Australia: The Land Where Time Began |
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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).
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Author: M.H.Monroe Email: admin@austhrutime.com Sources & Further reading |