From the southern Mediterranean to the northern half of Europe, the distinctive colour of the chalk-hill blue butterfly Polyommatus coridon is unmissable — in contrast to the species itself, which has a rather patchy distribution. In the context of general concern over the effect that changes in climate and land use might have on biodiversity, T. Schmitt and G. M. Hewitt used this species, and another — Erebia medusa, the woodland ringlet — to test ideas about a species' history, its present-day patterns of genetic diversity and its robustness.
As they report in Molecular Ecology (13, 21–31; 2004), Schmitt and Hewitt have drawn on various datasets to conclude that the butterflies' location in Europe, determined by past climatic events, reflects their genetic diversity, and also, they speculate, their predisposition to population decline.
There are two major lineages of the chalk-hill blue in Europe, one in the east, the other in the west. After the last ice age, these groups probably emerged from two regions in the southern Mediterranean, and then underwent a northward postglacial colonization. Although the two lineages show equivalent genetic diversity, when the authors compared northern and southern populations of each, those further north were considered genetically poor, with respect to diversity, and less stable demographically, than their southern counterparts. A similar pattern emerged for the woodland ringlet, only with a west–east axis.
So for both the woodland ringlets and the chalk-hill blues, the least stable populations — and so those with apparently less adaptability to change — are those furthest away from their former refugial areas. On the face of it, then, the butterflies' ability to adapt is a correlate of genetic diversity. If matters are to be taken further in examining the relationship between ‘phylogeographical’ patterns and population stability in European butterflies, more detailed genetic surveys will be required of these as well as other species — in addition to an assessment of the relative importance of genetic constitution and ecological change.