Evolution is ultimately an unpredictable process. Although it can be predicted in the short term through knowledge of natural selection and inheritance, long term evolution is randomly altered by the interaction of highly variable factors. Such factors include the randomness of genetic diversity within a species and the process of natural selection acting upon this. Also significantly altering evolution is the unpredictable movement of tectonic plates which often leads to the isolation of large areas of land, such as Australia.
Stemming from this arises various other substantial factors such as a lack of competition and predation and considerable changes in climate and ecosystems. The interaction of these forces (the unpredictable changes) is exactly what happened to Australia and as a consequence, drove the native Australian animals onto a very unique evolutionary path different from any other area land- no matter how similar their environments. It is widely accepted that between 260-180 million years ago, all of the Earth’s land was a part of one single large landmass called ‘Pangaea’.
However, 180 million years ago Pangaea is believed to have split into two smaller ‘supercontinents’, with the land we call Australia connected to the southern landmass of ‘Gondwana’. This came as a result of the random movement of tectonic plates; the outer crust of the Earth consists of about 12 of these plates which are able to move due to convection currents in the mantle. As a result of this continual plate movement, Australia completely rifted from the last Gondwanan continent of Antarctica approximately 45 million years ago.
Since then, up until 5 million years ago, Australia remained as a completely isolated continent. This 40 million-year period of isolation drove Australia’s plants and animals into a unique evolutionary path. Due to Australia’s isolation, fauna from other continents were unable to travel across to Australia. As a result of this, there was a significant lack of competition for Australian species for a very long time. Natural selection in the absence of competition enabled them to evolve in very unique ways.
Natural selection is the evolutionary process whereby heritable traits that enable organisms to more effectively live and reproduce become more prominent within the population. Thus, they increase an organism’s chance of survival. Natural selection is based upon genetic variability producing these favourable traits. Moreover, biotic and abiotic features in the environment such as competition, predation, disease and environmental change impact upon an organism’s need to adapt.
Thus, Australia’s isolation led to an organism’s genetic variation and adaptation becoming more focused on adapting to the environment rather than avoiding predation. As this unique natural selection continued, Australian flora and fauna were precisely adapting to their niche environments; they were adapting to their adaptations. For example, the Koala has many adaptations for its environment but it has limited adaptations for predation; its large paws and claws are adapted for climbing trees and its stomach has adapted to digest the abundant food source of eucalyptus leaves.
However, this slow moving marsupial would not be able to withstand any competition or depletion of its food source or predation. If the Koala and most other Australian organisms were subject to more competition, evolution would have been ‘balanced’ in Australia such as it is in other continents around the world. Its impact on natural selection would have caused adaptations to predators and not just solely to their environmental conditions. In effect Australia’s isolation removed a significant variable from the process of natural selection.
Thus, they were able to adapt precisely to their uncontended niche environments.. One example of this is the monotreme, the platypus. It has adapted so precisely to its environment that today it can only be found in clean and flowing water bodies. Monotremes are one of the most distinct orders of living mammals, native only to Australia and surrounding islands. Some have considered these animals to be reptiles and others classify them into a subclass with extinct lineages and separate from all other living mammals today. Consisting of only five different species, including T. culeatus, the Monotremes are the epitome of unique Australian fauna. Australian organisms have become so adapted to the environment that they have lost the genetic diversity to deal with change. For example, Australia’s isolation protected its species from the threat of disease. Today the Tasmanian devil is threatened by a single virus, Devil facial tumour disease, due to its inability to adapt-stemming back to Australia’s isolation. It is estimated that since 1996, it has caused a 20-50% decrease in the devil population, spreading across over 65% of Tasmania.
Thus it can be argued that Australia’s unique flora and fauna is in fact an evolutionary anomaly and a disadvantage for the future survival of Australian species. This unique evolutionary pathway can also be traced back to changes in Australia’s environment as it floated over the southern regions of Earth. As Australia’s climate changed, the environment underwent a dramatic transformation. As Australia drifted northwards, the closed rainforests that once dominated the continent began to shrink as large areas of the continent started to die.
By 2 million years ago, more than half of the continent was arid desert or semi-arid scrubland, consisting of open forests, woodlands and scrublands. Today, Australia is characterised by its large environmental diversity. It experiences around 6 different types of climatic conditions. The vast corridors once provided by the extensive rainforests allowed animals to distribute around Australia. But with the changing environments, these corridors disappeared and populations became isolated as harsh desert land prevented them from moving through Australia.
With isolated populations, the diversity within Australian species also increased as different populations adapted to their own climatic conditions. For example, Koalas in southern areas of Australia have thick, wool-like waterproof fur to keep them warm in cold weather and dry in rain but Koalas in northern regions are smaller, with less dense fur to keep them cooler in the tropical climate. In conclusion, because Australia was an isolated continent subject to dramatic changing environments it led to the unique Australian biota that we know today.
One technology that has been used to investigate evolutionary relationships is the ‘molecular clock’. The molecular clock tells time on an evolutionary scale and allows scientists to calculate the timing of evolutionary events, in particular when new biological species developed through the splitting or ‘divergence’ of a lineage. This tool is used to measure the number of mutations within a species’ gene sequence that has happened over time. From this, we are able to work out how species have evolved, and more importantly, it allows us to determine the date when two species separated.
Thus, the molecular clock is significantly beneficial to evolutionary biology as it allows an evolutionary timeline to be constructed. Through comparing the gene sequences of various species, scientists are able to determine when they last shared a common ancestor. Therefore, they are able to construct an evolutionary timeline. For example Australian biologists have recently applied the molecular clock to the echidna and the platypus, coming to the conclusion that the two species shared a common ancestor approximately 30 million years ago.
The molecular clock has been able to provide an explanation for why there are no echidna fossils older than 13 million years- they simply had not evolved yet. It has thus proven useful for filling in and explaining some of the many holes of evolution. It is a very useful tool as finding common ancestors as using other methods such as studying fossils can often be very difficult, especially when an organism does not have a fossil record. For example, the fungi do not form fossils well due to their “soft and squishy” bodies.
In effect, it allows science to draw up the family trees of various species. The molecular clock works because mutations within a specific gene occur at fairly regular rates, with each gene having its own particular rate of mutation or change. The theory teaches that the number of differences between two DNA sequences increases over time. Thus, using the molecular clock is a very straightforward process. The DNA sequences of the species are compared, allowing for the number of differing amino acid or nucleic acid bases to be counted.
This number of differences is then plotted against the species’ known rate of mutation, which allows the time at which these two species shared a common ancestor to be determined. Let us look at this diagram; changes or mutations in the DNA sequence are highlighted by a shaded circle. Looking at this example it is clear that after 25 million years, the DNA sequences had diverged and differed by 2 bases. After 50 million years, they differed by 4 bases. Thus, the DNA sequence of each species changes at a rate of one base every 25 million years.
So today, the two DNA sequences now differ by 100 million years of evolution; 50 million years each. It can thus be approximated that they once shared a common ancester 50 million years ago. However, using the molecular clock has some very significant limitations and is still a very controversial topic. The molecular clock theory does not take into account the effects that natural selection and smaller populations can have upon genetic change within a species; thus, the reliability of the molecular clock is significantly reduced. This is evidenced in the various contradictions between dates produced by molecular clocks and those found by ore traditional sources such as the fossil record. For example, molecular clocks produced a date of 1000-900 million years ago for the emergence of animals, but palaeontologists widely accept the date of 570 million years ago for such event. The molecular clock is also limited by the types of molecules it can use as it has very specific requirements. For example, the particular gene chosen must be common to all organisms being compared and it must also be under strong functional constraint so as to ensure it remains highly conserved. This can limit which species are able to be compared.
The development of molecular biology over the past fifty years has helped shed light on earth’s evolutionary history. Through examining the changes in the important proteins and other genes of a species, evolutionary biologists using the molecular clock are helping to piece together earth’s evolutionary timeline.
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