Previous classifications based on morphology recognized a distinctive but poorly delimited group of species from western North America as Oenothera subg. Pachylophus. In earlier papers, I split this group into four sections more comparable to others in the genus, but nevertheless assumed the whole group was monophyletic based on morphological analyses. Recent molecular analyses indicate that the morphological analysis was incorrect, however, and suggest instead that the sections are part of a basal grade within the genus. The two yellow flowered sections are shown by the molecular study to be sister taxa, yet because of highly distinctive morphology, especially of the capsules and seeds, I maintain them as separate sections, sect. Contortae (O. xylocarpa) and sect. Eremia (O. primiveris) comparable to others in the genus. The two white flowered sections, sect. Pachylophus (O. brandegeei, O. cespitosa, O. cavernae, O. harringtonii, and O. psammophila) and sect. Ravenia (O. muelleri, O. riskindii, and O. tubifera with two subspecies, tubifera and macrocarpa), are not so closely related and the shift to white petals appears to have been independent. Detailed taxonomic descriptions are given for the species of sects. Contortae, Eremia, and Ravenia. Taxonomic treatment of the species of sect. Pachylophus was given elsewhere.
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