AQA AS Level Biology Paper 2, June 2022: Question 8

7 marks · Medium difficulty · Short Answer

Suggest guppy behavior affected by brightly colored stones, propose a further investigation with a null hypothesis, and explain directional selection caused by predators on guppy spot number.

Practise this question

Question

Three-part question about guppy breeding experiments. Figure 9 shows a graph of mean number of spots per fish increasing over generations. Question 08.1 asks to suggest one type of guppy behavior affected by brightly colored stones. Question 08.2 asks to suggest a further investigation and null hypothesis. Question 08.3 includes Figure 10, a graph showing an increase then decrease in mean number of spots after predators are added, and asks to name the type of selection and explain its effect on allele frequencies.
Question text

08 Guppies are small fish with brightly coloured spots on their body.

The number of spots on a guppy is controlled by genes.

A scientist investigated guppy breeding in a large population of guppies in a fish tank.

The fish tank contained brightly coloured stones. After each generation of breeding,

she counted the number of spots on every guppy.

Figure 9 shows her results.

Figure 9

08.1 The scientist concluded that the mean number of spots on the guppies changed in this

fish tank because the brightly coloured stones had affected the behaviour of the

guppies.

The guppies did not behave aggressively towards each other and their feeding

behaviour did not change.

Suggest one type of guppy behaviour that could be affected by the presence of

brightly coloured stones.

[1 mark]

08.2 Suggest:

• a further investigation the scientist could do to confirm that the brightly coloured

stones had affected guppy behaviour

• a null hypothesis for the new investigation.

[2 marks]

Further investigation

Null hypothesis

08.3 The scientist repeated the investigation of guppy breeding in an identical fish tank, but

added predators of the guppies into the fish tank at the fourth generation of breeding.

Figure 10 shows her results.

Figure 10

Name the type of selection the scientist investigated in this fish tank.

Explain why this selection affected the frequency of alleles in the population of

guppies in this fish tank.

[4 marks]

Type of selection

Explanation

Mark scheme

Show the mark scheme Mark scheme providing acceptable answers for questions 08.1, 08.2, and 08.3, including marking guidance and acceptable alternatives such as mating or courtship behavior, repeating investigation without stones for null hypothesis, and directional selection explanation.

Question Marking Guidance Mark Comments

Mating/courtship/sexual behaviour; 1 Ignore breeding/

08.1 reproduction

(AO2)

1. Repeat (the investigation) without stones Accept

dull/grey/one/same for

OR uniformly coloured

Repeat (the investigation) with uniformly

coloured stones; 2

08.2

(2 x AO3)

2. Presence of stones has no effect on behaviour

OR

Colour of stones has no effect on behaviour`;

1. Directional; 2. 3. and 4. Accept

converse

2. Fish with more spots are more likely to be 2. Accept killed/eaten

predated; for ‘predated’

3. Alleles for (more) spots not passed on; 2. Accept more of

4. (So) frequency of (more) spots alleles them killed/eaten, for

08.3 decreases; ‘more likely’

(4 x AO2)

3. and 4. Reject ‘gene’

once

4. Accept ‘proportion’

for frequency

Ignore ‘number of

alleles decreases’

How to answer it

Guppy Evolution, Selection Pressures, and Experimental Design

What this question tests

This question assesses your understanding of evolutionary mechanisms (specifically natural/directional selection), experimental design principles (controlling variables, forming null hypotheses), and applying biological knowledge (AO2/AO3) to unfamiliar contexts involving animal behaviour and predation.

Question 0.8.1

Identifying Affected Animal Behaviours

✅ Correct Answer

Mating / courtship / sexual behaviour

1 mark (AO2)

💡 Key Knowledge

Brightly coloured spots in guppies are typically evolved through sexual selection to attract mates. Changing the background environment (coloured stones) can disrupt visual signaling required for courtship.

❌ Common Errors

Students often lose this mark by writing vague terms like "breeding" or "reproduction". The mark scheme explicitly states to ignore breeding/reproduction and requires specific behavioural terminology like courtship or mating.

Question 0.8.2

Experimental Design & Null Hypotheses

✅ Correct Answers

Further investigation: Repeat the investigation without stones OR repeat with uniformly coloured (dull/grey/same) stones.

Null hypothesis: Presence of stones (or colour of stones) has no effect on behaviour.

2 marks (2 x AO3)

🧠 Exam Technique

A null hypothesis must always state that there is no significant difference or no effect. Avoid phrasing it as a question or predicting a positive outcome.

Question 0.8.3

Selection Pressures and Allele Frequencies

✅ Correct Answers

1. Type of selection: Directional selection.

2. Explanation: Fish with more spots are more likely to be predated. Therefore, alleles for (more) spots are not passed on, causing the frequency of (more) spots alleles to decrease.

4 marks (4 x AO2)

💡 Key Knowledge

When predators are introduced, highly visible traits (like lots of spots) become a disadvantage. The mechanism of natural selection relies on differential reproductive success: differential predation leads to differential survival and allele propagation.

❌ Common Errors

Examiners note that students frequently confuse "allele frequency" with "number of alleles" or incorrectly use the word "gene" instead of "allele". Make sure to refer specifically to allele frequencies decreasing rather than genes being "lost" entirely.

Topics

Biology · Practical skills · 3.4 Genetic information, variation and relationships between organisms · Experimental design

Question and mark scheme from the AQA AS Level Biology examination, Paper 2, June 2022. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.