The aervoid clade of Amaranthaceae includes the genera Aerva, Kelita, Nothosaerva, and Omegandra and the diverse Australian genus Ptilotus. Unresolved phylogenetic relationships among these genera have hindered investigations into their evolution. These relationships were tested in this thesis, which led to a re-circumscription of their taxonomy. The morphological and ecological partitioning within two widespread species complexes of Ptilotus were also evaluated, resulting in new species. A better understanding of relationships within the aervoids enabled their chronological, ecological and geographic origins to be tested, indicating that Ptilotus originated in Miocene Australia and underwent rapid cladogensis during widespread aridification in Australia.