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Ecologically specialized plants are expected to be at greater risk of extinction than generalists due to climate change. Such risk is greatest in biodiversity hotspots such as the Greater Cape Floristic Region (GCFR), which accommodates both ecological specialists and generalists. Thesium L., a genus with the highest number of species in Santalaceae and the most diverse in Africa, offers an appropriate system for evaluating both the correlates of range extent and specialization and the relative extinction risks associated with both. We hypothesized that range size, ecological specialization, and consequent climatically modulated extinction risks are all phylogenetically structured, such that climate change will precipitate a disproportionate loss of phylogenetic diversity in the GCFR Thesium. Past and future species distribution ranges were predicted using MaxEnt models based on present‐day occurrences and environmental conditions. Of the 101 Thesium species modeled, 70% have had large range sizes during the Last Glacial Maximum (LGM), 50% currently have a large range size, and future conditions are predicted to allow 40% to obtain large range sizes. Between the LGM and the present, 17% of species are thought to have undergone a contraction of available range space in the present time whereas 37% are expected to expand their ranges into the future, while 51% of species will experience range contractions. Of the 65 species currently ranked as Least Concern in the South African Red List, 24% will likely shift into higher extinction risk categories. Interestingly, 8.5% of ecological specialists, although having experienced a range reduction from the LGM to the present, are predicted to persist in the face of future climate change. However, the range extent, ecological specialization, and extinction risk are phylogenetically random and therefore should have a negligible impact on the phylogenetic diversity of the GCFR Thesium. [ABSTRACT FROM AUTHOR]