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Date	November 2021	Marks available	1	Reference code	21N.1.HL.TZ1.36
Level	Higher level	Paper	Paper 1	Time zone	Time zone 1
Command term		Question number	36	Adapted from	N/A

Question

The graph shows the relationship between mass at birth and the percentage that die shortly after birth for Scottish Blackface lambs.

[Source: Dwyer, C.M., Conington, J., Corbiere, F., Holmoy, I.H., Muri, K., Nowak, R., Rooke, J., Vipond, J. and Gautier,
J.-M., 2016. Invited review: Improving neonatal survival in small ruminants: science into practice. Animal, 10(3),
pp. 449–459.]

What type of selection for the lambs is shown in the graph?

A. Disruptive selection, as there is a drop in mortality at intermediate birth masses

B. Stabilizing selection, as lambs with low or high birth mass are less likely to survive

C. Directional selection, as lambs with a high birth mass have high mortality

D. There is no evidence in the graph of selection, as survival frequency is not shown

Markscheme

Examiners report

Two thirds of candidates chose the correct answer but the discrimination index was lower than expected. Perhaps some candidates were tiring by this stage of the exam and therefore did not think carefully enough. The shape of the graph looks superficially like disruptive selection, but the y-axis shows mortality, not survival.

Syllabus sections

Additional higher level » Topic 10: Genetics and evolution » 10.3 Gene pools and speciation

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