Cardamine hirsuta L. is an emerging model system in developmental genetics, where natural genetic variation within C. hirsuta provides the means to investigate the genetic basis of morphological traits. This thesis investigates the geographical structure and genealogical history of genetic variation within C. hirsuta and identifies its closest relatives. This will enable the accurate selection of species for comparison with C. hirsuta when making interpretations of evolutionary processes, and provide a better understanding of morphological character evolution in C. hirsuta. The phylogeographic history of C. hirsuta was reconstructed using multiple chloroplast and nuclear markers and widespread accession sampling from across its native range. A distinct group was identified within C. hirsuta, restricted to the high mountains of East Africa. Climate suitability modelling showed that Pleistocene glacial dynamics have had a strong effect on the distribution of genetic variation within C. hirsuta. The phylogeographical data generated here was used to investigate the origin of C. hirsuta in the Azores, an oceanic archipelago. The Azores are dominated by an endemic chloroplast haplotype which is associated with an endemic phenotype. Thus, C. hirsuta appears to have diversified in situ in the Azores. Phylogenetic analyses of Cardamine, restricted to diploid species to remove the confounding effects of polyploids, found that C. hirsuta is most closely related to C. oligosperma, a western North American species. Multiple loci and extensive intraspecific sampling were brought to bear to demonstrate that C. hirsuta and C. oligosperma are reciprocally monophyletic. Cardamine pattersonii, a restricted endemic from north-west Oregon is likely to be an allopolyploid, with C. oligosperma as the maternal parent and possibly C. nuttallii as the paternal parent.