The Euphorbia section Monadenium comprises approximately 90 species that are restricted to tropical Africa, with the greatest diversity found in East Africa. Members of this section range from geophytes and perennial herbs to succulent shrubs and small trees, many of which are endemic to arid or seasonally dry habitats. However, their evolutionary and biogeographical history remains poorly understood.

Researchers from the Wuhan Botanical Garden of the Chinese Academy of Sciences have reconstructed a significant part of the evolutionary history of E. sect. Monadenium. The researchers analyzed 81 accessions, including 63 plastomes representing 56 species, which account for about 62% of the recognized species in the section.

Their findings were published in Palaeogeography, Palaeoclimatology, Palaeoecology.

According to the researchers, phylogenetic analyses strongly supported the monophyly of E. sect. Monadenium and identified eight major clades. The results also showed that the formerly recognized genera Monadenium, Synadenium, and Endadenium do not form separate monophyletic lineages.

Divergence-time estimates indicate that the crown diversification of E. sect. Monadenium began approximately 16.69 million years ago in the Early Miocene. Ancestral-range reconstruction suggests an origin in the Zambezian region, followed by repeated dispersal and vicariance across eastern, south-central, and southern Africa.

Although the ancestral range was determined to be in the Zambezian region, the study notes that the current species richness is concentrated in East Africa, particularly around the East African Rift. Changes in rainfall seasonality and precipitation during climatically stressful periods played an important role in shaping ecological differentiation within the section.

Ecological niche modeling revealed fluctuations in suitable habitats, including contractions during the Last Glacial Maximum and expansions during the Middle Holocene. Present-day suitable areas are concentrated mainly in East Africa.

These findings suggest that late Neogene climatic drying, increasing rainfall seasonality, and the heterogeneous landscapes of African jointly promoted the dispersal, isolation, and diversification of this succulent lineage.

Species richness pattern of Euphorbia sect. Monadenium across Africa based on 100 km × 100 km grid cells (Image by WBG)