Australia: The Land Where Time Began

A biography of the Australian continent 

Australasian Biogeography – Conclusions

Revolutions in all the life sciences have resulted from molecular work. It has contributed a large amount of valuable information in biogeography which is summarised in phylogenies, though most interpretations of the phylogenies and explanations of the evolution across space and time rely on old concepts that have been inherited from the Modern Synthesis. A program such as DIVA or DEC in LAGRANGE are the framework in which many papers have been written with the aim of finding a centre of origin for a particular group, and to transmogrify estimates of the minimum age of clades into estimates of the maximum age of the clade when a  Bayesian program such as BEAST is used. According to Heads1 both these procedures and the concepts they are based on are flawed and can be dropped; there is no need to find a centre of origin or to transmogrify dates of fossils outside the Modern Synthesis.

Modern molecular studies are moving away from the Modern Synthesis model in the approach to structural evolution, while still relying on older ideas about evolutionary space and time. Monomorphic ancestors that were undifferentiated were assumed by the Modern Synthesis, with evolution proceeding by random mutation, and groups that are defined by characters that are uniquely defined. Polymorphic ancestral complexes have instead been proposed by new ideas, descent with lineage sorting that is incomplete and evolution being determined by prior genomic architecture and mutational bias instead of selection (Heads, 2012a, Chapter 10).

Evolution – space

Basic to the neo-Darwinian concept of evolution in space and time is the idea that structure develops by growing out from a centre of origin, and not by a series of differentiation events in a widespread entity. Methods and programs such DIVA and DEC/LAGRANGE that were developed in this research tradition assume that for most of the distribution patterns there is a centre of origin model. An area phylogeny A(A(A,B)) will always be interpreted in both programs as dispersal from A to B, though the pattern can also be explained by an ancestor that is widespread and is already present in A and B with a centre of differentiation in A (not a centre of origin), together with subsequent local overlap there. Any dispersal between A and B in the group, though it could possibly have occurred in an ancestor, is not required by the phylogeny.

Evolution does not proceed by radiation from a centre if there is no centre of origin of the groups, but by differentiation at phylogenetic and biogeographic nodes. Range expansion and overlap that is subsequent may or may not occur. Unexpected levels of biogeographic structure at all space and time scales have been found in molecular data. Also, the repetition of breaks at the same localities in many different groups is an indication that distribution of these localities is not random. However, most breaks in modern topography or ecology do not correspond with the obvious breaks, instead showing a general relationship with features that have previously been zones of tectonic instability, such as spreading zones, subduction zones, basins, transform faults, mobile belts and orogens.

Evolution – time

For many modern groups such as the orders of mammals and birds the oldest fossils date from the Cainozoic, and any of these groups that actually originated at that time would have arisen after the Breakup of Gondwana and so not be affected by the breakup. According to Heads1 either chance dispersal results in biogeography, in which case the fossil record is an accurate representation, more or less, of evolution, or the development of patterns of biogeography have not developed randomly, in which case a literal reading of the fossil record, where fossil ages are treated as clade ages, is misleading, at least for most terrestrial groups. According to Heads1 there is a lot of direct evidence in the fossil record of large gaps; an example presented by Heads1 being large extant groups for which there are no known fossil representations in the Cainozoic though they have representative fossils in the Mesozoic (S.W. Heads, 2008). Fossils are useful as they indicate minimum ages, though not as positive evidence of the actual ages of clades, but because they can eliminate origins that are related to younger geological events. Heads1 suggests there is no real reason to assume that groups for which the oldest know fossils are from the Cainozoic could not have been present in the Mesozoic. There are many chronograms of evolution that have been produced using Bayesian methods that assign 95% credibility intervals (higher posterior densities) to the ages of clades. The analysis is based, however, on the prior transmogrification of estimates that are fossil-based of minimum clade ages into estimates of maximum clade ages (with confidence intervals that are narrow), which are then used for calibration.

Fossil identification is another problem with calibration that is fossil-based. Molecular studies have, in many cases, contradicted morphological analyses of phylogeny in extant groups, though it is even more difficult to carry out morphological analysis of fossil groups, and even more likely to be flawed, with placement of pre-Neogene fossils of many groups in a phylogeny is dubious and controversial. In the case of older or fragmentary fossils or which are greatly different from modern forms there are increased doubts. Given the dramatic rearrangements of phylogenies that have occurred in the molecular revolution, especially in groups that are regarded as ‘difficult’, many of the Mesozoic forms that are more unusual are misplaced and actually belong in extant orders.

The method adopted by Heads1 is not against the fossil record, and doesn’t neglect it, as fossil records must be integrated with the extant forms, and he cites many fossils in his book. Biogeographic evidence that he has mentioned has suggested that some brachiopods and crustaceans have fossil records indicating the approximate ages of clades, though the general conclusions reached by heads1  conflict with a literal reading of the fossil record, where the date of the fossil gives the age of the clade. ‘The fossil record is evidence but it cannot be taken at face value’ (A.B. Smith, 2007).

Sources & Further reading

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


Author: M. H. Monroe
Last Updated 13/08/2014
Journey Back Through Time
Experience Australia
Aboriginal Australia
National Parks
Photo Galleries
Site Map
                                                                                           Author: M.H.Monroe  Email:     Sources & Further reading