| Date | May 2011 | Marks available | 2 | Reference code | 11M.3.SL.TZ1.10 |
| Level | Standard level | Paper | Paper 3 | Time zone | Time zone 1 |
| Command term | Analyse | Question number | 10 | Adapted from | N/A |
Question
Populations of threespine sticklebacks (Gasterosteus sp.), a fish living in small freshwater lakes in British Columbia, Canada, are derived from the marine threespine stickleback (Gasterosteus aculeatus). In order to investigate the process of speciation in these populations, three small lakes were studied. Each lake contained two varieties of stickleback: a large, bottom-dwelling variety that fed on invertebrates near the shore and a small, plankton-eating variety that lived in the open water. The probability of breeding between pairs of individuals was measured under laboratory conditions in the following breeding combinations:
I different varieties (small × large) from the same lake
II different varieties from different lakes
III same variety (small × small) and (large × large) from different lakes
IV same variety from the same lake.
The data are summarized below.
Identify the highest and lowest probabilities of breeding for individuals of the same variety from different lakes.
Highest probability: ...................................................
Lowest probability: ....................................................
Identify the breeding combination that results in the lowest probability of breeding.
Analyse the probability of breeding between individuals from the same lake.
Scientists concluded that speciation is taking place in these populations. Discuss the evidence for speciation provided by the data.
Markscheme
Highest probability: 0.58 (Allow answers from 0.57–0.59)
Lowest probability: 0.25 (Allow answers from 0.24–0.26)
Both required for the mark.
different varieties from same lake / I
individuals are more likely to breed if they are the same variety / individuals of different varieties have a low probability of breeding;
the probability of breeding between individuals of the same variety shows a large range of values / narrow range if of different variety;
the probability of breeding between any two individuals is always less than 0.6/correct numerical value;
data provides (strong) evidence for reproductive isolation between the varieties in each lake;
different sizes/feeding habits/habitat (shore versus open water) seem to contribute (strongly) to low breeding probability;
this could lead to speciation/formation of separate species in each lake;
same varieties from different lakes do not show strong reproductive isolation/ geographical isolation is a weak factor in speciation / no evidence of allopatric speciation;
sympatric speciation seems to be taking place because different varieties from the same lake have a low probability of breeding;
Examiners report
Most candidates answered this correctly.
The majority of candidates identified the correct breeding combination.
A good proportion of candidates gained two marks, showing their ability to correctly analyse the data given.
This was a good discriminator as it allowed the better students to connect the data with their knowledge of speciation, although very few mentioned allopatric speciation, and few gained all three marks. Many weaker candidates appear to have little understanding of the process of speciation however, and answers were often difficult to interpret.
