High endemism of the genus Petalidium (Acanthaceae) in the highlands and escarpments of Angola and Namibia

Abstract

Petalidium (Acanthaceae) is a speciose genus of woody shrubs with its diversity centred in arid regions of central to northwestern Namibia and southwestern Angola. Here, we compiled distribution information for all accepted species in the genus from herbarium specimens and our own fieldwork to determine the number of endemics and near-endemic species in the highlands and escarpments of Angola and Namibia (HEAN). In the process, we provide an informal overview of all accepted species of Petalidium, which we hope will facilitate further research on the genus. We found that 22 of 36 species are endemic or near-endemic to the HEAN, clearly indicating the importance of this area for plant conservation. Furthermore, our results presented here, coupled with evolutionary studies that show the genus to be rapidly diversifying, suggest that the highlands and escarpments have played a significant role in plant speciation. We conclude by highlighting the advantages of studying this genus further as a model system for plant ecology and evolution in arid environments.