The phylogeny and biogeography of Thamnochortus (Restionaceae)

Abstract

Parsimony analysis of morphological data was used to demonstrate the existence of five groups in Thamnochorius (Restionaceae). Although the most parsimonious trees have a resolved relationship among these groups, there appears to be little support for this resolution. The composition of the terminal groups, and the relationships among the species making up these groups, is more robust. The distribution patterns of Thamnochortus differ only in detail from the general patterns ascribed to the Cape Flora (southern South Africa). Within the Cape Floristic Region four centres can be recognized, and more than half of the Thamnochortus species are endemic to these centres. Embedded in these centres (or phytochoria) are small centres of endemism: in the Cedarberg, Bokkeveld mountains, Cape Peninsula, Overberg, Bredasdorp plain and the Langeberg. These centres are best demonstrated by mapping the distributions of range-restricted species, rather than using parsimony analysis of endemicity. There are two major patterns within the phytogeographical elements: an arid group, which ranges from the West Coast to the Klein Swartberg, and a mesic coastal group. The coastal group can be further subdivided. A cladistic biogeographic analysis indicates that the first division follows the divide between all-year rainfall and summer drought, and the second division suggests greater aridity. This implies that geographical differentiation within the genus has followed climatic patterns, suggesting that some of the speciation may be a consequence of climatic change in southern Africa. Curiously, centres of endemism appear to be defined too narrowly for effective cladistic biogeographic analysis, and more success is obtained using wider areas. (C) 1998 The Linnean Society of London