Australia: The Land Where Time Began

A biography of the Australian continent 

Evolution and Ecology - Darwinian and neo-Darwinian Models

In 1859 Darwin reasoned that as species that are highly adapted arise, the less adapted species that are relatives that are less competitive are forced to move out of the area occupied by the newly evolved species. When more recent, advances forms arose in the north, the more primitive forms, such as marsupials and ratites were displaced further south to the ‘ends of the Earth’: regions of southern Africa, Patagonia and the Tasman region. This logical account integrates adaptation, ecology at the community level, speciation and global biogeography. The ‘neo-Darwinian’ Modern Synthesis (Mathew, 1915; Mayr, 1942) that according to Heads1 supported a model of evolution ‘that was the complete opposite of Darwin’s view, and is now almost universal’. Possibly the last authors to support Darwin’s 1859 early analysis of centres of origin and dispersal were Frey, 1993; Briggs, 2005). According to the neo-Darwinian model new clades, rather than old clades, moved to new habitats by the mechanism of chance dispersal across a barrier and are therefore separated from their relatives. Extrinsic needs that are imposed on the migrating species by the new conditions (either habitat or habitats) are forced to respond by evolving to adapt to the new conditions where they adapt by natural selection acting on random mutations.

The pan-selectionist viewpoint was adopted by Darwin, as supported by the neo-Darwinians, in his earlier publications, including the first edition of the Origin of Species (1859), though in his more profound later work his emphasis shifted as he began investigating ‘laws of growth’. The reductions and fusions of organs in phylogenetic series are examples of these series, as well as the concomitant development of 5-fold and bilateral symmetry (Heads, 2009c). Darwin wrote, 23 years after the Origin was published (1882: 61) that he admitted that in the earlier editions of Origin of Species he perhaps attributed too much to the action of natural selection as he was not able to annul the influence of his former belief, that as he said, was universal at the time, that individual species had been created purposely which led to his assumption that all details of structure, apart from the rudiments, had a special, though unrecognised, function. So he stated that that anyone with this assumption in his mind would extend the action of natural selection too far.

Therefore, in Darwin’s later work he regarded the structure of an organ as being determined by its previous evolutionary trends, rather than its current function. A similar conclusion has been reached by current work in genomic evolution, which has rejected the selectionist model of evolution and stressing specific structure of genomic architecture and the effects of mutation (Stoltzfus, 2006; Lynch, 2007; Heads, 2012a, Chapter 10). According to Heads1 these are equivalent to Darwin’s laws of growth.

In summary, the key points of Darwin’s approach have been reversed by the ‘neo-Darwinian’ Modern Synthesis:

Darwin suggested that advanced groups are found to be in their centre of origin, which is the location which is accepted by the neo-Darwinian synthesis, this is the region where the primitive, basal groups are present.

Darwin adopted a relativistic species concept that assumed that there were no qualitative differences between species and clades with other ranks – species are not special, rejecting the criterion of reproductive isolation as essentialistic (See pp. 183-184 of Heads’ book1). In the neo-Darwinian synthesis proposed instead that only species are real and that they have a value that is absolute; they are defined by reproductive isolation.

According to Darwin prior trends in variation that are non-random (laws of growth) provide the material for selection to work upon. It is assumed, instead, by the neo-Darwinian synthesis that adaptation results from the selection of random mutations and therefore the structure of an organ is explained by its required function.

Evolution and Ecology – Beyond the CODA Model

According to the view of the Modern Synthesis evolution and ecology are linked with the idea of structural evolution occurring by the species adapting to extrinsic needs. This approach is the CODA paradigm, as it is based on centres of origin, dispersal and adaptation (Lomolino & Brown, 2009) with evolution proceeding by adaptation and the ‘penetration’ or ‘wedging’  of new species into a ‘niche’ that is not occupied. Ecology has been dominated since the 1950s by the paradigm, according to which, species packing and the filling of any available niche spaces limits diversity (Ricklefs & Jenkins, 2011).

According to the neo-Darwinian model the morphology and physiology of a group are determined by environmental conditions. A new habitat is invaded by a new group where it survives before it adapts to the new conditions. According to an alternative model, a group evolves at the time of its distribution, its ecology and habitat being determined by its biogeography, morphology and physiology. Morphological trends, such as reduction-fusion-suppression series, result from mutations that are non-random and long-term trends in genomic evolution, such as biased gene conversion, and these structural changes lead to shifts in habitat and habits. An example is the lengthening of the neck of an ungulate; in this case the animal would tend to feed on taller trees. In animals with sight that has been compromised by reduction of the optic system, or with eyes that have become oversensitive, may of necessity change to a nocturnal way of life, or even a subterranean lifestyle. Animals may change to softer food if their teeth have been reduced or completely lost.

Tectonics and regional biogeography can often determine ecology, such as in the case of many aspects of the ecology of a community being determined by their elevational range, which is in tern determined by uplift, subsidence or erosion. Date palms and related fan palms have been isolated, stranded around desert oases, as well as in montane rainforests and on coral atolls in the Pacific Ocean. An ecology has been inherited, along with their distribution. According to the CODA model the palms have invaded the extreme habitats in the desert of Arabia or the atolls of the Pacific, following which they have adapted to their new situation by evolving features that allow them to survive there.

The ‘marsupial mole’, Notoryctes, the last extant member of the order Notoryctemorphia, is restricted to the sand deserts of central Australia. According to the CODA model this animal invaded the underground habitat of the sand deserts from a centre of origin, and then developed the features that are regarded as bizarre as an adaptation to the needs of the new conditions of its habitat. Fossils from Queensland, that have been dated to about 20 Ma, have demonstrated that rainforest members of the order, had developed the defining features of the order long before the sand deserts existed (Archer et al., 2011). It is believed that rather than the order dispersing to the desert after which they adapted to it, earlier forms were living in the region when it was still covered by rainforest, only becoming desert specialists after the reafforests had been replaced by deserts, at a time before the modern clades had evolved.

According to this model ancestral landscapes were already occupied by polymorphic ancestral complexes, with aspects of modern biogeography and biodiversity already present for a long time prior to the development of extant groups or the landscapes they occupy. It is often believed that parallelisms concentrated in a particular area are the result of particular needs imposed by the environment, though they are just as likely to be a reflection of ancestral polymorphism. An example is that many unrelated groups that produce trees in the land areas of the Pacific that elsewhere produce herbs, which is an indication of the existence of prior concentrations of ‘tree genes’ in the Pacific Basin, before the rise of modern clades. The centre of diversity of divaricate plants around the Indian and Atlantic Oceans, as well as their rarity on and to the east of New Guinea and the Chatham Islands, is another example.

Heads1 suggests either chance dispersal or vicariance can explain any pattern, and predictions made by the 2 models can be explored. It is predicted by a vicariance model that the distribution of clades in a group will be highly structured and will reflect phylogeny in a simple way. Another prediction made by it is that the phylogenetic/geographic breaks present in one group will also be found in other groups that have ecology that is different and that have different dispersal mechanisms. The same revolutions from the Mesozoic have all affected ‘old’ groups such as bryophytes and arthropods, and ‘young’ groups, e.g., birds, angiosperms and mammals. Distribution in groups of invertebrates or lower plants could shed light on the evolution of other groups, including mammals, birds and angiosperms, if dispersal means are not directly relevant to biogeography.

For biogeography there are 2 possibilities, either all the possibilities that have been discussed in Heads’ book1 could result from chance dispersal and chance extinction. There would not be much more to say about biogeography if this were true; all patterns that were of coincidental distribution would be illusory pseudopatterns, because the different components would have developed at different times as the result of different chance events. Dispersal means would appear to be puzzling and enigmatic, and any biogeographic distribution that was not related to ecology would appear more or less meaningless, and the model would be compatible with a literal reading of the ages of clades in the fossil record.

Alternatively, simple vicariance could be the cause of evolution at intercontinental, regional and local scales and the expansion of range of communities by normal dispersal means (geodispersal), both processes being triggered by geological changes. In this view accurate estimates of evolutionary chronology for most terrestrial groups is not provided by the fossil record, and distribution and dispersal are neither by chance nor are they enigmatic, being capable of being treated as evidence in the own right, with normal observations and comparisons being used to analyse them.

Distribution and dispersal are indicated by the molecular phylogenies themselves to be not random, in spite of the wide range of variation in details, coherent patterns emerging that are shared with groups of different ecology. Sequencing studies are said by Heads1 to be revealing interlocking, geographic structures at all scales, with many of the patterns being based on simple recombinations of just a few nodes. The underlying paradigm of dispersal by chance can be rejected as ‘a conceptual roadblock impeding further progress’1. Everyone is now in agreement ‘that geographic distribution has turned out to have a special phylogenetic importance. It is, in fact, what Darwin (1887) thought it was – a keystone of the laws of creation’.

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
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