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It has been proposed that the diversification of the MADS-box gene family of transcription factors has played a major role in the radiation of land plants. This suggestion is based on the critical roles that these genes play in plant development and the apparent coincidence of key duplication events with major radiations, such as the establishment of the B and C lineages concurrent with the evolution of the seed plants. On a more recent scale, it is also possible that subsequent duplication events have contributed to later morphological diversifications. In order to investigate this possibility, we are studying the evolution of homologs of the petal and stamen identity genes APETALA3 (AP3) and PISTILLATA (PI) in the Ranunculaceae. In this family, the AP3 and PI lineages have undergone many duplication events at every phylogenetic level. Early duplications gave rise to three paralogous AP3 lineages, which are found throughout the family. In contrast, numerous duplications have occurred relatively recently in the PI lineage. We outline a hypothesis that these duplications have played a role in the evolution of the unique types of petaloid organs in the Ranunculaceae and present preliminary expression data supporting such a scenario.</p>