* Premise of the study: To understand the origins of C4 grasslands, we must have a better interpretation of plant traits via phylogenetic reconstruction. Muhlenbergiinae, the largest subtribe of C4 grasses in Mexico and the southwestern United States (with 176 species), is taxonomically poorly understood. * Methods: We conducted a phylogenetic analysis of 47 genera and 174 species using six plastid regions (ndhA intron, ndhF, rps16-trnK, rps16 intron, rps3, and rpl32-trnL) and the nuclear ITS 1 and 2 (ribosomal internal transcribed spacer) regions to infer evolutionary relationships and revise the classification. * Key results: In our analyses, Muhlenbergia (ca. 153 species) is paraphyletic, with nine genera (Aegopogon, Bealia, Blepharoneuron, Chaboissaea, Lycurus, Muhlenbergia, Pereilema, Redfieldia, Schaffnerella, and Schedonnardus) found nested within. We recognized the following five well-supported monophyletic lineages within Muhlenbergia: subg. Muhlenbergia, with species that have phosphoenolpyruvate carboxykinase-like leaf anatomy and long, scaly rhizomes; subg. Trichochloa with long-lived species that are relatively tall (up to 3 m); subg. Clomena with 3-nerved upper glumes; sect. Pseudosporobolus species with narrow panicles and plumbeous spikelets; and sect. Bealia species with lemmas with hairy margins and midveins. * Conclusions: We propose expanding the circumscription of Muhlenbergia to include the other nine genera in this subtribe and make the following new combinations: Muhlenbergia subg. Bealia, M. diandra, M. geminiflora, M. paniculata, M. phleoides, M. subg. Pseudosporobolus (also lectotipified), M. solisii, M. tricholepis. We also propose several new names: M. ammophila, M. columbi, M. plumosa. Our phylograms suggest that Muhlenbergia originated in North America because the sister (Sohnsia filifolia and Scleropogoninae) is composed of predominantly North American species.
