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Date May 2017 Marks available 2 Reference code 17M.3.HL.TZ2.1
Level Higher level Paper Paper 3 Time zone Time zone 2
Command term Describe Question number 1 Adapted from N/A

Question

The larval stage of the fly Eurosta solidaginis develops in the plant Solidago altissima. The larva secretes a chemical which causes plant tissue to grow around it forming a swelling called a gall. The gall provides the developing insect with protection from predators.

The E. solidaginis fly is preyed upon by the parasitic wasp Eurytoma gigantea. The graph shows the relationship between gall diameter and the percentage of flies that avoid predation by E. gigantea.

In order to form galls, the insects choose a location where cell division occurs at a high rate. State the term for a region of rapid cell division within a plant.

[1]
a.

Describe the relationship between gall diameter and percentage survival of E. solidaginis.

[2]
b.

Explain the concept of directional selection with respect to this example.

[2]
c.

Markscheme

«apical» meristem/shoot apex

a.

a. percentage survival is higher with larger diameter galls
OR
positive relationship  

b. variation/outlier at the lower diameters  OWTTE

c. little variation in survival percentage at highest diameters OWTTE

b.

a. directional selection is when an extreme phenotype/characteristic is favoured  OWTTE

b. flies that form small galls will be selectively predated  OWTTE – accept vice versa

c. over time, flies that produce small galls will become rarer
OR
mean gall size will increase

c.

Examiners report

[N/A]
a.
[N/A]
b.
[N/A]
c.

Syllabus sections

Core » Topic 4: Ecology
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