‘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.