Based on the most recent revision, the Bouteloua curtipendula complex is a group of 11 grass species and 5 varieties native to the New World. Non-coding nuclear ribosomal and chloroplast DNA sequences (ITS and trnT-L-F regions, respectively) subjected to parsimony cladistic analysis show that the complex is monophyletic, though most of the species are para- or polyphyletic. Both phylogenies have three major clades. The largest clade includes mostly polyploids with more than one spikelet per branch and usually orange, red, or purple anthers. The other two clades group mostly diploid taxa with yellow anthers, however the number of spikelets per branch varies from one to many. The phylogenies suggest that diploidy is ancestral. Samples showing incongruencies between the ITS and trnT-L-F phylogenies were found to be polyploids and in possession of several different copies of the ITS region, indicating that hybridization via allopolyploidy has played a role in the diversification of the complex. Homoplasy, hybridization, polyploidy, and the reported occurrence of apomixis combine to make circumscription of taxa in this group challenging.

Key words: allopolyploidy, Bouteloua curtipendula, evolution, gramineae