The Cape element in the Afrotemperate flora: from Cape to Cairo?

Abstract

The build-up of biodiversity is the result of immigration and in situ speciation. We investigate these two processes for four lineages (Disa, Irideae p.p., the Pentaschistis clade and Restionaceae) that are widespread in the Afrotemperate flora. These four lineages may be representative of the numerous clades which are species rich in the Cape and also occur in the highlands of tropical Africa. It is as yet unclear in which direction the lineages spread. Three hypotheses have been proposed: (i) a tropical origin with a southward migration towards the Cape, (ii) a Cape origin with a northward migration into tropical Africa, and (iii) vicariance. None of these hypotheses has been thoroughly tested. We reconstruct the historical biogeography of the four lineages using likelihood optimization onto molecular phylogenies. We find that tropical taxa are nested within a predominantly Cape clade. There is unidirectional migration from the Cape into the Drakensberg and from there northwards into tropical Africa. The amount of in situ diversification differs between areas and clades. Dating estimates show that the migration into tropical East Africa has occurred in the last 17 Myr, consistent with the Mio-Pliocene formation of the mountains in this area.

Figure 1

Map indicating areas used in analysis. ‘Western Africa’ including: A the Fouta Djalon in Guinea and B Mount Cameroon; ‘Eastern Africa’ including: C Ethiopian highlands, D Virunga and Rwenzori mountains and E North Tanzanian/West and Central Kenyan/East Ugandan mountains; ‘South Central African centre’ including: F Nyika Plateau and southern Tanzanian highlands and G Mount Mulanje; ‘Zimbabwe overlap region’ including: H Chimanimani Mountains and Nyanga Plateau; the ‘Drakensberg range’ I; and the Greater Cape Floristic Region J. Areas north of the Limpopo from White (1978). Map adapted from (Linder 1990).

Figure 2

Optimization of ancestral node distribution for the Irideae p.p. including proportional likelihoods of areas for nodes that do not optimize unambiguously. Area: C, Cape; DR, Drakensberg range; ZOR, Zimbabwe overlap region; SCA, South Central Africa; EA, East Africa and Med, Mediterranean.

Figure 3

Diagram showing the number of migration events between the Greater Cape Floristic Region, the Drakensberg range and the north of the Limpopo River. Unambiguous migrations only were considered.