Evolutionary relationships among species within the large plant genus Ruellia (Acanthaceae) have never been studied. With approximately 300 species, Ruellia is geographically widespread and morphologically diverse. Molecular data for almost half of the genus in addition to 16 closely related genera in Ruellieae were used to reconstruct phylogenetic relationships and to understand morphological evolution. Ruellia s.l. (including Blechum spp., Eusiphon geayi, and Polylychnis fulgens) is monophyletic: Old World species form an early-diverging grade from which New World species are derived and monophyletic. Several New World lineages are informally recognized and supported by morphology. Two new taxonomic combinations Ruellia geayi and Ruellia fulgens are made. Additional study of morphology and relationships within the New World Blechum lineage of Ruellia resulted in a transfer of the entire genus Blechum to Ruellia. New lectotypifications, combinations (Ruellia haughtii and Ruellia panamensis), synonymies, and further nomenclatural recommendations are made. Another lineage of New World Ruellia, Ruellia section Chiropterophila, is composed of rare species that are endemic to Mexico. To better understand these poorly known species, a taxonomic revision of sect. Chiropterophila was conducted. This revision includes phylogenetic analyses and thorough elevation, floristic, habitat, rainfall, and temperature maps generated via GIS. A key to all 12 known species of section Chiropterophila including one species described as new to science, Ruellia laslobasensis, is provided. Using the comprehensive species phylogeny of Ruellia, the evolution of pollination systems was studied. Phylogenetic analyses, morphological ordinations, ancestral state reconstructions, and a character mapping simulation were conducted to reveal key patterns in the direction and lability of floral characters associated with pollination. Significant floral morphological differences among species were found, and these were generally associated with different groups of floral visitors. Floral evolution has been highly labile and also directional. Some specialized systems such as hawkmoth or bat pollination are likely evolutionary dead-ends. In contrast, specialized pollination by hummingbirds is clearly not a dead-end. There is strong evidence for multiple reverse transitions from presumed ancestral hummingbird pollination to more derived bee or insect pollination. These repeated origins of insect pollination from hummingbird-pollinated ancestors have not evolved without historical baggage. Flowers of insect-pollinated species derived from hummingbird-pollinated ancestors are morphologically more similar to hummingbird flowers than they are to other more distantly related insect-pollinated flowers. Finally, some pollinator switches were concomitant with changes in floral morphology that are associated with those pollinators. These observations are consistent with the hypothesis that some transitions have been adaptive in the evolution of Ruellia.