| Date | May 2010 | Marks available | 1 | Reference code | 10M.3.HL.TZ1.1 |
| Level | Higher level | Paper | Paper 3 | Time zone | Time zone 1 |
| Command term | Identify | Question number | 1 | Adapted from | N/A |
Question
Drosophila subobscura (shown in photograph below) is a species of fruit fly native to Europe. The sample on the left is from Spain, latitude 39°, and the one on the right is from Denmark, latitude 56°. The species was introduced into both South America and North America approximately 20 years ago. The graph below shows the wing size in arbitrary units of D. subobscura at different latitudes in the three locations.
Identify the relationship between wing size and latitude shown by D. subobscura in Europe.
Compare the data for wing size of D. subobscura in North and South America with wing size in Europe.
Suggest one reason for the differences.
Predict, with a reason, what might happen to D. subobscura in the future as a result of its introduction to new areas.
Markscheme
as latitude increases so does wing size / direct relationship / positive correlation
(do not accept directly proportional as line does not pass through the origin)
at higher latitudes/above 45° European species have larger wings than American species/largest wing sizes seen in European flies;
European flies show more variation than American flies (steeper curve);
at lower latitudes/below 45° North American flies have larger wings than European species;
South American flies have smaller wing size than European/ N. American flies;
American species show less variation because they have had less time to diverge / evolve;
size of American flies closer to that of flies recently introduced/founder effect / European founder population from low latitudes;
American flies exposed to different selective pressures;
isolated populations diverge (genetically) / separation of gene pools;
may become a new species / allopatric speciation;
wing size will follow same trend as in Europe as population spreads (to higher latitudes because larger size favored by higher latitudes/colder climate);
wing size may stay small due to smaller gene pool/different climate;
Examiners report
Almost every candidate was able to identify the relationship shown in the graph. Candidates sometimes confused latitude with altitude which caused problems when answering the other sections of D1.
Many obtained one mark for correctly indicating that the European flies showed more variation than the American flies but few were able to get a second point in (i) or to suggest an acceptable reason for this in (ii). Those that did answer (ii) got a mark for understanding the role of selection pressures. Candidates need to be sure to make comparisons when asked rather than write descriptions.
Many obtained one mark for correctly indicating that the European flies showed more variation than the American flies but few were able to get a second point in (i) or to suggest an acceptable reason for this in (ii). Those that did answer (ii) got a mark for understanding the role of selection pressures. Candidates need to be sure to make comparisons when asked rather than write descriptions.
Many were able to get one mark for indicating that a new species may form and some indicated this was a result of separation of gene pools due to geographic isolation.
