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Mass Extinction Events

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One of the most significant biological debates of this generation regards the possibility of a forthcoming, human-induced mass extinction event and what it’s effects would be upon the evolution rate of species. In the search for understanding this possibility and its effects, scientists must look to the past where it is widely accepted that there have been 5 previous periods of mass extinction. The term ‘mass extinction event’ is defined as any period when abnormally large numbers of species die out simultaneously or within a limited time frame. Such occurrences have been observed at 5 places in history- the Ordovician-Silurian, Late Devonian, Permian, Triassic Jurassic and Cretaceous periods. What was the cause of these extinction periods? Well... no one can say for sure. Although there are a number of plausible hypotheses, the sheer amount of time and space between today and the events in question makes it very difficult to place a theory as anything more than conjecture. In saying this, it is known that these cataclysms were random occurrences and not caused by the actions of any one single species. The extinction of species is not normally considered an important element of Neodarwinian theory, in contrast to the opposite phenomenon, speciation. In the wake of mass extinction periods, the fossil records provide an accurate and reliable source of information invaluable to the study of evolution. Today, there is evidence mounting to suggest that the sixth mass extinction may be beginning- but the apocalypse this time is us. Human activities such as pollution, land clearing and overfishing may drive vast numbers of the worlds’ marine and land species to extinction. The following report will evaluate these claims and whether they hold biological significance, especially in terms of evolution.

In the Origin (1859), Darwin made his view of extinction, and its role in evolution, quite clear. He saw four essential features.
1. Extinctions of species have occurred gradually and continuously throughout the history of life.
“Species and groups of species gradually disappear, one after another, first from one spot, then from another, and finally from the world. The complete extinction of the species of a group is generally a slower process than their production: if the appearance and disappearance be represented by a vertical line of varying thickness the line is found to taper more gradually at its upper end, which marks the progress of extermination.” – Origin, Charles Darwin
2. Sudden disappearances of many species (mass extinctions) did not actually occur. Darwin believed that sudden disappearances of species from the fossil record were due solely to unrecognized gaps in the temporal record.
3. Species extinction is usually, though not always, caused by the failure of a species in competition with other species. That is, causes of extinction are generally biological, not physical.
“The inhabitants of each successive period in the world's history have beaten their predecessors in the race for life, and are, insofar, higher in the scale of nature.If extinct inhabitants were put into competition with the existing inhabitants, the extinct fauna or flora would certainly be beaten and exterminated; each new variety, and ultimately each new species, is produced and maintained by having some advantage over those with which it comes into competition; and the consequent extinction of the less-favoured forms almost inevitably follows.” – Origin, Charles Darwin

4. The extinction of species (and larger groups) is an inevitable outcome but closely tied to the process of natural selection and thus a major component of evolution.
Today, 99.9% of all species that have ever existed on Earth are extinct. Extinction has both destructive and generative consequences. On one hand, extinction terminates genetic lineages, reduces biodiversity and eliminates evolutionary possibilities. In terms of evolution, the extinction of one species has broad effects for other species. As new species evolve to fit ever changing ecological niches, older species fade away. However, the rate of extinction is far from constant. Earth has existed for approximately 4.54 billion years and each of the 5 greatest periods of extinction spanned for just a few thousand years each- a geological blink of the eye. Though these mass extinctions are deadly events, they open up the planet for new life-forms to emerge through the demise of dominant groups and the post extinction sorting of survivors. For example, the fourth mass extinction (which occurred 205 million years ago) destroyed many non-dinosaur reptiles. When these potential competitors of dinosaurs were killed, the dinosaurs were then able to thrive. The dinosaurs evolved and dominated life on Earth for 140 million years until the fifth mass extinction- making room for mammals to rapidly diversify and evolve.
TABLE 1 Comparison of species extinction levels for the Big Five mass extinctions
Extinction episode Age, Myr before present Percent extinction
Cretaceous (K–T) 65 76
Triassic 208 76
Permian 245 96
Devonian 367 82
Ordovician 439 85
Extinction data are from Jablonski (1991).

Rises in evolutionary progression correlate with extinction events. The fossil record contains many well-documented examples of the transition from one species into another. Evidence from the fossil record is unique, because it provides a time perspective for understanding the evolution of life on Earth. This is known as the evolution rate of species. It can be easily seen that mass extinction periods coincide with changes in evolution rates until, in time, equilibrium is reached. The equilibrium reached after a mass extinction is often very different to the equilibrium which predated it. On a larger scale this equilibrium can be seen in the rate of species extinction vs. the rate of speciation. This is largely due to the complex interplay between both consequent aspects (destructive and generative) of extinction upon evolution. The Fossil Record supports Darwins theory that survivorship during mass extinction events is not entirely random, but rather coincides with favourable characteristics.
Scientists have long speculated over the precise causes of these events to no definite avail. The very occurrence of non-random clusters of extinctions (mass extinction events) goes against Darwinian theory. Species become extinct when the environment changes too quickly for them to adapt to the new conditions. Possible causes include climate change, catastrophic methane release, flood basalt eruptions and impact events. However, it is general consensus that in the case of every extinction period there was more than one overall contributing cause.
It appears that of recent times, species have been becoming extinct at an unprecedented rate and the blame for this falls firmly on the shoulders of human beings. Scientists have identified a "background rate" of species extinctions from the fossil record, which allows for a comparison to the current extinction rate, thus allowing us to assess the human impact on the rate of species loss. Species are currently going extinct 100 to 1,000 times (possibly even 1,000 to 10,000 times) faster than at the normal background equilibrium rate, which is about 10 to 25 species per year. It has been found that these figures are

With the current rate of population decrease and extinctions which have been listed, many ecologists have drawn the conclusion that we are in the early stages of a sixth mass extinction, caused almost solely by the actions of humans, It has been recorded that since the year 1500, more than 320 species of land dwelling vertebrates have become extinct, and over only the past 35 years, invertebrate numbers have dropped 45%. On par with previous mass extinctions, mega fauna are facing the most severe and rapid rate of decline, outside the already high average rate of population abundance decline of 25%. Some who argue the claim of a sixth mass extinction mentions the fact that the actual amount of species extinct does not yet qualify as a mass extinction. This argument is negligible as approximately 16-33% of just vertebrates are considered to be endangered, many which may go extinct within a short period of time at this rate. Also, the claims suggest we are at the beginning of a mass extinction event rather than in the middle or approaching the end of one. The extinction rates of mass extinction events do pick up pace as they progress, as with the annihilation of species, many other species are affected in a chain reaction, due to a lack of food source, an alteration in environment and many other reasons. The extreme drops in numbers of populations and species can be directly attributed to human activities such as pest control (mass-destruction of invertebrate populations), hunting/poaching, habitat loss from pollution and destruction, decreased food sources and climate change (a factor which is not 100% due to human activities, but is increased by).

It is true that this proposed mass extinction may not negatively affect each species- for rodents in particular it may prove beneficial for a period of time- however on a larger scale, the effects would be severe and perhaps the most irreversible and negative to have come from any mass extinction. Each of the previous mass extinction events have occurred over a period of thousands of years, whereas this mass extinction is predicted to have begun and finished within a matter of possibly 200 years which is a drastic change from previous events. While there is the argument that, as evolution and mass extinction events are caused by random occurrences, the fact it is caused by humans is insignificant in an evolutionary sense. This would be true, if humans were also wiped out during this mass extinction. However, as humans will almost definitely remain as the major dominant species of the planet after this mass extinction, factors unseen before now will be added to the recovery of the planet. Re-diversification is a massive process which has been noted to take up to 5 million years, and occurs in an almost new planet; many of the larger animals and planet species go extinct, allowing the smaller less prominent species to take advantage of the lack of predators and grow and expand. A major difference in this mass extinction is that both main contributor to the extinction event and the most dominant, large and widespread species- humans- will survive. This will mean that the planet will not have the opportunity to revive itself as it would have should the human race have been wiped out as there is a little doubt the activities and the destructive nature of humans will change.

From analysing data collected from past mass extinctions and noting trends which came before and followed them, it is quite clear that we are in the early stages of another, this mas extinction can also be directly attributed to the actions of humans; without the presence of human on the planet the climate change would not be so rapid, the habitats of many animals would not be destroyed and ecosystems would bit be polluted so horrifically. The fact that is human induced is of great significance, not just immediate effects but in the long term of re-diversification, as many whole biomes may be eliminated and the humans will continue to contribute in similar ways as in the present. It can be safely said that id we wish to avoid destroying this planet, on an irreversible level, action needs to be taken now. Waste needs to be properly disposed of – both domestic and industrial- climate change needs to be curbed and destruction of habitats must cease. We need to be as unobtrusive as possible to allow the planet earth and all the living organisms which inhabit it to recover.

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