The stickleback fish, Gasterosteus aculeatus, occurs 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 carry armour plates along each side of the body. These plates are made of bone 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 major armour-plating morphs have been identified: complete morph armour plating, partial morph armour plating, and low morph armour plating.
(a)[2]
Explain why armour plating variation in stickleback fish is described as discontinuous.
(b)[1]
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.
(c)[5]
From 1990, annual sampling was carried out in the lake. Each year there was a decline in the number of individuals with complete morph armour plating (from head to tail on each side). This change happened over a relatively short timescale.
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.
Worked solution & mark scheme
This 8-mark question has a full step-by-step worked solution and mark scheme. One marking point: “Separate / clearly different categories or phenotypes” …