The high dispersal rate observed between northwest Africa–Macaronesia and the eastern Africa–southern Arabia ( and ) gives some support to the vicariance hypothesis (a
), which postulates the fragmentation of a continuous, subtropical flora in northern Africa that was divided by climatic events into eastern and western refuges (Axelrod & Raven 1978
). However, rather than one vicariant event across multiple groups, this rate is likely to reflect repeated events of dispersal and vicariance between east and west Africa following the alternation of cycles of arid and humid periods in the Saharan Desert since the Late Miocene/Pliocene (Thiv et al. 2010
The high dispersal rate between northern Africa and the eastern region stands in constrast with the low dispersal rate between this region and southern Africa ( and ). This can be interpreted as either a historical low rate of biotic exchange between southern and northern Africa, or, alternatively, that this exchange is too old to leave a signal in our phylogenetic data, that is, the south–north African disjunctions may be older than the northwest-eastern African disjunctions and therefore more likely to have been obscured or wiped-out by later extinction events. Indeed, there is some support for this hypothesis. The fossil record suggests that forests covering the whole of tropical/central Africa appeared in the Oligocene–Miocene, and geological data give the same time frame for mountain formation and the establishment of drier areas in northern Africa. In contrast, direct contact between the African continent and Eurasia was not established until the Miocene, when the Arabian Plate collided with the Eurasian Plate 16 Ma ago. If southern Africa was a part of a continuous Rand Flora, the region is likely to have been separated by climatological barriers from the northern areas earlier than the start of major biotic exchange between northern Africa and Eurasia.
Interestingly, northwest Africa plus Macaronesia show the smallest carrying capacity but present the highest dispersal rate (), suggesting that the flora of this region was built up by immigration of new lineages. Migration from the Mediterranean region was probably the dominant route (), but dispersal from west Asia via the Arabian Plate and across the Saharan desert is another possible route supported by our data.
Conversely, the high carrying capacity of southern Africa () gives some support to the hypothesis that the highly diverse South African Cape flora has diverged in situ
, facilitated by the relative climatological stability of the area from the Miocene onwards (Linder 2005
). Similarly, the low dispersal rate with all other regions but eastern Africa agrees well with the idea that, following the formation of the eastern African mountains in the Pliocene, some south African lineages migrated to the north (via the Grand Rift and the Drakensberg mountains) and gave rise to the highly endemic eastern African mountain flora (Linder 2005
; Galley et al. 2007
The results from this analysis should be considered preliminary for several reasons: the dataset only represents a sample of groups showing this disjunction (Andrus et al. 2004
); all phylogenies are not complete and some of the relevant disjunct taxa are missing (e.g. Monsonia
, see the electronic supplementary material); finally, there is a potential underestimation of groups with southern African distributions (e.g. Euphorbia
). Despite these drawbacks, our results suggest that the Bayesian island model may be useful in a continental setting, where the number of inferred dispersal/migration events between areas is low.
Although originally designed to study dispersal patterns in islands, the BIB method may also be used to detect possible vicariance events, by incorporating estimates of absolute divergence times. Vicariance predicts biogeographical and temporal congruence in disjunct distribution patterns across different plant groups, and that the disjunct distribution is at least as old as the geological barrier that caused it. By plotting the rate of dispersal over time, the appearance of a vicariance event can be detected: a decrease in dispersal frequency between two areas would suggest the formation of a new geographical barrier between them. BIB can even be used to estimate the timing of the paleogeographic barrier from the dated phylogenies, by comparing a one-rate model, in which the rate of biotic exchange is constant before and after the barrier, with a two-rate model in which there are two parameters, the rate of exchange before and after the barrier.
Future work should focus on testing asymmetric dispersal patterns, e.g. northward versus southward migration between southern Africa and northern–eastern Africa, as well as on incorporating absolute times to the inference, either directly as molecular divergence time estimates or indirectly through the use of fossil and paleogeographical information (Ree & Sanmartín 2009