Phylogenetics of the grass genus Ehrharta: evidence for radiation in the summer-arid zone of the South African Cape.

Climatic and geological change may play a key role in stimulating biological radiations. Here, we use phylogenetic data to test whether the comparatively high diversity of ehrharteoid grasses in the Cape region of South Africa is the result of rapid radiation associated with the onset of a seasonally arid climate during the late Miocene. A phylogenetic hypothesis based on morphological and nucleotide sequence (nuclear ITS1 and plastid trnL-F) data confirms the monophyly of the African Ehrharta species and shows that the diversification of this lineage was centered in the Cape region. Sequence divergence data (ITS1 + trnL-F) indicate a pulse of rapid speciation, which may explain poor phylogenetic resolution within the African Ehrharta clade. Alternative calibrations yield a broad range of time estimates for the start and end of this radiation, most of which indicate a radiation inside the last 11 million years. A calibration based on the age of Ehrhartoideae suggests that radiation started 9.82 +/- 0.20 million years ago and ended 8.74 +/- 0.21 million years ago. Under alternative calibrations, estimated speciation rates during the period of radiation range between 0.87 and 4.18 species per million years. Parsimony optimization of habitat parameters reveals that radiation was correlated with the occupation of seasonally arid succulent karoo environments, wet heathy (fynbos) environments being ancestral. These data support earlier suggestions that late Miocene climatic change stimulated floristic radiation at the Cape, and highlight the potential importance of environmental change in powering diversification in continental floras.