The stickleback fish, Gasterosteus aculeatus, has two distinct forms, the saltwater form and the freshwater form. The larger, freshwater form is thought to have evolved from the smaller, saltwater form. Both forms have armour plating on each side of the body. The plates are made of bone and contain a high proportion of calcium. The ectodysplasin gene, EDA, codes for a protein involved in the development 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: • is found mainly in the saltwater form • has many plates from head to tail to cover most of the body • provides defence against large, predatory fish • limits the growth of the fish. Partial morph armour plating: • is found mainly in the freshwater form • has a reduced number of plates to cover only part of the body. Low morph armour plating: • is found mainly in the freshwater form • has very few, undeveloped plates and no body cover.

1. (a)Explain why the variation in armour plating in stickleback fish can be described as discontinuous.[2]
2. (b)In 1982, at Loberg Lake in Southern Alaska, the entire freshwater stickleback fish population was accidentally destroyed by humans. In 1990, a new population of stickleback fish was found 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, despite living in freshwater.[1]
3. (c)From 1990, annual sampling took place in the lake. Each year showed a reduction in the number of individuals with complete morph armour plating (from head to tail on each side). This change took place in a relatively short period of time. • 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 occurred in this new stickleback fish population.[5]