Photoperiodism refers to the organism’s ability to detect and respond to seasonal changes in the daily duration of light and dark and thus constitutes one of the most significant and complex examples of the interaction between the organism and its environment. This thesis attempts to describe the prevalence of variation in a photoperiodic response, its adaptive value, and its putative genetic basis in a common cruciferous weed, Capsella bursa-pastoris (Brassicaceae). Furthermore, the thesis presents a first comprehensive comparative overview of the circadian clock mechanism in an early land plant, Physcomitrella patens (Bryophyta), thus providing insights into the evolution of the plant circadian system. In an introductory survey of global gene expression changes among early- and late flowering accessions of C. bursa-pastoris we found an enrichment of genes involved in photoperiodic response and regulation of the circadian clock. Secondly, by phenotyping circadian rhythm variation in a worldwide sample of accessions with known flowering time, we detected robust latitudinal clines in flowering time and circadian period length, which constitute strong indications of local adaptation to photoperiod in the shaping of flowering time variation in this species. In an attempt to elucidate putative genetic causes for the correlated variation between circadian rhythm and flowering time, we found that sequence variation and diverged expression in components regulating light input to the clock, PHYTOCHROME B (PHYB) and DE-ETIOLATED 1 (DET1) make them strong candidate genes. Finally, we present a comparative study of circadian network topology in the moss P. patens. Phylogenetic analyses and time series expression studies of putative clock homologues indicated that several core clock genes present in vascular plants appeared to be lacking in the moss. Consequently, while the clock mechanism in higher plants constitutes at least a three-loop system of interacting components, the moss clock appears to comprise only a single loop. We conclude that C. bursa-pastoris is a highly suitable model system for the further elucidation of the molecular variation that influences adaptive change in natural plant populations. Furthermore, we believe that the continuing study of the seemingly less complex circadian network of P. patens not only can provide insights into the evolution of the plant circadian system, but also may help to clarify some of the remaining issues of the circadian clock mechanism in higher plants.