Australia: The Land Where Time Began

A biography of the Australian continent 

Australian Biogeography

A previously unrecognised level of organic structure, from submicroscopic to continental scale has been revealed by molecular studies of the Australasian biota. Also, global biogeography and evolution studies have revealed by DNA sequencing that many large groups, such as flowering plants (angiosperms), passerine birds and squamates have their basal components in this area.

Michael Heads uses a number of examples, such as platypuses and kangaroos to kiwis and birds of paradise in his book1 to examine the patterns of distribution and evolution of the Australasian biodiversity, explaining them by referring to tectonic and climatic changes that have occurred in this region. It has been demonstrated, surprisingly, by molecular biogeography that it is essential to understand evolution in Australasia to understand the development of modern life on Earth.

Evolution – spatial component  

For most groups of plants and animals high levels of geographic structure have been demonstrated by molecular studies, and there are fundamental implications of this for the science of biogeography. According to Michael Heads1 there is fascinating depth and detail in the geographic structure, as well as pattern replication among groups that are unrelated, all of which, he claims, threaten the undermining of biogeography theory of the 20th century, in the form of speciation by chance dispersal. Dispersal theory has dominated the field ever since an influential study was carried out by Mathew in 1915. According to Heads1 the idea of dispersal as a mode of speciation, and not just the movement of organisms is still often accepted as fact at the present, including in molecular work. Chance dispersal has sometimes been suggested to have been “revealed” by biologists producing molecular phylogenies, though in scientific studies everyone agrees on the facts, not the inferences and interpretations, which have been revealed. Heads1 agrees that spectacular patterns of distribution have been revealed by molecular studies that can be accepted as ‘facts’ for the purpose of discussion, though he suggests that what is less obvious are the underlying causes of these facts.

In his book Heads1 interpreted the new molecular work that has shown the patterns of distribution with reference not to chance, but to a few geological and climatic revolutions. Among these are the pre-breakup tectonics of Gondwana, extension and magmatism, rifting of Gondwana, and the last, great flooding that occurred in the mid-Cretaceous, that was probably caused by a combination of tectonic and climatic events. Heads1 doesn’t deny the existence of chance dispersals, instead focusing here on patterns that are repeating, and not on idiosyncratic distributions that are present in only 1 or a few groups.

Chance dispersal is the main mode of speciation according to the Modern Synthesis. When a group arises by dispersal across a barrier it attains its distribution by a second dispersal, which a range expansion outwards. Different clades develop during a vicariance (fragmentation of a widespread ancestral distribution by the appearance of a new barrier) event, from a common ancestor that is already widespread, each new clade originating at the same time as its distribution, more or less in situ. According to Heads1 by the end of the Enlightenment, around the same time as the introduction of evolutionary theory, the relative importance of dispersal and vicariance was already being debated (White, 1789[1977: 65]; Willdenow, 1798: 430 [quoted by Weimarck, 1934). The ideas on evolution have changed in many ways since that time, though by the present the debate over vicariance versus dispersal has continued unabated.

It has been assumed by most writers that physical movement, ‘dispersal’, is the cause of the patterns of distribution. All organisms move, and every individual organism has moved to its present location, though there are a few exceptions. Also, a clade’s distribution is actually mobile, at least at a dynamic equilibrium, though the distribution is marked on maps by dots or lines. In a similar manner, if only vicariance was involved in evolution, there would be only a single type of organism in every small area on the surface of the Earth, and it would be endemic in that area, and as this is obviously not the case, movement must have occurred. Movement does not account for all aspects of distribution, though physical movement is universal in nearly all individuals, and it has occurred in many clades. Cessation or a reduction of the rate of interchange among populations has led to many phylogenetic/geographic breaks.

Heads1 suggests it is not so much whether vicariance or dispersal have taken place but whether dispersal and speciation have resulted from chance movement of individual organisms or general, underlying causes such as geological changes. A research programme (dispersal biogeography) that explains the geographical distribution of organisms, which is based on dispersal processes, and the distribution with reference to geological events is vicariance biogeography (Gillespie et al., 2012). Expansion of range and speciation, vicariance, both result from geological change, according to vicariance theory, examples being marine incursions or mountain building. According to dispersal theory, neither is caused by geological change, expansion of range and speciation with dispersal over long distances occur at different times in different groups, both being due to chance.

Sources & Further reading

Heads, Michael, 2014, Biogeography of Australasia: A Molecular Analysis, Cambridge University Press


Author: M. H. Monroe
Last updated  05/08/2014
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