| Date | May 2012 | Marks available | 1 | Reference code | 12M.3.HL.TZ1.1 |
| Level | Higher level | Paper | Paper 3 | Time zone | Time zone 1 |
| Command term | Estimate | Question number | 1 | Adapted from | N/A |
Question
A study of the changing frequencies of the melanic (dark) and non-melanic (light) forms of three moth species Biston betularia, Odontoptera bidentata and Apamea crenata was carried out in the north of England. Moths were captured using mercury vapour light traps throughout the flying season. All three of the moth species fly at night and rest in the day, when they would be vulnerable to predation by birds, if visible. B. betularia rests on tree branches and there is evidence that the change from melanic to non-melanic forms was associated with black soot disappearing from these branches as air pollution was reduced and pale coloured lichens started to grow. O. bidentata rests under the leaves of trees or in cracks in tree bark. A. crenata rests in long grasses. The graph shows the percentage of melanic moths caught between 1969 and 2002.
Identify the year with the highest frequency of the melanic form of O. bidentata.
Estimate the percentage of non-melanic forms of A. crenata in 1978.
Compare the trends for the three moth species during the study.
Suggest reasons for the differences in trends.
Markscheme
1972
32 (units not required) (Allow answers in the range 31–33)
Award [2 max] if at least 1 similarity and 1 difference not given.
Similarities:
in all species, melanics show a decline / non-melanics an increase in frequency;
up to 1987 percentage of melanics stable/slight decline for all three species;
Differences: Both parts of comparision required for [1].
after 1987 percentage of B. betularia melanics declines sharply whereas percentage for the other species declines slowly;
at start of investigation/1969/up to 1987 (very) few (less than 10 %) non-melanic forms of B. betularia whereas the percentage much higher (more than 20 %) for the other two species / converse;
melanic forms of B. betularia and O. bidentata drop below 50% (by 1996) but A. crenata does not;
change from polluted (industrial) environment to clean (post-industrial);
change in tree species allowing different habitats for resting moths;
change in predators as climate/habitats change;
changing selection pressures/mutation/migration/genetic drift;
grass grows quickly/colour not influenced by pollution so least change in/less selection pressure on A. crenata;
Examiners report
Almost every candidate was able to identify 1972 as the year with the highest frequency of the melanic form of O. bidentata.
Many obtained one mark for correctly estimating the percentage of non-melanic moths in 1978.
This part proved more discriminating. For (i), many candidates obtained one mark for indicating that in all species, melanics showed a decline. However, most did not compare the trends but simply stated what was happening for each of the three moth species over time. They also did not connect the fall in percentage melanics with a rise in percentage of nonmelanic forms of the species but instead incorrectly stated that the species number fell. .
Many were able to suggest the trends were due to a change from the polluted environment to a cleaner one for one mark but did not get a second point.
