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

The question

A population of beetles has two alleles for shell colour: B (black) and b (brown). Initially the B allele is rare. A predator can easily spot brown beetles against the dark soil, so brown beetles are eaten more often. Explain how the frequency of the B allele in the population is likely to change over several generations, and why. [Paraphrased for study — not reproduced from any exam paper.]

Mark scheme, decoded

What each mark is really for — in plain English — and the wording trap that loses it.

• S1

  Step 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.

• S2

  Step 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.

• S3

  Step 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.

• S4

  Step 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.

• S5

  Step 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.

Model answer

Worked through, with each step tagged to the mark it earns.

1. S1

   Step 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.

2. S2

   Step 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.

3. S3

   Step 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.

4. S4

   Step 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.

5. S5

   Step 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.

6. 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.

Common mistakes

• Confusing genetic variation with environmental factors. — 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. — 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. — 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. — 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. — 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. — 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

• Full worked solution (all marking points)5 marks

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