AQA AS Level Biology Paper 2, November 2020: Question 1

Mark scheme

Show the mark scheme

Question Marking Guidance Mark Comments

01.1 1. Select at random; 2 1. Accept description

of random

2. Large sample/number (of snails)/>10;

technique

2. Ignore ‘many’

01.2 Mean and standard deviation; 1 Accept 2 correct

statistical values for 1

mark

Accept any order

Accept average for

mean

Accept standard error

for standard deviation

Accept interquartile

range

01.3 1 Accept phonetic

(Littorina -) genus and

spelling

(littorea -) species;

Accept genera

01.4 1. No (significant) difference between (mean) 2 Accept suitable null

height (in these populations); hypothesis

2. (Students) t-test;

TOTAL 6

How to answer it

Investigation of Variation in Snail Shell Height

What this question tests

This question assesses core practical and analytical competencies required at AS Level Biology. It tests your understanding of sampling methodologies, descriptive statistics, biological nomenclature (binomial naming system), formulating null hypotheses, and selecting appropriate statistical tests for comparing data sets.

Question 01.1

Ensuring Reliable Samples

Give two ways in which the student could ensure his samples would provide a reliable measure of the variation between individuals in each population. [2 marks]

✅ Correct Answers

Select snails at random.
Use a large sample size (e.g., number of snails greater than 10).

🧠 Exam Technique

Random selection: Prevents sampling bias (avoiding picking only the largest or most visible snails). Accept a description of a random technique, such as using a random number generator to coordinate a grid.
Sample size: Examiners strictly look for a large sample or a specific numerical threshold (>10). Vague terms like "many" or "lots" will be denied marks.

❌ Common Errors

Writing "take a fair test" or "keep variables the same"—these are irrelevant for observational population variation studies.
Using vague quantities like "a few" or "several" instead of large sample parameters.

Mark breakdown: 1 mark for random selection + 1 mark for large sample size/number > 10.

Question 01.2

Statistical Values

The student could determine the median, mode and range from his measurement of shell heights in these populations. Give two other statistical values the student could calculate from his measurement of shell heights in these populations. [1 mark]

✅ Correct Answers

Mean
Standard deviation (or standard error)

💡 Key Knowledge

When data is continuous (like shell height), measures of spread and central tendency beyond basic counts are required.
Acceptable alternatives include interquartile range . Note that "average" is accepted for mean, but exact statistical terminology is preferred.

❌ Common Errors

Repeating terms already given in the stem (median, mode, range).
Listing non-statistical calculations like percentages or ratios.

Mark breakdown: 1 mark total for stating both correct statistical values (mean and standard deviation).

Question 01.3

Binomial Nomenclature

Name the taxon in the hierarchy of classification represented by: 1. Littorina, 2. littorea [1 mark]

✅ Correct Answers

1. Littorina = Genus
2. littorea = Species

💡 Key Knowledge

Binomial nomenclature uses a two-part naming system: the first word indicates the genus (always capitalised) and the second indicates the species (lowercase).
Phonetic spellings are typically accepted by examiners as long as the intent is clear.

❌ Common Errors

Confusing genus with family or domain.
Writing "specific epithet" instead of species (though scientifically correct, stick to the primary curriculum term: species).

Mark breakdown: 1 mark for correctly identifying both genus and species.

Question 01.4

Hypothesis Testing & Statistics

The student noticed there was a difference in shell height between these populations of snails. He wanted to investigate if the difference was significant. Give a suitable null hypothesis to use in his investigation and name the statistical test to use with these data. [2 marks]

✅ Correct Answers

Null hypothesis: There is no (significant) difference between the mean shell height in these two populations.
Statistical test: (Students) t-test.

🧠 Exam Technique

Null Hypothesis phrasing: Must explicitly state that there is no significant difference (do not say "no correlation" or "no association", as this is a comparison of means between populations). Including the word "mean" strengthens the answer.
Statistical test selection: Use a t-test when comparing two means from independent samples/populations.

❌ Common Errors

Failing to include the word "significant" in the null hypothesis.
Stating an experimental hypothesis instead of a null hypothesis.
Suggesting chi-squared (used for categorical/counted data) or Spearman's rank (used for correlations) instead of a t-test.

Mark breakdown: 1 mark for a valid null hypothesis + 1 mark for naming the Student's t-test.

Topics

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