AQA GCSE Combined Science: Trilogy Biology Paper 2 (Higher), 2023: Question 2

8 marks · High Demand difficulty · Extended Answer

Explain why the percentage of extinct species is only an estimate, identify the evidence used to study extinct species, and explain how the evolution of antibiotic-resistant C. difficile can be slowed down.

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Question

Question text

02 It is estimated that 99.9% of all species that have ever existed are now extinct.

02.1 Why is the percentage of species that are extinct only an estimate?

[1 mark]

Tick ( ) one box.

All individuals of one species have the same genes.

Extinction is always caused by humans.

Humans have not found evidence of every species.

02.2 What evidence is used to study species that have become extinct?

[1 mark]

02.3 A bacterium called Clostridioides difficile (C. difficile) can infect the

human digestive system.

C. difficile can multiply and produce toxins. The toxins cause diarrhoea.

Doctors are concerned that new strains of C. difficile may evolve.

Antibiotics may not be able to kill these new strains.

Explain how the evolution of antibiotic resistant C. difficile can be slowed down.

[6 marks]

Mark scheme

Show the mark scheme

Question 2

AO /

Question Answers Extra information Mark

Spec. Ref.

02.1 humans have not found 1 AO3

evidence of every species 4.6.3.2

4.6.3.3

AO /

Spec. Ref.

02.2 fossils allow fossil record 1 AO1

allow remains of organisms 4.6.3.1

allow DNA 4.6.3.2

do not accept fossil fuels 4.6.3.3

AO /

Question Answers Mark

Spec. Ref.

Level 3: Relevant points (reasons / causes) are identified, given in

02.3 5–6 AO2

detail and logically linked to form a clear account.

Level 2: Relevant points (reasons / causes) are identified, and 3–4 AO2

there are attempts at logical linking. The resulting account is not

fully clear.

Level 1: Points are identified and stated simply, but their relevance 1–2 AO1

is not clear and there is no attempt at logical linking.

No relevant content 0

4.6.3.4

Indicative content

4.6.3.1

Methods of reducing rate of evolution and linked explanation

4.6.2.2

• doctors should not prescribe antibiotics inappropriately

o so fewer C. difficile are exposed to antibiotic(s)

• do not use antibiotics to treat mild (bacterial) infection

o because the immune system can respond (to mild bacterial

infection)

• do not use antibiotics to treat (any) viral infections

o because antibiotics do not kill viruses

• patients should complete their course of antibiotics

o so (more likely that) all C. difficile are killed

o so none survive to mutate (and form resistant strains)

• the agricultural use of antibiotics should be restricted

o so fewer C. difficile are exposed to antibiotic(s)

10 • hand washing after going to toilet

o will reduce spread of C. difficile

• people with diarrhoea / C. difficile should stay away from school /

work

o to reduce spread of C. difficile

• develop new antibiotic against C. difficile

o so all C. difficile are killed

o so none survive to mutate and form (another) resistant

strain

• develop vaccine against C. difficile

o would decrease the use of antibiotics

For Level 3, answers must refer to method(s) and linked

explanation(s).

Total Question 2 8

How to answer it

Extinction Evidence and Antibiotic Resistance

What this question tests

Why estimates of extinct species are uncertain.
What evidence scientists use to study extinct organisms.
How antibiotic-resistant bacteria evolve and how to reduce the rate of this evolution.
How to build a 6-mark answer using linked ideas: method + explanation + effect.

Question 02.1 • 1 mark

Part (a): Why is the extinction percentage only an estimate?

✅ Correct answer

Tick: Humans have not found evidence of every species.

1 mark for selecting the correct box.

💡 Key knowledge

Scientists cannot know the exact percentage of extinct species because not every species that has ever lived has been discovered or recorded.

🧠 Exam technique

For a multiple-choice question, remove statements that are clearly wrong:

“Extinction is always caused by humans” is false.
“All individuals of one species have the same genes” is false.
The remaining option matches why the value is an estimate.

❌ Common errors

Choosing the genes option because it sounds scientific.
Thinking all extinctions are due to humans.
Forgetting the word estimate means there is incomplete evidence.

Question 02.2 • 1 mark

Part (b): What evidence is used to study extinct species?

✅ Correct answer

Fossils

1 mark for fossils .

💡 Key knowledge

Fossils provide evidence about organisms that lived in the past. This is called the fossil record.

Credit can be linked to remains of organisms or DNA in some cases, but the direct accepted answer here is fossils .

🧠 Exam technique

For a 1-mark recall question, give the exact scientific term and keep it short.

❌ Common errors

Writing fossil fuels — the mark scheme says this is not accepted.
Giving a vague answer like “rocks” instead of fossils.

Question 02.3 • 6 marks

Part (c): Slowing the evolution of antibiotic-resistant C. difficile

This is a level-of-response question, so linked explanations matter.

💡 Key knowledge

Bacteria can mutate.
Some mutations may make bacteria resistant to an antibiotic.
If antibiotics are used a lot, resistant bacteria are more likely to survive and reproduce.
Reducing unnecessary antibiotic use lowers selection pressure.
Reducing spread of infection means fewer bacteria and fewer opportunities for resistant strains to spread.

🧠 Exam technique

To reach Level 3 (5–6 marks) , do not just list ideas. You must link each method to why it slows evolution or spread.

Strong structure:

Method → why it helps → effect on resistant strains

✅ Model full-mark answer

Doctors should not prescribe antibiotics unnecessarily, so fewer C. difficile bacteria are exposed to antibiotics. This reduces selection pressure, so resistant bacteria are less likely to survive and reproduce.

Antibiotics should not be used to treat viral infections because antibiotics do not kill viruses. They should also not be used for mild infections if the immune system can deal with them, because this avoids unnecessary exposure of bacteria to antibiotics.

Patients should complete the full course of antibiotics so all the C. difficile are more likely to be killed. If treatment is stopped early, some bacteria may survive and resistant strains could remain and reproduce.

Good hygiene, such as hand washing after using the toilet, and keeping infected people away from school or work, reduces the spread of C. difficile. This means fewer bacteria are passed on and fewer resistant strains can spread.

The use of antibiotics in agriculture should be restricted so fewer bacteria are exposed to antibiotics. New antibiotics or a vaccine could also help, because they would kill the bacteria or reduce the need to use antibiotics.

This answer gains high marks because it gives several valid methods and links each one to how resistance evolution or spread is reduced.

✅ Points accepted by the mark scheme

Do not prescribe antibiotics inappropriately.
Do not use antibiotics for mild bacterial infections if the immune system can respond.
Do not use antibiotics for viral infections.
Patients should complete the full course of antibiotics.
Restrict agricultural use of antibiotics.
Hand washing after going to the toilet.
People with diarrhoea / C. difficile should stay away from school or work.
Develop new antibiotics against C. difficile.
Develop a vaccine against C. difficile.

❌ Common errors

Just listing “wash hands, use antibiotics properly” with no explanation.
Saying antibiotics kill viruses.
Saying bacteria become resistant because they choose to or get used to antibiotics.
Missing the idea of selection pressure — antibiotics kill non-resistant bacteria, leaving resistant ones to survive.
Talking only about curing the patient, not about slowing evolution of resistance.

🧠 Examiner insight

Top-level responses included methods plus linked reasons.
Students lost marks when they gave separate bullet-point facts with no clear chain of logic.
The best answers explained that reducing antibiotic use means fewer bacteria are exposed, so resistant ones are less likely to be selected.
Including both preventing unnecessary use and reducing spread makes the answer broader and stronger.

📐 How to build your 6-mark answer step by step

State one valid method, for example: doctors should not prescribe antibiotics unnecessarily.
Link it to exposure: fewer C. difficile are exposed to antibiotics.
Link that to evolution: there is less selection for resistant bacteria.
Add a second method, for example: patients should finish the course.
Explain the effect: more bacteria are killed, so fewer survive to reproduce.
Add another idea on reducing spread, such as hand washing or staying off school/work.

Important: This is not a maths question, but clear step-by-step logic helps you reach the top level.

Level guidance

Level	What it looks like	Marks
Level 3	Relevant points are given in detail and logically linked into a clear explanation.	5–6
Level 2	Some relevant points and some linking, but the explanation is not fully clear.	3–4
Level 1	Simple points stated, but little or no clear linking.	1–2
No relevant content	No creditworthy biology.	0

Quick full-mark checklist

✅ Must know answers

02.1: Humans have not found evidence of every species.
02.2: Fossils.
02.3: Use several methods and explain how each reduces antibiotic resistance evolution or spread.

❌ Avoid these slips

Do not write fossil fuels.
Do not say antibiotics treat viruses.
Do not give an unlinked list in the 6-marker.

Topics

Biology · B6: Inheritance, Variation and Evolution · B3: Infection and Response

Question and mark scheme from the AQA GCSE Combined Science: Trilogy examination, Biology Paper 2 (Higher), 2023. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.