The Onagraceae is well suited for systematic and evolutionary studies in view of its moderate size, distinct demarcation, and abundant and diverse information base from past studies. Members of the Onagraceae share a number of distinctive characters that tie them together as a monophyletic group and separate them from all other members in the Myrtales. The 15 genera (excluding Heterogaura and Boisduvalia which are now placed within Clarkia and Epilobium, respectively) of Onagraceae are grouped into 7 tribes and comprise about 650 species. Tribal relationships in Onagraceae have been the explicit object of cladistic analyses involving morphology, nrDNA and cpDNA restriction site mapping, and rbcL sequencing. An ITS sequence analysis of all genera and encompassing over 200 species provided the most detailed and explicit phylogenetic hypothesis to date for the Onagraceae. The basic results of previous molecular studies are supported by the ITS analysis, with the ITS data supporting the near basal placement of Lopezia suggested by nrDNA but not the cpDNA data sets. The previous, surprising relationship of Circaea and Fuchsia is again strongly supported. The Onagreae is paraphyletic with the tribe Epilobieae imbedded within this large tribe of 10 genera. Gonglyocarpus is sister to the rest of the tribe, followed by Epilobium. Clarkia and Gayophytum are sister genera. Camissonia appears paraphyletic in most analyses to a larger clade comprising Oenothera, Calylophus, Gaura, and Stenosiphon. Chromosomal and morphological evolution are examined on the basis of the emerging Onagraceae phylogeny.

Key words: ITS, Onagraceae, Onagreae, phylogenetics