The theory of speciation thought to be the most widely applicable to all organisms is that of allopatry. Allopatry occurs when a peripheral population is geographically and reproductively isolated from the main population. Often these peripheries are at the edge of ecological tolerance for a species, so variation that extends a fitness advantage is more rapidly taken up by a population, with the usually smaller population size aiding in this genetic dispersal.(Freeman and Herron, 2007).
In Petrogale, allopatric speciation appears to have been the mode of speciation resposible for diversity (Sharman et al, 1990) but in Dendrolagus, the mode of speciation was unclear, with ancestral forms (the two Australian species) seemingly on the outer edge of the stronghold of the more derived forms (most New Guinea species) (Martin, 2005). This conundrum was even met with a proposal of a new pattern of speciation, called centrifugal (Groves, 1990).
In light of the recent discovery of the dry-forest Bohra fossils, the dry-forest incursions of present-day D. bennettianus and taking into account the relationship between dry country Petrogale and Dendrolagus, it may well be that tree kangaroos originated in Australia under dry conditions (Martin, 2005). If this is the case, then the pattern of allopatric speciation fits, with the more derived species occuring in very different environments to the ancestral group, with different groups becoming periodically isolated by contraction of their montane forest habitats (Winter, 1997).
Despite the general paucity of knowledge about these animals, it seems that we are piecing together a picture of the evolution of an amazing creature.
 |
| An example of the dry habitat inhabited by a Petrogale species (P. xanthopus) in Western Queensland. Source: www.ehp.qld.gov.au Retrieved 24/4/2015 |
Darwin, C. (1859). The origin of species by means of natural selection: or, the preservation of favored races in the struggle for life. Reprinted 2008. Ed. Quammen, D. Sterling. New York
Eldridge, N. and Gould, S. J. (1972). Punctuated equilibria: an alternative to phyletic gradualism, in Essential Readings in Evolutionary Biology. John Hopkins University Press. Baltimore
Freeman, S. and Herron, J.C. (2007). Evolutionary Analysis. Pearson Educational.
Groves, C. P. (1990). The centrifugal pattern of speciation in Meganesian rainforest mammals. Memoirs of the Queensland Museum, 28, 325-328.
Martin, R. (2005). Tree-kangaroos of Australia and New Guinea. CSIRO Publishing, Melbourne.
Sharman, G. B., Close, R.
L., & Maynes, G. M. (1990). Chromosome evolution, phylogeny and
speciation of rock wallabies (Petrogale, Macropodidae). Australian Journal of Zoology, 37(3), 351-363.
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.
An interesting set of evolutionary ideas. Could you expand a little on this idea of centrifugal speciation? If allopatry is in operation, then the peripheries are areas considered more disturbed. Would this suggest that these newer allopatric species would cope better with disturbance than older more established populations? There is a concern (from what I hear) that tree kangaroos are threatened by forest fragmentation, but if these are indeed allopatrically derived and more capable of coping with the disturbances of periphery-type effects, could they actually just cope with this fragmentation? A bit far-fetched I suppose, but interesting food for thought.
ReplyDeleteThis comment has been removed by the author.
ReplyDeleteHi Tasmin, firstly, the idea of centrifugal evolution has been around for quite a while and didn't originate with Groves as I erroneously said in the post. To summarise a paper from William Brown (1957), centrifugal speciation occurs when a species expands to the limits of its geographic range, then contracts for some reason, leaving behind refugial populations that then speciate in their own ways.
ReplyDeleteOn the allopatry question, if the peripheries are not necessarily considered more disturbed, but merely different from the ancestrally tolerable conditions, then perhaps we would expect to see these peripheral populations moving further towards adaptive peaks in order to thrive in the new conditions. I would expect that a higher degree of specialisation would in fact lower the derived species' tolerance for disturbance. This is my understanding, but please correct me if I'm wrong, which is always highly likely ;)
For Lumholtz's tree-kangaroo, which has probably been affected most by habitat clearing, Graham Newell's study in 1999 found that they show remarkable site tenacity. This means that they don't handle displacement well and will persist in fragments rather than relocating. From my own recent experience and discussions with researchers currently trying to quantify fragment use by this species, though, it seems that breeding females are thriving quite happily in very small (<1ha) fragments. Of course, long term genetic effects of this fragmentation are unknown as yet, and likely to be deleterious. Unfortunately I couldn't find much on the effects of fragmentation on New Guinea tree-roos, so I'm unsure how these derived species are reacting. Food for thought indeed :)