Introduction
Genetic erosion is one of the most important threats to the survival of many wild species. Nowadays these species are subjected to many stress factors, most of which are due to human activities: namely environment alteration, habitat fragmentation, urbanisation, pollution, irrational forest management and the introduction of allochthonous species. Current climate changes will probably determine a decline in growth and survival, and an alteration of genetic structure and diversity for many wild species. This pressure may be strengthened in marginal and peripheral populations of the species distribution, mainly in European southern populations of plants [1]. In order to survive these threats, and to persist over time, a high adaptive potential is fundamental: this is mainly determined by the within-species genetic diversity [2]. Genetic differentiation derives from several evolutionary forces, among which selection pressure as a response to different ecological conditions and genetic drift are the most important. Gene flow, in contrast to this pattern, leads to homogenisation of allelic frequencies [3].