We show strong geographical patterning in the morphological and especially the anatomical variation in the widespread southern African danthonioid grass species, Tenaxia disticha. Four varieties can be discriminated, and are formally recognized here as Tenaxia disticha var. disticha, var. vlokii H. P. Linder, var. dracomontana H. P. Linder and var. lustricola H. P. Linder. Tenaxia disticha var. disticha is widespread in eastern southern Africa, from Cape Agulhas at the southern tip of Africa to Inyangani in Zimbabwe; the other three varieties are highly localized: var. vlokii in the southern Cape and vars. dracomontana and lustricola on the high plateau of the Drakensberg. Plastid DNA nucleotide variation revealed only limited geographical pattern, but all regions and all varieties have numerous haplotypes, with no haplotype-poor areas. This is consistent with a model wherein this grass species has occupied its current distribution range for a long time, possibly since the Late Miocene, thus allowing the accumulation of local haplotype diversity. The divergence of the Drakensberg varieties may have been stimulated by the Pliocene uplift of these mountains resulting in a sub-alpine climate. This phylogeographical model is consistent with the hypothesis that southern Africa was not subjected to the dramatic Pleistocene climatic fluctuations driving extensive range changes which were typical of the temperate Northern Hemisphere phylogeographical patterns.
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Vol. 39 • No. 2