However, species-wide Atropine sulfate samples are not suitable for investigating population processes of divergence, demographic change, and local adaptation. To understand the maintenance of variation within species, and the importance of local selection and demography in natural populations, both within- and amongpopulation sampling are needed. Despite the importance of local population processes to plant evolution, surprisingly little attention has been given to the distribution of DNA sequence diversity among plant populations. Although many studies have used genetic markers to study genetic differentiation among plant populations, so far only a handful have examined DNA sequence diversity within and among natural plant populations. Unlike other molecular marker systems, DNA sequence data provide information about recombination and linkage disequilibrium, which can be highly sensitive to demographic history. Work done to date has demonstrated the need for local sampling to accurately describe patterns of LD, diversity, and the frequency spectrum of polymorphisms in local populations, and shown that even simple demographic processes can better explain observed data than can the assumption of neutral equilibrium. Nonetheless, many of these studies have relied on small samples of loci or groups of candidate genes, neither of which is likely sufficient to capture patterns of genome-wide variation or provide insight into the relative roles of demographic history and selection. Here we present a large-scale population-genetic analysis of sequence diversity in natural populations of Arabidopsis lyrata. A. lyrata is a predominantly self-incompatible, perennial species with a circumpolar distribution across northern and central Europe, Asia, and North America. A. lyrata appears to maintain large, stable populations, particularly in Central Europe, where populations are hypothesized to have served as refugia during the most recent Ice Age. A. lyrata has become a model system for plant molecular population genetics and for investigating local adaptation. For example, divergent selection on trichome production has been found among phenotypically differentiated Swedish A. lyrata populations. Flowering time and floral display also appear to be under strong selection, with large differences in day-length Ginsenoside-F4 requirements between Northern and Southern populations. A. lyrata is 
also of great interest because it is a close relative of A.