A-Level · Biology · AQA · Mark scheme decoded

AQA A-Level Biology: Genetic Diversity and Natural Selection — mark scheme explained

Machine-verifiedchecked against the AQA A-Level Biology specificationlast verified 3 July 2026

The short answer

Understanding genetic diversity and natural selection is fundamental to grasping the mechanisms of evolution. This section delves into how genetic variation within a population enables natural selection, leading to the adaptation and survival of species over time.

The question

A population of bacteria is exposed to an antibiotic. Initially, the frequency of the resistant allele (R) is 0.1. After several generations, the frequency increases to 0.8. Explain how this change can be attributed to natural selection.

[Paraphrased for study — not reproduced from any exam paper.]

4 marks

Mark scheme, decoded

How the examiner actually awards the marks on this topic.

Gradora's own decode of the marking approach — not the exam board's published mark scheme.

How marks are awarded

For conceptual questions, marks are typically awarded for clear and concise explanations that demonstrate a deep understanding of genetic diversity and natural selection. For data interpretation questions, marks are given for accurately analyzing the data and relating it to the principles of natural selection. Always provide specific examples when required.

What the command words demand

Explain: Provide a detailed description of the concept, including relevant principles and relationships.
Describe: Give a clear account of the process or phenomenon without necessarily providing an explanation.
Analyze: Examine and interpret data or graphs to draw conclusions about the behavior of the system.
Predict: Use given information to forecast future outcomes based on the principles of natural selection.

Model answer

A full-mark response to the question above, worked through step by step.

Timing: Allocate approximately 5-7 minutes per question to ensure you have enough time to carefully read the problem, analyze data if provided, and construct a well-supported answer.

Identify the initial and final frequencies of the resistant allele (R).0 marks
Initial frequency of R = 0.1Final frequency of R = 0.8
Note that the resistant allele arose by random mutation before antibiotic exposure, and explain how the antibiotic affects the population.", "The resistant allele (R) was already present in the population due to a random mutation that occurred before exposure; the antibiotic does not create or direct resistance but acts as a selection pressure.2 marks
The antibiotic kills bacteria that do not have the resistant allele, reducing their reproductive success.
Describe the impact on the resistant allele.1 mark
Bacteria with the resistant allele (R) survive and reproduce more successfully, passing the allele to the next generation.
Conclude how natural selection leads to an increase in the frequency of the resistant allele.1 mark
Over several generations, the frequency of the resistant allele increases from 0.1 to 0.8 due to directional selection favoring antibiotic resistance.

Final answer: The frequency of the resistant allele (R) increased from 0.1 to 0.8 due to natural selection favoring antibiotic resistance.

Work through every step correctly and you earn all 4 marks.

Another worked example

A population of birds has a range of beak sizes. In a drought, birds with larger beaks are better able to crack open tough seeds and survive. Explain how this scenario exemplifies directional selection.

4 marks

Identify the environmental change and its impact on the population.1 mark
The drought makes tough seeds more common, favoring birds with larger beaks.
Describe the survival advantage of birds with larger beaks.1 mark
Birds with larger beaks can crack open tough seeds more easily, leading to higher survival rates and reproductive success.
Explain how this leads to directional selection.1 mark
Over time, the frequency of alleles for larger beaks increases in the population as these birds have a survival advantage.
Conclude with an explanation of directional selection.1 mark
This scenario exemplifies directional selection because it favors one extreme (larger beaks) over the other (smaller beaks).

Final answer: The drought favors birds with larger beaks, leading to an increase in the frequency of alleles for larger beaks through directional selection.

Work through every step correctly and you earn all 4 marks.

Common mistakes

Confusing genetic diversity with genetic drift.
Why it happens: Students sometimes mix up the concepts of genetic diversity and genetic drift, leading to incorrect explanations in problems involving natural selection.
Fix: Remember that genetic diversity refers to the variety of alleles in a population, while genetic drift is a random change in allele frequencies due to chance events.
Forgetting that mutations are random and not directed by need.
Why it happens: Students may mistakenly believe that mutations occur because an organism needs them, rather than understanding that they are random events.
Fix: Always emphasize that mutations are random changes in DNA and do not occur to meet specific needs of the organism.
Confusing directional selection with stabilizing selection.
Why it happens: Students sometimes mix up the definitions and effects of directional and stabilizing selection, leading to incorrect answers in problems involving these types of selection.
Fix: Practice distinguishing between directional selection (favoring one extreme) and stabilizing selection (favoring intermediate forms). Use examples to reinforce the differences.
Failing to link genetic diversity to evolutionary outcomes.
Why it happens: Students may not fully understand how genetic diversity enables natural selection and leads to evolutionary changes over time.
Fix: Practice explaining the importance of genetic diversity in providing a range of traits that can be selected for or against, leading to adaptation and evolution.
Incorrectly interpreting data on allele frequencies.
Why it happens: Students may misinterpret data showing changes in allele frequencies over time, leading to incorrect conclusions about natural selection.
Fix: Practice analyzing data sets and graphs that show allele frequency changes. Relate these changes to the principles of natural selection and explain how they indicate evolutionary processes.
Failing to provide specific examples when explaining types of selection.
Why it happens: Students may struggle to provide concrete examples of directional or stabilizing selection, leading to vague or incorrect explanations.
Fix: Practice providing specific examples for each type of selection. For instance, use antibiotic resistance in bacteria for directional selection and human birth weights for stabilizing selection.

Where the marks go

The question types you’ll meet on this topic and the marks each one carries — so you know what to expect and where to focus.

Question type	What you’re asked to do	Marks
Natural Selection Explanation	Explain how natural selection increased the resistant allele frequency in a bacterial population.	4
Directional Selection Explanation	Explain how a drought scenario drives directional selection towards larger beak size in birds.	4
Stabilizing Selection Explanation	Explain how selection against extreme birth weights favours intermediate phenotypes and reduces variation.	4
Directional Selection	Explain how selection pressure shifts allele frequencies and predict its impact on genetic diversity.	5
Total across these question types		17

Question types and mark tariffs are Gradora’s guidance based on how this topic is typically examined — not the board’s official paper structure.