Question 20N.2.HL.TZ0.c
| Date | November 2020 | Marks available | [Maximum mark: 2] | Reference code | 20N.2.HL.TZ0.c |
| Level | HL | Paper | 2 | Time zone | TZ0 |
| Command term | Discuss | Question number | c | Adapted from | N/A |
White clover (Trifolium repens) is native to Eurasia but is now a common plant found worldwide in lawns, next to roads, in pastures and similar habitats.
[Source: © International Baccalaureate Organization 2020.]
Some T. repens plants are able to produce the toxin hydrogen cyanide (HCN) by cyanogenesis. A study at 128 sites (2509 plants) in Toronto (Canada) looked at the proportion of T. repens plants producing HCN. The sites were at regular intervals from the city centre towards rural areas.
[Source: Adapted from Thompson, K.A., Renaudin, M. and Johnson, M.T.J., 2016. Urbanization drives the
evolution of parallel clines in plant populations. Proceedings of the Royal Society B, [e-journal] 283.
https://doi.org/10.1098/rspb.2016.2180.]
To determine whether similar patterns in cyanogenesis were seen in other cities, studies were also carried out around New York City and Boston (USA) and Montreal (Canada).
[Source: Adapted from Thompson, K.A., Renaudin, M. and Johnson, M.T.J., 2016. Urbanization drives the
evolution of parallel clines in plant populations. Proceedings of the Royal Society B, [e-journal] 283.
https://doi.org/10.1098/rspb.2016.2180.]
The researchers considered two possible ecological causes for the observed gradients in cyanogenesis. First, the researchers assessed the extent of herbivory of T. repens plants in Toronto by measuring the percentage of leaf area that was eaten. The graph shows the results for both cyanogenic and non-cyanogenic plants.
[Source: Adapted from Thompson, K.A., Renaudin, M. and Johnson, M.T.J., 2016. Urbanization drives the
evolution of parallel clines in plant populations. Proceedings of the Royal Society B, [e-journal] 283.
https://doi.org/10.1098/rspb.2016.2180.]
Discuss whether the data supports the hypothesis that the gradient in cyanogenesis is due to its benefits against herbivory in rural areas.
[2]
a. (hypothesis) not supported;
b. large overlap/little difference between cyanogenic and non-cyanogenic (in herbivory);
c. smaller difference between cyanogenic and non-cyanogenic in rural areas;
d. both show negative correlation between herbivory and distance from city center/same trend;
e. some support/hypothesis partly supported by lower herbivory in cyanogenic (at all distances).
Do not accept ‘No’ unqualified as an answer.
Do not award mpe if the answer states that the hypothesis is supported without doubt/fully.
This was possibly the hardest part of the data-based question. Candidates were spread approximately equally between 0, 1 and 2 marks. Many candidates did not realise that the differences between herbivory in cyanogenic and non-cyanogenic plants were unlikely to be significant, especially in the rural areas that were furthest from the city. The hypothesis stated in the question was therefore not supported. Candidates should be encouraged to consider both the closeness of means and also the spread, when analysing data.
