Natural and artificial selection — practice question

Fig. 3.1 presents a red squirrel, $\textit{Sciurus vulgaris}$. This species is native to the British Isles, which means it has been present there for at least 10000 years. During the 1800s, a closely related but slightly larger species, the grey squirrel, $\textit{Sciurus carolinensis}$, was brought in from North America. The history of interactions between red squirrels and grey squirrels includes the following points: - Grey squirrels, which occupy a similar niche to red squirrels, spread through their range very quickly after introduction. - Grey squirrels carry a virus that does not affect them, but which is lethal to red squirrels. - Red squirrels died out in many areas of the British Isles. - More recently, some places have shown the opposite pattern. Grey squirrel numbers have fallen. Red squirrels have returned to their former habitats. - This has occurred in areas where the native predator, the pine marten, $\textit{Martes martes}$, has been protected, causing the number of these predators to rise. - Examination of pine marten faeces shows that they capture and eat many more grey squirrels than red squirrels.