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duration of isolation are reflected in the proportion of endemics in the flora (Table 4.2.4).

Areas are also differentiated by their form, particularly whether they are closed areas, i.e. the species is established in a single, clearly delimited space, or whether the area consists of several partial areas and is thus disjunct (see Chap. 4.1.2). In closed areas, gaps between individual growing sites are so small that they may be bridged easily and quickly by transport of propagules or pollen. In disjunct areas this is no longer possible. It may be assumed that poly-phyletic origin of the same species does not occur and therefore other explanations for the genesis of disjunctive areas must be found. One explanation might be an extremely rare distribution event, e.g. by migrating birds, or atmospheric anomalies. In many cases, however, it is known that present disjunct areas were once closed and were either separated by tectonic events (e.g. continental drift), formation of mountains, or climatic changes (e.g. change in cold and warm periods). For the latter there are many examples with Arcto-Alpine species (e.g. Draba aizoides).

With the separation of once closed areas, so-called vicarious areas may develop. In the partial areas, the populations of once uniform species developed further in different ways. This would be called geographical vicariance with examples of the genera Kleinia and Aeonium (Fig. 4.2.5). The centres of diversity of these species in southern and eastern tropical Africa as well as in Macronesia and Cap Verde were

I Fig. 4.2.5. Vicariance of the two genera Kleinia and Aeonium. (After Deil and Müller-Hohenstein 1984, from Richter 1997)

separated by the north African dry regions. If subspecies or other species of a genus are established in the same space at different sites, this is called ecological vicariance; examples are the two alpine rhododendrons, Rhododendron hirsu-tum and R. ferrugineum, respectively, on limestone and silicate rocks.

If many related species are distributed closely together, it may be concluded that they have developed within the same space and that the area is a centre of diversity (genetic centre, central zone of related groups) or at least a maintenance centre of this genus. Plant breeders aim to find such centres for economically important plants, as they hope to find important gene reserves.

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