Knowledge about current levels of genetic biodiversity is paramount to identify the appropriate units for conservation and to develop accurate conservation programs. This project is aimed at analysing the population genetic structure and the role of natural selection on phenotypic differentiation among populations of Donacosa merlini Alderweireldt & Jocqué 1991 (Araneae, Lycosidae), an endemic and potentially endangered wolf spider species, which distribution is limited to the sandy habitats of the Doñana area. The species is known to show local variation in morphology, life-history and behaviour, which is potentially related to a limited gene flow and a pronounced population genetic structure. We will sample DNA from 14-18 populations located through the species distribution area. A 1000 bp portion of CO1 will be sequenced from 10 individuals per population, in order to identify mtDNA haplotype diversity. Genetic distances among populations will be related to both geographic and environmental distances, in order to identify the role of stochastic and deterministic processes in the genetic differentiation among populations of this singular species.