A-LevelBiologySelection and evolution9700/43 · May/June 2021 · Q89 Marks

Explain what is meant by stabilising selection.

(a)Explain what is meant by stabilising selection.[3]
(b)(i)Fig. 8.1 shows the distribution of fur colour in a population of mice. The colour of the soil where the mice lived was mid-brown. [Figure 8.1] Suggest why mice with white fur and mice with black fur were present in the lowest numbers in Fig. 8.1.[2]
(b)(ii)An environmental event caused the colour of the soil to change to dark brown. On Fig. 8.1, sketch a curve to show the distribution of fur colour in the mouse population after many generations.[2]
(c)Explain what is meant by genetic drift.[2]

This question uses a diagram from the original paper, which isn't shown here. Open the full paper from the past papers archive to see it.

Mark scheme

Marking points from the official mark scheme (May/June 2021). The mark or mark type is shown in brackets.

[1] (a) individuals in a population with, intermediate phenotypes / AW, more likely to, survive / reproduce or individuals in a population with, intermediate phenotypes / AW, are selected for ;
[1] (a) individuals in a population with extreme phenotypes, die or individuals in a population with extreme phenotypes are selected against ;
[1] (a) no change in environment ;
[1] (b)(i) poor camouflage / AW ;
[1] (b)(i) predation;
[1] (b)(ii) narrower curve ;
[1] (b)(ii) curve to right with apex at dark brown ;
[1] (c) random/chance, change in allele frequency or description of alleles not passed onto the next generation ;
[1] (c) larger influence in smaller population / AW;

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