The British swallowtail butterfly, Papilio machaon britannicus, is limited to the fens of the Norfolk Broads. Its caterpillars feed solely on milk parsley, Peucidanum palustre, which only grows in the East Anglian wetlands.
Yet in France and the Low Countries, the subspecies P. m. gorganus is common and widespread, availing itself to a range of host plants.
Like many organisms at the edge of a range, our swallowtail has a narrow set of very precise habitat requirements. Genetic diversity is the key factor: in southern Europe, intermingling populations keep genetic variability high, resulting in a greater diversity of habits, host plants and ranges.
Outlier populations, such as those in Norfolk, were founded by a small number of individuals – possibly just one – following the last Ice Age. With a shallower gene pool, their members are more sedentary, less experimental when it comes to egg-laying and less capable of colonising new ecological niches.
There are fears that rising numbers of gorganus migrants into Britain may well establish colonies here and interbreed with the Fenland race, diluting it to the point of non-existence.
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