AQA GCSE Combined Science: Trilogy Biology Paper 2 (Higher), November 2021: Question 4

📐 Step-by-Step Calculation

1 Read values from graph:
• Year 1984 = 13.55 mm
• Year 2000 = 13.60 mm

2 Calculate the change in value:
Δy = 13.60 − 13.55 = 0.05 mm
Δx = 2000 − 1984 = 16 years

3 Calculate gradient (rate):
Rate = 0.05 / 16 = 0.003125 mm/year
(or in standard form: 3.125 × 10⁻³ mm/year)

❌ Common Errors & Traps

• Misreading the y-axis: The y-axis starts at 13.50, not zero! Each small square on the vertical axis represents 0.005 mm (or 10 squares = 0.05 mm). Check grid intervals carefully.
• Inverting the fraction: Dividing time by length change (16 / 0.05 = 320) gives years per mm instead of mm per year. Rate is always change in y ÷ change in x.
• Ignoring the specified years: Calculating across the entire line (1982 to 2007) instead of the range stated in the question (1984 to 2000).

✅ Model 6-Mark Answer (Full Level 3)

Within the bird population, there is genetic variation in beak length caused by random mutations.

Birds with longer beaks can reach nuts deeper inside the bird feeders more easily, allowing them to outcompete birds with shorter beaks for food.

Because they obtain more food and energy, these longer-beaked birds are more likely to survive and reproduce (natural selection).

When they reproduce, they pass on the alleles for longer beaks to their offspring.

This process is repeated over many generations, leading to an increase in the mean beak length of the population over time.

🧠 Top-Grade Exam Technique

• The "V.A.S.P.G" Template: Use this proven 5-step structure for ANY natural selection question:
  1. Variation: due to random mutation.
  2. Adaptation: why long beaks help (get nuts from feeder).
  3. Survival: survive & reproduce more ("survival of the fittest").
  4. Pass on alleles: favourable genes passed to offspring.
  5. Generations: repeated over many generations.
• Level 3 Requirement: The mark scheme explicitly states that to get 5–6 marks, you must mention that this is repeated over many generations and link it directly to beak length.

💡 Key Examiner Insight: Lamarck vs Darwin

Never say "the birds stretched their beaks to reach the food" or "their beaks grew because they needed food". This is Lamarckian evolution and gets zero credit. Evolution occurs because individuals with pre-existing advantageous mutations survive better.

❌ Lost Marks Checklist

• Saying "passing on traits" instead of specifying passing on genes/alleles.
• Forgetting to state that this happens over many generations (caps your score at Level 2, maximum 4 marks).
• Failing to mention the food source (nuts in feeder) provided in the question.

✅ Correct Marking Points (Any 3)

• Birds have a shorter life cycle / life span (than humans). [1 mark]
• Birds produce more offspring / larger number of young (than humans). [1 mark]
• Therefore, the genetics of the population changes faster / mutations and adaptations appear in fewer human years / scientists can observe changes across many generations in a single lifetime. [1 mark]

🧠 Exam Technique & Keyword Precautions

• Use the stimulus: The question gives you two specific numbers (3 years lifespan, 24 eggs). Translate each number into an evolutionary advantage for researchers:
  • 3 years → short generation time / life span
  • 24 eggs → large number of offspring
• Examiner rule: The mark scheme says "ignore shorter life" if unqualified — make sure you say shorter life cycle or shorter lifespan / faster generational turnover.

✅ 3 Distinct Sources of Evidence

• Breeding test: They can reproduce / interbreed to produce fertile offspring. [1 mark]
• Genetics: They have similar DNA profile / genotype / base sequences. [1 mark]
• Appearance: They share similar phenotype / physical characteristics / anatomy. [1 mark]

❌ Common Errors

• Missing the word "fertile": Just stating "they can mate and have babies" is incomplete. Mules are offspring of horses and donkeys, but horses and donkeys are different species because mules are sterile! You must state fertile offspring.
• Vague answers: "They live in the same place" or "they eat the same food" is NOT evidence of being the same species. Different species often share the same habitat and diet.