AQA A-Level Biology: Variation, Natural Selection, and Speciation — mark scheme explained

The question

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.

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, perform calculations, and check your work.

1. Identify the source of variation. The population shows variation in shell colour caused by the two alleles B (black) and b (brown), arising originally from mutation.0 marks
2. Identify the selection pressure. The predator can easily spot brown beetles against the dark soil, so brown beetles are eaten more often than black beetles. This is the selective pressure acting on the population.1 mark
3. Explain differential survival and reproduction. Black beetles (carrying the B allele) are better camouflaged, so they are more likely to survive, reach reproductive age and breed.1 mark
4. Explain the effect on alleles. Surviving black beetles pass on the advantageous B allele to their offspring, while fewer brown beetles survive to pass on the b allele.1 mark
5. Describe the change over generations. Over several generations the frequency of the B allele increases and the frequency of the b allele decreases within the population's gene pool. This is an example of directional selection.2 marks

Final answer: The frequency of the B (black) allele increases over successive generations. Brown beetles are more easily seen and eaten, so black beetles (carrying B) have a selective advantage, survive and reproduce more, and pass on the B allele. This is directional selection, which shifts allele frequencies in the gene pool over time.

✓Work through every step correctly and you earn all 5 marks.

Common mistakes

• Confusing genetic variation with environmental factors.

  Why it happens: Students may not clearly distinguish between genetic and environmental sources of variation, leading to incorrect explanations.

  Fix: Always specify whether a factor is genetic (e.g., mutations, sexual reproduction) or environmental (e.g., climate, diet).

• Failing to explain how natural selection leads to changes in allele frequencies.

  Why it happens: Students may describe the process of natural selection without linking it to changes in allele frequencies.

  Fix: Clearly state that natural selection favours organisms with advantageous phenotypes, leading to increased survival and reproduction. Over time, this results in changes in the frequencies of different alleles within a population's gene pool.

• Confusing stabilising, directional, and disruptive selection.

  Why it happens: Students may mix up the definitions and effects of different types of selection, leading to incorrect answers.

  Fix: Practice identifying and describing each type of selection from given scenarios or data sets. Stabilising selection favours intermediate phenotypes, directional selection favours one extreme phenotype, and disruptive selection favours both extremes.

• Failing to explain how reproductive isolation leads to speciation.

  Why it happens: Students may not fully understand the role of reproductive isolation in the formation of new species.

  Fix: Explain that reproductive isolation occurs when two populations become unable to interbreed and produce fertile offspring. Over time, isolated populations accumulate genetic differences that can lead to the formation of new species.

• Not understanding the role of genetic drift in small populations.

  Why it happens: Students may not recognize the importance of random events in changing allele frequencies, especially in small populations.

  Fix: Explain that genetic drift is a mechanism of evolution where allele frequencies change due to random events rather than natural selection. It is particularly important in small populations because random sampling effects can have a significant impact on the gene pool.

• Confusing the founder effect with the bottleneck effect.

  Why it happens: Students may mix up these two mechanisms of genetic drift, leading to incorrect explanations.

  Fix: Practice describing each mechanism clearly. The founder effect occurs when a new population is established by a small number of individuals from a larger population, leading to a different gene pool. The bottleneck effect occurs when a significant reduction in population size leads to a loss of genetic variation and changes in allele frequencies due to random sampling effects.

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

Related questions

• AQA A-Level Biology: Monomers, Polymers, and Chemical Reactions in Biological Molecules — mark scheme explained →
• AQA A-Level Biology: Triglycerides and Phospholipids: Structure and Properties — mark scheme explained →