Volume 26, Number 4, April 2014
Activity
Synopticity
Bill Indge
Possums
Brush-tail possums are mammals that are native to Australia. They were introduced to New Zealand
mainly in the years 1890 to 1900. The total number of possums released was between 200 and 300.
Possums are now widespread in New Zealand and, in suitable habitats, reach population densities of
up to 30 animals per hectare.
The data in the table show lengths of male possums from different parts of New Zealand.
Location
Latitude*
Total length / mm
Tail length /mm
Auckland
36o 37’
786
321
Hawke’s Bay
39o 43’
787
322
Wairurapa
40o 54’
780
297
Orongorongo Valley
41o 22’
789
328
Nelson
41o 55’
831
356
Taramakau River
42o 45’
840
n/a**
*Latitude is expressed in degrees and minutes. It is a measure of the distance from the equator. The
greater the latitude of a particular place, the farther it is from the equator.
**Data for tail length of the possums from Taramakau River are not available.
1 (a) The measurements in the table were obtained from labels attached to museum skins. It would
not be reliable to measure the actual skins. Explain why. (2 marks)
(b) Suggest why all the data were collected from the animals of one sex. (2 marks)
2 How would you expect mean annual temperature to vary with latitude? (1 mark)
3 The scientists who collected the data in the table predicted that body size would be positively
correlated with latitude.
(a) Suggest why larger body size would be an advantage to a possum further south.
(3 marks)
(b) Use the data in the table to evaluate this prediction. (3 marks)
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Eggs and pesticides
Although it is now illegal to take the eggs of wild birds, many museums contain collections of eggs
made last century. When the eggs were originally collected, they had living contents. A small hole was
made in the shell and the contents were removed.
Partial ban and reduced use of
organochlorine insecticides
Organochlorine insecticides used
to treat seeds
Organochlorine insecticides in
common use to treat crops
The graph shows the relationship between the thickness of egg shells and the use of some
organochlorine insecticides.
2.0
1.8
Egg shell
thickness
index
1.6
Peregrine
1.4
1.2
Sparrowhawk
1.0
0.8
1900
1920
1940
Date
1960
1980
1 The egg shell thickness index was calculated from the formula:
egg shell thickness index =
mass of egg shell (mg)
length × breadth of egg (mm)
(a) Suggest why this index was used rather than measuring the actual thickness of the egg shell.
(2 marks)
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(b) The mass of the egg shell was divided by its length × breadth. Explain why it was necessary to
divide by the length × breadth. (1 mark)
2 Both the peregrine and the sparrowhawk feed on birds. Use your knowledge of food chains and food
webs to explain:
(a) how organochlorine insecticides used to treat seeds enter the tissues of a sparrowhawk (1 mark)
(b) why the concentration of organochlorine insecticide in the tissues of a peregrine falcon is much
higher than the concentration on the seeds.
(2 marks)
3 In a scientific paper published in 1977, the biologist R. K. Murton pointed out that many biologists
assumed egg shell thinning was harmful. They considered that if a population lays eggs with thin
shells and at the same time is declining, there must be a causal relationship.
(a) Suggest how egg shell thinning may lead to lower breeding success. (1 mark)
(b) Explain why a decline in number of birds may not necessarily be caused by egg shell thinning. (2
marks)
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Tigers
Tigers are critically endangered. Scientists are concerned about the genetic diversity of the remaining
wild tigers.
1 Preservation of genetic variability in rare or endangered species may help them to survive. Use your
knowledge of natural selection to explain why. (2 marks)
Scientists assessed genetic variation in tigers. They obtained samples of DNA from a number of tigers
in zoos or from museum specimens. They knew the geographical origin of each of these tigers or they
knew the geographical origin of the particular tiger’s female parent.
2 In order to assess tiger genetic variation, it was important that samples were taken from tigers that
originally came from different geographical areas. Explain why it was important that these tigers did
not come from the same geographical area.
(2 marks)
The scientists extracted DNA from mitochondria of tiger cells. Mitochondrial DNA (mtDNA) is a circular
molecule. The mutation rate of mtDNA is between 5 and 10 times faster than that of coding nuclear
DNA.
3 The mutation rate in mtDNA is faster than that in nuclear DNA because of differences in the DNA
repair systems. Suggest how these differences could result in a slower mutation rate in nuclear DNA.
(2 marks)
4 If a tiger was born in a zoo, it may be useful to know the origin of the female parent if its mtDNA is to
be studied. Explain why.
(2 marks)
The scientists studied the mtDNA in tigers and in other members of the cat family. They used a
particular restriction enzyme to cut the mtDNA molecules. This produced fragments of DNA of different
length.
5 Suppose that there were five sites on an mtDNA molecule where it could be cut by the restriction
enzyme. How many pieces of DNA would result? Explain your answer. (2 marks)
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50
40
30
20
10
Tiger
Jaguar
Ocelot
Geoffroy’s
cat
Margay
Puma
Oncilla
0
Leopard
Number of polymorphic sites for this enzyme
The graph shows some of the results of the scientists’ work.
6 A polymorphic site is a site on a DNA molecule where the sequence of bases is different in different
animals.
(a) A particular restriction enzyme may cut the DNA from one leopard but not from another at a
particular site. Explain why.
(3 marks)
(b) The scientists who carried out the work described in this question suggested that, around 10 000
years ago, the world tiger population fell and there were very few animals left. All of the tigers that are
alive today have descended from these few animals.
Explain how the graph supports the suggestion that the tigers that are alive today have descended
from very few animals. (3 marks)
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Suggested marking guidelines
Possums
1 (a) The process of preparation is likely to lead to a change in length of the actual skin;
May be stretched/may shrink (as it dries);
2
Allow 1 mark for idea of change in length and a second for a specific, more detailed answer. The
second point on its own is worth 2 marks.
(b) Ensures that the data will be comparable;
Because sex is likely to influence size;
2
2 Mean annual temperature should decrease with latitude;
1
3 (a) A larger animal would have a smaller surface-area-to-volume ratio;
Would lose less heat;
In colder conditions;
3
Once the principle has been established, this question should be straightforward.
(b) There is a positive correlation between total length and latitude/total length increases with latitude;
(but)
If tail length is subtracted from total length;
There is little/no correlation between body length and latitude;
Other factors/factors such as altitude may also affect mean temperature; max 3
The first marking point should be straightforward but the second and third are intentionally more
challenging.
Eggs and pesticides
1 (a) The eggs are museum specimens/valuable/cannot be replaced;
The index does not involve breaking them/measuring the thickness involves breaking them;
2
This question requires the application of a little common sense and an appreciation of the value of
museum specimens.
(b) The mass of the egg shell will depend on the size of the egg/length×breadth gives a measure of
the egg size;
1
The answer to question 1(b) might be expressed in a variety of ways. Other answers that reflect an
understanding of the denominator in the expression should be accepted.
2 (a) (Transfer through the food chain) via an intermediate seed-eating bird/appropriate example; 1
A straightforward question but elementary errors may occur.
(b)
Insecticide taken up by primary consumer/example from part (a) and accumulates/not broken
down in tissues;
Primary consumer eats many seeds so higher concentration in its tissues;
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Process repeated in secondary consumer;
max 2
This question tests the principle of bioaccumulation. Allow full credit if points 1 and 2 are answered in
terms of secondary consumer and primary consumer.
3 (a) Eggs with thinner shells are more likely to be broken (during incubation);
(b)
1
Another factor may be involved;
Which might be linked to both decline in birds and egg-shell thinning;
2
The last question in this section tests another important principle, namely that correlation does not
necessarily mean causation. However, it requires students to go a little further than simply stating this
principle. Accept an example of another factor provided there is a plausible link with both egg-shell
thinning and the decline in population.
Tigers
1 Some forms may be at an advantage/be better adapted;
If there is a change in the environment of the organism; 2
Although there are other reasonable suggestions that might be made about the advantages of genetic
variability, the question requires students to link this to natural selection.
2 If they came from the same area they would be more likely to be genetically similar;
Because they might be related/might be adapted to similar conditions/might have had the same
ancestors;
We need to know about all tigers;
max 2
3 Mutations in nuclear DNA may be removed/repaired (by the repair system);
System only found in nucleus/not found in mitochondria; 2
Note that this question requires students to explain the slower rate in nuclear DNA. It should be more
straightforward this way.
4 This will tell you the geographical origin of its mtDNA;
Because it is inherited from its mother;
2
5 Five;
Because mtDNA is a circular;
2
Provided students have taken on board the fact that mtDNA is circular, they should appreciate that
there will be as many pieces as there are cleavage sites. A diagram may help.
6 (a) Sequence of bases is different;
Only a specific sequence fits/binds;
With active site of a particular enzyme; 3
The key here is to note the mark allocation. 3 marks give an idea of the depth of answer required.
(b) Alleles/different forms of gene may be lost from population as it gets smaller;
Tigers descended from this population will have little genetic diversity;
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Graph shows no polymorphic sites;
10 000 years is a short time so few mutations producing differences in DNA;
max 3
Note that in this question the term allele should be used. Gene is not acceptable in this context.
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