over and independent segregation [1]; random fertilisation means any sperm fuses with any egg [1]; mutation may have produced new alleles in a gamete [1]. Environmental: differences in diet, disease or other conditions affect how genes are expressed, so the phenotype reflects both genotype and environment [1]. [4] Examiner insight: Name a genetic cause and an environmental cause. Repeating “genes and environment” without naming processes is vague.

6. (a) p = 0.6; RR = 0.36 × 2000 = 720 [1]; Rr = 2 × 0.6 × 0.4 × 2000 = 960 and rr = 0.16 × 2000 = 320 [1]. (b) Survivors = 960 + 320 = 1280 [1]; r alleles = 960 + (2 × 320) = 1600 of 2560 [1]; frequency = 0.625 [1]. (c) Directional selection [1]. [6] Examiner insight: Count alleles, not individuals. Dividing 1600 by 1280 gives a value above 1, which should alert you to the error.

7. (a) 7 ÷ 40 = 0.175; change = 0.30 − 0.175 = 0.125 (a decrease) [1]. (b) The mainland population has 40 000 gene copies [1]; 5 ÷ 40 000 = 0.000 125 [1]. (c) Chance events affect a few copies, which is a large share of a small gene pool, so frequencies change a lot or an allele is lost [1]. In a large population the same chance events are a tiny share and average out [1]. [5] Examiner insight: Support the explanation with the figures. Saying “small populations have fewer alleles” does not explain why chance matters.

8. (a) Stabilising [1]. (b) Disruptive [1]. (c) Directional [1]. (d) Large birds are more likely to survive and reproduce in the cold [1]; they pass on the alleles for large size to offspring [1]; the frequency of these alleles in the gene pool rises across generations [1]. [6] Examiner insight: An explanation of selection should end with the allele frequency rising, not just “large birds survive”.

9. The mountain range is a geographical barrier, so there is no gene flow between the two populations [1]. Mutations arise independently in each [1]. The environments differ, so natural selection favours different alleles in each [1]. Genetic drift may also change allele frequencies in either population [1]. Over many generations the gene pools accumulate differences [1]. The populations can no longer interbreed to produce fertile offspring, so they are now two species [1]. [6] Examiner insight: Allopatric speciation is a sequence. Name the barrier, the lack of gene flow, the different selection, and finish with the failure of fertile interbreeding.

10. Allopatric: populations are separated by a geographical barrier [1]. Sympatric: the populations share the same area [1]. Allopatric isolation comes from physical separation, while sympatric speciation needs another barrier, such as different breeding times, behaviour or a change in chromosome number [1]. In both, gene pools diverge until members cannot interbreed to produce fertile offspring [1]. [4] Examiner insight: A comparison needs a similarity or difference stated in the same sentence for each feature. Two lists with no link tend to gain less.

11. (a) Allele a falls overall from 0.50 to 0.00 [1]; the change is irregular, with a rise in generation 3 (0.35 to 0.45) [1]. (b) Population Y is small, so chance events have a large effect on allele frequency (genetic drift) [1]. In Population X (large) chance effects average out, so the frequency stays near 0.50 [1]. Drift is random, so it shows no steady direction [1]. (c) Allele a has been lost from Population Y [1]; it cannot reappear unless by mutation or migration, so the population is now fixed for the other allele [1]. [7] Examiner insight: Quote figures in a describe answer. A reference to a size difference alone does not explain the pattern; link the small size to chance.

12. Variation arises by mutation, meiosis and random fertilisation [1]. Selection pressures such as predation, disease or competition favour some phenotypes, so favourable alleles become more frequent [1]. Where populations are isolated, their gene pools diverge by different selection and drift [1]. Over a long time, repeated cycles of divergence lead to groups that cannot interbreed to produce fertile offspring, so new species arise from existing species, increasing diversity [1]. [4] Examiner insight: Use the words “allele frequency”, “isolated” and “fertile offspring”. An answer that says species “adapted to their environment” does not describe a mechanism.

Where marks are usually lost

• Describing evolution as something individuals do rather than a change in a population’s allele frequencies.
• Using the dominant phenotype as q² instead of the recessive.
• Counting individuals instead of alleles when finding p and q.
• Giving a calculated frequency without a final answer, or mixing frequencies with numbers.
• Not naming a broken condition when observed genotypes differ from expected.
• Leaving out the passing on of the allele in a natural selection explanation.
• Describing drift as if it improves survival.
• Omitting “fertile” from the end point of speciation.

Next steps

• Re-read the revision notes and the study guide.
• Visit the AQA A-level Biology hub and the printable checklist.
• Take one of the free 10-minute diagnostics.
• Book a free trial class.

Official syllabus

AQA AS and A-level Biology specification (7401 and 7402), for first teaching 2015, published by AQA: sections 3.7.2 and 3.7.3.