‘Inclusion species’ – evidence for sporadic ancient dispersal?
Among the 64 delineated species, we identified only 11 (17%) that were collected in a different zoogeographic realm than the majority of species in their respective clades (we termed such species as ‘inclusion species’ as their geographic localities stand out from the localities of other species in their clades; Table 4). Given that all these species diverged from their closest relatives (as identified in our dataset) ca. 13–91 Mya (47 Mya, on average), we interpret them as examples of ancient dispersal. Although we do not know when the dispersal occurred, the deep divergences observed between ‘inclusion species’ and their most close relatives suggest that the colonisations of new zoogeographic realms occurred in the deep past rather than recently. Alternatively, ‘inclusion species’ may be artefacts resulting from undersampling. Specifically, it is possible that if more populations of these species are collected (currently, all except three are singletons), these new populations will represent the dominating zoogeographic realm in the clade. In such a case, ‘inclusion species’ would become examples of species with wide geographic ranges, indicating recent dispersal events. Thus, more intense sampling is required to test these alternative hypotheses. Nevertheless, current data support the first hypothesis about ancient dispersal and although the existence of ‘inclusion species’ constitutes evidence that LDD occurs in Milnesium , the low percentage of such incidents suggests that LDD is rare. Moreover, the classification of one of the ‘inclusion species’, the OrientalMilnesium sp. nov. ID.947 (species #53) is questionable, because it was collected in Sulawesi, i.e. in the proximity of the Australian realm in which all other species in clade D were found, therefore it is an example of a short-distance dispersal (SDD) rather than LDD, even though the new locality lies in a different realm. Another examples of a potential SDD could be the two Madagascan species (#33 and #46) embedded in the otherwise South African clade C.