Biology 9700 · AS & A Level · Variation

Variation — practice question

The stickleback fish, Gasterosteus aculeatus, exists in two separate forms: a saltwater form and a freshwater form. The larger freshwater form is believed to have evolved from the smaller saltwater form. Both forms have armour plating on either side of the body. These plates are bony and contain a large amount of calcium. The ectodysplasin gene, EDA, codes for a protein that is involved in the formation of armour plates. The EDA gene has two alleles, low armour and high armour. Three main morphs of armour plating have been described: complete morph armour plating, partial morph armour plating, and low morph armour plating.
(a)[2]

Explain why the variation in armour plating in stickleback fish can be described as discontinuous.

(b)[1]

In 1982, at Loberg Lake in Southern Alaska, the whole freshwater stickleback fish population was accidentally eliminated by humans. In 1990, a new population of stickleback fish was discovered in the lake. Most of these fish had armour plates from head to tail on each side. Suggest why these new stickleback fish have armour plates from head to tail on each side, even though they live in freshwater.

(c)[5]

From 1990, the lake was sampled every year. In each year, the number of individuals with complete morph armour plating (from head to tail on each side) fell. This happened over a fairly short time span. - In 1990, $96\%$ of the stickleback fish population had complete morph armour plating. - In 1993, $39\%$ of the stickleback fish population had complete morph armour plating. Explain how natural selection has taken place in this new stickleback fish population.

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