Over half the named species of tree-kangaroos in existence today occur in the high montane forests of New Guinea.  These all fall in the category of the highly-derived species and some are reported to be quite specialised in diet and behaviour (Martin, 2005). New Guinea's highlands are very young, with most tectonic uplift occurring over the last 5 million years, with evidence that uplift is still happening (Riker-Coleman et al, 2006). 

Ancestral tree-kangaroos would have happily moved into these lush, well-forested mountain tops and from there, just like Petrogale species isolated on rocky outcrops, speciated.  It may be that this speciation occurred purely as a result of long isolation by time: it's quite a long way from one mountain-top to another, but equally likely is vicariance brought about by periods of glaciation in the Pleistocene. These glacial periods are well-known to have affected the distribution of vertebrates in Australia's Wet Tropics (Winter, 1997).

Palynology tells us that over the past 190,000 years, the rainforests of the area have undergone repeated expansions and contractions and that as a result, mammal distributions have become determined by the persistence of refugia (Winter, 1997).  In the Wet Tropics region, for example, mammal distribution can be divided into two discrete refugia: the Thornton Unit to the North and the Atherton unit to the south, divided by an 80km wide gap of dry forest known as the Black Mountain Corridor (Bell et al, 1987).  It is thought that mammals were confined to either one or the other of these two refugia and a subsequent warm and wet vicariant phase determined the distribution of cool-adapted upland isolate species (Winter, 1997).  While isolation into these refugia has not been cited as a determining factor for tree-kangaroo distribution in the Wet Tropics, it is interesting that the ranges of Australia’s two species do fall generally into one of these two units.  It is therefore not entirely unlikely that the same basic pressures that cause speciation in many organisms, i.e. geographic and reproductive isolation brought about by vicariance, have been the cause of the diversity of tree-kangaroo species in existence today.

Figure 1.  Wet Tropics region showing the Black Mountain Corridor (black bar). Source: Taberlet, 1998

 

 





Figure 2.  Comparative distribution maps of the two Australian species of tree-kangaroo. Source: www.tenkile.com Accessed 22/5/15






Bell, F. C. (1987, May). Distribution, area and tenure of rainforest in northeastern Australia. Royal Society of Queensland.

Martin, R. (2005).  Tree-kangaroos of  Australia and New Guinea.  CSIRO Publishing, Melbourne. 

Riker‐Coleman, K. E., Gallup, C. D., Wallace, L. M., Webster, J. M., Cheng, H., & Edwards, R. L. (2006). Evidence of Holocene uplift in east New Britain, Papua New Guinea. Geophysical research letters, 33(18).

Taberlet, P. (1998). Biodiversity at the intraspecific level: the comparative phylogeographic approach. Journal of Biotechnology, 64(1), 91-100.

Winter, J. W. (1997). Responses of non-volant mammals to late Quaternary climatic changes in the wet tropics region of north-eastern Australia. Wildlife Research, 24(5), 493-511.