[Figure 3.1] shows a red squirrel, Sciurus vulgaris. This species is native to the British Isles, meaning it has lived there for at least 10000 years. In the 1800s a related but slightly larger species, the grey squirrel, Sciurus carolinensis, was introduced from North America. A history of the interaction between red squirrels and grey squirrels includes these facts: • Grey squirrels, which occupy a similar niche to red squirrels, expanded their range rapidly after introduction. • Grey squirrels carry a virus that does not harm them, but which kills red squirrels. • Red squirrels became extinct in many parts of the British Isles. • Recently some areas have seen a reversal of this trend. There has been a decrease in numbers of grey squirrels. Red squirrels have re-colonised their former areas of habitat. • This has happened in areas where protection of a native predator species, the pine marten, Martes martes, has resulted in an increase in the number of these predators. • Analysis of pine marten faeces shows that they catch and eat many more grey squirrels than red squirrels. (a) Use the information given to identify the causes of the extinction of red squirrels in many parts of the British Isles. (b) Describe how the level of molecular similarity between the two species, S. vulgaris and S. carolinensis, can be investigated. (c) Explain how the differing effect of pine marten predation on red and grey squirrel populations is due to natural selection and the separate evolution of each squirrel species.