Different Causes of Mass Extinctions
How to kill (almost) all life: the end-Permian extinction event (Benton and Twitchett 2003)
In this paper, the end-Permian extinction and its potential causes are discussed. Timing was a problem when first discussing the end-Permian extinction, there was no precise time that the extinction occurred. Another problem was dating the boundary and shape of the extinction however this was made clearer by Jin Yugan and his study in China that showed the beds where there was evidence of the end-Permian extinction. The first theory for causation of the end-Permian is the impact of an extraterrestrial object. The evidence for this theory is all based off the KT impact, which includes the candidate crater, the iridium spike, and shocked quartz. All three of these phenomena were reported for the Permo-Triassic (PTr) boundary however all three have since been rejected or very little interest has been given to them. More support for the evidence of an impact came from fullerenes (large molecules of carbon, comprising of 60-20 carbon atoms arranged as regular hexagons around a hollow ball). The fullerenes found at the PTr boundary were identical isotopically to helium and argon that is derived from meteorites. It was then argued that they must have come from the impact of a meteorite. However, this was tested again and they were unable to get the same result from the first time in the fullerenes and it was never demonstrated that the helium and argon were trapped in the fullerenes. Next came the theory of an eruption causing the end-Permian extinction which was supported by the fact that there were gigantic volcanic eruptions at the end of the Permian occurring in Siberia. However, at first the dating of these eruptions was difficult and there was a large span of dates ranging from 160 to 280 million years ago for its occurrence. With newer radiometric methods, the date of the eruptions was yielded to be exactly on the PTr boundary and the range from the bottom to the top was about 600,000 years which matches the evidence from China of rapid extinction. Also, the records through the extinction event of fossiliferous rock sections is much more complete now than originally had been thought. The rocks contain a huge diversity of organisms both from the ocean and on land and comparing the two shows that the timing of species loss on both coincide to one another. The last possible causation was the runaway greenhouse which is the idea that initial global warming at the PTr boundary, caused by the Siberian eruptions, melted frozen gas hydrate bodies and large volumes of methane rich in 12C rose to the surface of the ocean in giant bubbles. These enormous amounts of methane caused more warming which could have melted more gas hydrate reservoirs that led to a positive feedback spiral where natural systems couldn’t operate properly leading to the end-Permian extinction. Understanding how the few taxa that survived this extinction recovered from this is essential for learning the subsequent evolution of those taxa.
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Questions:
What do you think is the most plausible cause for the end-Permian mass extinction? Are there any other causes that could be possible that this paper doesn’t touch?
Is the runaway greenhouse model something that should be explored further? Why?
With current conditions of climate change on Earth and the debate about global warming, can this model help us to learn how much global warming can be sustained? And how far we are from another runaway greenhouse?
In determining the cause of a mass extinction, what would the benefits be in examining the taxa that survived rather the strictly analyzing the taxa the experienced extinction?
What details of the surviving taxa will help narrow plausible causes?
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The role of extinction in evolution
By David M. Raup
Extinction of species is typically ignored in contrast to speciation. This is surprising because the number of species extinction is almost the same as the number of originations. There is an increase in the interest of extinction in the past decade, although research in the field is unadvanced and the knowledge is “weak.” Raup explains in the article that extinction is probably due to factors outside of natural selection, extinctions produce major restructuring of the biosphere, and extinction is almost impossible to predict.
When contemplating extinction, it is oftentimes believed that mass extinctions are due to a physical impact on Earth in which a species abruptly stops existing. Charles Darwin in the Origin explains that extinctions of species occur gradually and continuously throughout life. He explained that they are rarely caused by great catastrophes, but oftentimes caused by a failure of competition with other species. George Gaylord Simpson is referenced in the article as disagreeing with Darwin’s theory, viewing the replacement of one species by another as a passive process.
The fossil record is also very limited, leaving much room for contemplation as to causes of extinction. Raup concludes that extinction of a widespread species requires an environmental shock, in which survivors are adapted for the “unexpected stress,” which have a profound influence on evolution and changing the biosphere.
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Questions:
1.) In your own opinion, has Earth's biodiversity radiated more from mass extinction events or from natural selection? Why?
2.) Why are extinctions expected to affect successful species more than those who are not as successful?
In this paper, the end-Permian extinction and its potential causes are discussed. Timing was a problem when first discussing the end-Permian extinction, there was no precise time that the extinction occurred. Another problem was dating the boundary and shape of the extinction however this was made clearer by Jin Yugan and his study in China that showed the beds where there was evidence of the end-Permian extinction. The first theory for causation of the end-Permian is the impact of an extraterrestrial object. The evidence for this theory is all based off the KT impact, which includes the candidate crater, the iridium spike, and shocked quartz. All three of these phenomena were reported for the Permo-Triassic (PTr) boundary however all three have since been rejected or very little interest has been given to them. More support for the evidence of an impact came from fullerenes (large molecules of carbon, comprising of 60-20 carbon atoms arranged as regular hexagons around a hollow ball). The fullerenes found at the PTr boundary were identical isotopically to helium and argon that is derived from meteorites. It was then argued that they must have come from the impact of a meteorite. However, this was tested again and they were unable to get the same result from the first time in the fullerenes and it was never demonstrated that the helium and argon were trapped in the fullerenes. Next came the theory of an eruption causing the end-Permian extinction which was supported by the fact that there were gigantic volcanic eruptions at the end of the Permian occurring in Siberia. However, at first the dating of these eruptions was difficult and there was a large span of dates ranging from 160 to 280 million years ago for its occurrence. With newer radiometric methods, the date of the eruptions was yielded to be exactly on the PTr boundary and the range from the bottom to the top was about 600,000 years which matches the evidence from China of rapid extinction. Also, the records through the extinction event of fossiliferous rock sections is much more complete now than originally had been thought. The rocks contain a huge diversity of organisms both from the ocean and on land and comparing the two shows that the timing of species loss on both coincide to one another. The last possible causation was the runaway greenhouse which is the idea that initial global warming at the PTr boundary, caused by the Siberian eruptions, melted frozen gas hydrate bodies and large volumes of methane rich in 12C rose to the surface of the ocean in giant bubbles. These enormous amounts of methane caused more warming which could have melted more gas hydrate reservoirs that led to a positive feedback spiral where natural systems couldn’t operate properly leading to the end-Permian extinction. Understanding how the few taxa that survived this extinction recovered from this is essential for learning the subsequent evolution of those taxa.
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What do you think is the most plausible cause for the end-Permian mass extinction? Are there any other causes that could be possible that this paper doesn’t touch?
Is the runaway greenhouse model something that should be explored further? Why?
With current conditions of climate change on Earth and the debate about global warming, can this model help us to learn how much global warming can be sustained? And how far we are from another runaway greenhouse?
In determining the cause of a mass extinction, what would the benefits be in examining the taxa that survived rather the strictly analyzing the taxa the experienced extinction?
What details of the surviving taxa will help narrow plausible causes?
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The role of extinction in evolution
By David M. Raup
Extinction of species is typically ignored in contrast to speciation. This is surprising because the number of species extinction is almost the same as the number of originations. There is an increase in the interest of extinction in the past decade, although research in the field is unadvanced and the knowledge is “weak.” Raup explains in the article that extinction is probably due to factors outside of natural selection, extinctions produce major restructuring of the biosphere, and extinction is almost impossible to predict.
When contemplating extinction, it is oftentimes believed that mass extinctions are due to a physical impact on Earth in which a species abruptly stops existing. Charles Darwin in the Origin explains that extinctions of species occur gradually and continuously throughout life. He explained that they are rarely caused by great catastrophes, but oftentimes caused by a failure of competition with other species. George Gaylord Simpson is referenced in the article as disagreeing with Darwin’s theory, viewing the replacement of one species by another as a passive process.
The fossil record is also very limited, leaving much room for contemplation as to causes of extinction. Raup concludes that extinction of a widespread species requires an environmental shock, in which survivors are adapted for the “unexpected stress,” which have a profound influence on evolution and changing the biosphere.
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Questions:
1.) In your own opinion, has Earth's biodiversity radiated more from mass extinction events or from natural selection? Why?
2.) Why are extinctions expected to affect successful species more than those who are not as successful?
1. Honestly, looking at this paper and all the "possible" causes for the end-Permian mass extinction, it's pretty possible that it wasn't just one thing. It could have been a plethora of different kill mechanisms, or one triggering after the other- like we heard in class, a meteor impact CAUSING earthquakes and volcanic eruptions, which then cause climate issues, etc.
ReplyDeleteI'm sure there are other causes (like gamma rays) that the paper doesn't touch, but the ones discussed that could be plausible seemed to be mostly covered.
2. The runaway greenhouse model is definitely something to be further explored considering the current state of greenhouse gases on the planet. However, the current issue with global warming is human caused rather than natural, so it might not give us the exact solutions that we want. And the planet has also been hotter AND cooler than we are at currently, so we can definitely say that what happened during the end-Permian was something natural.
However, knowledge is power! The model could help us learn how much global warming can be sustained before a mass extinction happens, but for the most part I think biologists have a good understanding of what the future holds due to climate change. We are probably pretty far away from the runaway greenhouse effect though, considering our oceans probably won't be boiling away for some time...
3. In determining what caused a mass extinction, it would be beneficial to study the taxa that survived and map the traits they have in common. For example, if everything that went extinct was aquatic and everything that survived was terrestrial, it might be because the kill mechanism was in/mostly affected the water of the planet. That also shows in fresh water vs salt water. So the traits of the surviving taxa would be able to narrow down causes based on what the theorized causes were.
For example, if it was theorized that a mass extinction was caused by either acidic oceans or warming temperatures, and the surviving taxa were adapted for warm temperatures, it would probably be that the taxa that went extinct were unable to thrive in warmer temperatures.
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1. In class it was mentioned that a mass extinction can be followed by mass radiation- with all the niches in nature being opened up suddenly, the species left over can quickly adapt and radiate to fill them. It leaves a lot of room for change, so in my opinion, I would guess that biodiversity radiated more from mass extinction events than from natural selection and things became more different in less time than usual.
However...mass extinctions are pretty few and far between, and life doesn't need billions of years to recover, so it is possible that since natural selection has (probably) acted on species more than mass extinction recovery has, natural selection could be more responsible too.
2. Non successful species are more expected to have fewer numbers and populations in general, and successful species are well adapted and widespread. So if a non successful species of 1000 individuals go extinct and a successful species of 100,000 individuals go extinct, the successful species disappears from more area than the non successful species. Being successful and widespread means a higher probability of being affected.
The paper suggests that it was a huge volcanic event. This is most likely the major cause of the extinction in the end Permian. There is a lot of evidence to support the eruption over a bolide impact. Although there could have been an impact, it probably did not have the same devastation as the eruption in Siberia or we would have more substantial evidence of it. It is possible that other factors came into play, but most of them probably lead back to a massive eruption that not only caused massive global warming but ocean acidification and anoxia as well.
ReplyDeleteAs for studying the surviving taxa I think that it would be important to know what type of habitats an niches these organisms occupied. Mainly because if an organism is not restricted to one certain environment or habitat then it is possible that they were not as easily affected by the extinction event.
For the Earth biodiversity question. I think that both of those factors have a major role to play. Not only does natural selection lead to massive amounts of diversity among organisms, but mass extinctions opened up tons of niches that would have been otherwise occupied by something else and preventing the diversification of life.
I believe that the most plausible cause for the end-Permian mass extinction would be the volcanic activity completely changing the environment in such a short time period over multiple 100 million years. With gases and particles being sent into the air in large quantities, such as ash and sulphur dioxide, the environmental effects will be large. There will be a cooling effect and/or a warming effect on temperature that would affect both marine and terrestrial organisms to a great extent. There is lots of evidence to support this including the global spreading of ash.
ReplyDeleteWhen looking at surviving taxa after a mass extinction, their environment, body system, and behavioral patterns can be a great factor to look into to tell what they did to survive compared to others that did not. This could shed some light on what exactly caused the mass extinction based on the surviving taxa’s environment and habitat location. If only organisms that burrow survived the mass extinction, than maybe the cause had something to do with air pollution or an impact blast of some sort.
Both mass extinctions and natural selection can cause radiation. I fell that mass extinctions would have less of a radiation event compared to natural selections because mass extinctions open up the world for many different niches to be occupied, but natural selection narrows those niches down further as the species or genera adapt over time.
1. I think it seems plausible that the end Permian mass extinction was caused by a chain of eruptions of Siberian volcanoes which then lead to a runaway greenhouse effect. I would need more definitive evidence to be convinced that an impact on earth also played a role in the extinction. It seems that that hypothesis was largely refuted because of errors in evidence. However, it seems that eruptions and greenhouse effects are currently well supported by data and the experts.
ReplyDelete2. The runaway greenhouse model definitely should be studied in much greater depth. Something that was mentioned in the paper was that the warming of the earth caused the melting of frozen gas hydrate bodies. I wonder if there are still frozen hydrates in our oceans today and if so how many? Having an estimate of them would be helpful in determining the risk we run by letting global warming continue.
3. It seems that examining taxa that survived mass extinctions may help to narrow down the kill mechanism. Examining the features of the taxa that distinguished them from the extinct taxa may give insight on some aspect that was unaffected by the kill mechanism of the mass extinction.
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1. I think it is difficult to make a distinction between mass extinctions and natural selection in terms of evolution simply because the kill mechanisms in all extinctions are natural and therefore I think natural selection ranges between extremely harsh selective forces ( such that would cause mass extinction events) and relatively lax selective forces through the history of earth. I think that whether a species survives and evolves is due to a combination of advantageous characteristic and luck. Therefore I truly think that the biodiversity we see throughout earth’s history has to be attributed to both rapid adaptive radiation after an extinction or during colonization of a new geographical niches and to gradual evolution through less extreme natural selection processes.
2. Extinctions may sometimes effect successful species more than less successful ones because the successful ones may be more widespread and may be extremely well adapted to the particular environment Since mass extinction events are almost always caused by a drastic change to the environment the extremely successful and well adapted species would be more greatly impacted by such a drastic change. Also, if they are more widespread there is a greater probability of being affected by some global phenomena.
1)I think the most plausible cause for the mass extinction is the extraterrestrial impact model. I think that this model best accounts for the elements introduced and the environmental changes that took place. AS for other causes there are not many others I can really think about that would explain the sudden climate changes and debris that was most likely in the air.
ReplyDelete4)I think the runaway greenhouse model is something that should be explored further as today we are facing another global climate change. I believe in studying the past where greenhouse gases where also elevated will help us to understand possible changes we may face today with elevated gas levels. I think that the past model should be taken with a grain of salt though as it is not the same exact conditions so there will be variation in the events that unfold. The past model might help us to control our current change as again it could help us learn what might happen and give us insight on what stage we are in and foresight to possibly control future problems or consequences. I believe we are not that far away from another runaway greenhouse effect, if we are not already starting to experience one. I think the biggest indication of the current change is the rapid melting of the ice caps.
3)Determining the taxa that survived would help us to determine which characteristic(s) was the most predominate in survivors. Identifying this characteristic can help us as we can hypothesis what this characteristic is resilient against. This would help us narrow down possible causes, as there would be less to sort through as compared to taxa that died off had multiple characteristics that we would have to sort through first. Surviving characteristic though would be relatively few in comparison so sorting through those would I believe take less time. I don’t think that one group though should be used over the other, as both together I believe will provide a clearer picture.
1)I think earth’s biodiversity has radiated more from mass extinction than natural selection as mass extinctions has freed up different areas niches allowing species to diversify with more freedom to fill those niches as before they might not have done due to intense competition. Will the free space I think species had more freedom to really take chances as there was not really that many other groups within that niche placing large selection pressure on them so it was “safe” for a species to make changes.
2)I think extinction events affect successful species the most than unsuccessful species is because extinction events are usually a sudden change in the environment around the species. That species is so well adapted to the environment that when the environment changes the species is not longer able to cope as the environment is not something they really “prepared” for, as unsuccessful species are not really doing well in general so when the change happens it might be a change that favors the reason they didn’t do so well and then allows them to flourish. In addition there is the prevalence of more successful species appearing in the fossil record than un successful simple to distribution.
Benton Paper:
ReplyDeleteUnderstanding taxa that survived extinction events is important because if one only examines the taxa that went extinct, perhaps you would see the similarities in the taxa which did not survive (aquatic, arboreal, localized to a specific region, etc) but you may not see the or recognize the qualities in taxa that allowed them to survive the events. Analyzing the similarities in organisms that survived the extinction events allows researchers to rule out causes of the mass extinctions. This also gives researchers the oppootunity to ask why some organisms could survive the changes while others couldnt. Where they better adapted to the warmth? Could they handle the changes in the ocean oxygen content or acidification? etc.This better allows us to examine the evolution and adaptive radiations that follow mass exctinctions, and not just solely focus on the species which went extinct and what caused them go extinct.
Raup Paper:
Successful species are typically more impacted by extinctions when the causal mechanism is rapid and leaves no time for natural selection or adaptation. This could be seen as global climate change or enviomental change in which these large populations cannot adapt to the new circumstances (hotter,cooler, more acidic, ect) . They may depend heavily on a prey species or plant that has disappeared or reduced in numbers, leaving the large population with a defecit in food. Typically these successful species are widespread, meaning that it must be a global phenomenon rather than an issue in one geographical location. Less successful species may die off from less rapidly occuring processes, such as habitat loss or hunting (by predators) and therefore may allow for the more successful members of the group to survive, allowing for natural selection, and therefore evolution to occur- despite population numbes dwindling.
I think the most plausible cause of this particular mass extinction is a combination of volcanism and run away greenhouse effects. It makes sense that volcanism would lead to global climate change which would only amplify the devastation of this mass extinction. The extraterrestrial impact seems unlikely given the lack of evidence and support for this theory. I think the greenhouse effect should be explored further but I'm not sure how much of a proxy it can be to understanding modern global climate change. The environment on earth was simply too different during this time period compared to today. It's important to look at the traits and adaptations of surviving taxa from this mass extinction because these factors can shed light on the cause of the extinction event. If everything shared a quality or niche it can be used as clues or support for some of the many theories behind this extinction.
ReplyDeleteI believe that mass extinctions have opened up niche space and have provided space for diversity to expand over time. If we look at the Cambrian explosion the biggest jump in diversity you find evidence that ediacrian fauna needed to go extinct to make that jump in diversity possible. More successful species may have wider ranges of habitat that they live in so they would be more prone or more likely to be effected by a mass extinction in comparison to a less successful species that inhabits a smaller region or niche space which may be isolated from the causes of a mass extinction.
Yes, I believe the runaway greenhouse model is something that should be further explored today. Although it may not be directly applicable to the present day change in climate, there are major implications of drastic changes in climate, regardless of the exact methodology. Analyzing previous models like this may help us to understand how extensive the damage of an event like this would be on our current climate, and how we can attempt to either prepare or remedy these changes so as to not enable the start of a catastrophic event.
ReplyDeleteIt would be beneficial to analyze the taxa that survived the extinction because then you can attempt to determine what characteristics these organisms had that the organisms who were affected by the extinction did not. This could include not only physiological characteristics that may have put them at an advantage, but also information about their environment that may have enabled them to survive through various events. This may be beneficial in narrowing down the type of event that led to the mass extinction as it provides more information about what niches were able to tolerate the catastrophe. If you know there are certain environments that were most likely destroyed by, for example, LIPs, you may be able to infer that a mass extinction may not have occurred by these if taxa in that type of environment still persisted after the proposed major event.
In my opinion, I think it is hard to distinguish whether or not earth’s biodiversity comes more from mass extinctions or from natural selection. I want to say mass extinctions would cause more biodiversity because so many organisms get knocked out of the system, while just a few may be more successful at evolving and being able to survive the newer conditions that are present which then they would allow species to have more space or niches to occupy than before. Natural selection on the other hand has allowed for species to adapt to the environment naturally in a way that allows organisms to be able to diversify. Overall, I think mass extinctions have contributed more diversity in the case that being able to occupy more niches would give more opportunity to diversify and explore different areas.
ReplyDeleteI believe more successful species are likely to be affected from extinctions more than less successful species because the successful species are more than likely spread out into a larger range as they should have adapted more easily to the earth’s conditions presented to them. Less successful species do not have such a widespread range because they have not adapted as well and are not able to explore as much area or niches with the successful species being around. In this sense, successful species are more likely to be wiped out by some very large global change like volcanism or meteorites because they have a much larger amount of earth occupied where the change can easily affect them and not the less successful species which has a smaller amount of area to deal with.
1. This paper specifically makes a strong case for three plausible causes for the extinction at the end of the Permian period. In my option the biggest factor would be the eruptions happening in Siberia during this time frame. The evidence supporting this claim appears to be the major contributor and subsequently led to a runaway greenhouse effect. In considering this claim, I don't think it would be outlandish to think a combination of eruptions, runaway greenhouse effects, and being hit by a extraterrestrial object, could all factor in the mass extinction at the same time. However, because the data for a meteorite is vague, it would require further examination to truly determine if this particular factor would be a major contributor. I think it is definitely safe to conclude that volcanic eruptions and the runaway greenhouse effect were major contributors to the Permian mass extinction.
ReplyDelete2. I believe the runaway greenhouse model is something that should be studied further in relation to the Permian and other mass extinctions. Although the enviroments may have differed in each mass extinction, by studying the Permian runaway greenhouse, we can examine which species were affected the most and if there may be a commonality in all the species had that led to their demise. Did these organisms have certain traits that helped them if any to survive? Was the ability to be highly dispersed and adaptive to different environments a key factor to help the organism survive the extinction or ultimately lead to their downfall? These are some of the questions I would pose in regards to the runaway greenhouse effect. Although the runaway greenhouse effect may provides us with ideas or views as to how much environmental warming can be sustained, the ultimate causes for the runaway greenhouse effect are very different and may not be the best model to use.
1. I believe that when asked about radiation of earths biodiversity, I don't believe you could make a definitive case for natural selection or mass extinction radiation being more important than one another. Both of these processes are very important for species diversification and both fill many ecological niches. I believe a fundamental factor to consider here is the atmosphere at which these organisms live. Speciation allows for organisms to adapt to where they are living and fill certain niches. Where as, if you have a mass extinction, without radiation happening ecological niches cannot be filled. Both processes are very important, but over time, I believe we will see natural selection be more prevailing than mass extinction radiation.
2. The underlying reason for me in why extinctions will affect more successful species as opposed to those who are not successful is as follows. I believe it is because the more successful species are adaptive to their enviroment, and as soon as there is a environmental change, these species cannot adapt to the change quickly enough causing a large number of them to die out. Perhaps, species like bacteria who can survive in a multitude of different environmental conditions, are most likely to be prevailing during mass extinctions.
I think the most significant aspects of these papers to think about are the questions of whether runaway greenhouse models should be explored in correlation to modern-day climate change, and whether we should place more importance on the analysis of species that survive mass extinctions than those that disappear.
ReplyDeleteThe runaway greenhouse model, while definitely applicable in some respects to the modern climate, is greater suited to naturally occurring climates of the past. Throughout the planet’s history, instances of both significantly hotter and cooler average temperature have occurred naturally. Changes in living conditions have caused extinction for species that could not adapt, and we often view our modern climate crisis as what could potentially become the “sixth mass extinction,” even though we must remember that today’s circumstances are influenced by our own choices as well as natural processes. Scientists have been trying for years to develop climate models based on our specific circumstances, and so perhaps the runaway greenhouse model should be left to analyze past environments.
In regard to placing focus on species that survived over those that went extinct during mass extinction events, I think there are reasons to examine both sides equally. Species that disappeared during mass extinctions are likely reflective of potential kill mechanisms because researchers can infer whether species with similar living conditions or morphology were affected. Species that survive extinctions can be analyzed for the same reasons and should definitely be looked at in terms of whether or not that group of species has survived to present day.
Based on these papers and the discussion from Thursday, I think an important question to think about is why some species go extinct and others don't. It brings up an interesting point: that we tend to focus more on extinct species rather than those that have survived extinction events. I think the question of why extinction is expected to more significantly effect successful species rather than generalized species is a fascinating question that takes many factors into regard. Species that are extremely well-adapted to their environment and ecological niche are more successful than those who have more generalized traits and can't compete quite as well. However, these successful adaptations depend entirely on a constant environment; perhaps an organism is adapted to an arid environment, or a tropical or marine one. As soon as that environment changes, they don't have the proper traits to survive and can then be out-competed by other organisms. By studying Earth history, we can see that Earth's environment is always changing. Therefore, any species well-adapted to one specific environment is going to have a tough time competing when that environment inevitably changes.
ReplyDeletePublication contributors of Different Causes of Mass Extinction: Claire Lynch, Sarah Youna Park, and Patrick Zedalis.
ReplyDeleteIt is difficult to determine what the “smoking gun” was for the Permian extinction. Benton and Twitchett offer several contributing factors to the greatest mass extinction in the geologic record, but do not give us a definitive cause. Meteor impacts, volcanic activity, and increases in atmospheric carbon dioxide would all have an impact on the benthic zone. And, their illustration of the seabed in southern China (Fig I. p. 360) shows that species above and below the seafloor went extinct. They lead us to believe that the kill mechanism was of a chemical nature, i.e., benthic anoxia. This makes sense to me, as the most plausible cause for the end-Permian extinction. Speculatively, a particularly virulent virus, such as a retrovirus, might have a similar effect. Perhaps, as genomics progresses and sequences are cross-referenced, we will find a viral link among the taxa that survived the extinction.
ReplyDeleteIn my opinion, Earth’s biodiversity is more a result of natural selection than extinction. Extinction may have opened a niche for early tetrapods, but it was natural selection that brought them on land. Extinction made niches available for mammals, after the K-T extinction, but natural selection took them to the ocean and air. Without natural selection, there is no adaptive radiation.
Benton and Twitchett 2003
ReplyDeleteThe extensive volcanism and resulting increases in greenhouse gases is the most well supported theory of the cause of the end Permian mass extinction, at least in this article. Benton and Twitchett cite carbon isotope data to support this theory, stating the drop in carbon 13 accompanied by the increase in carbon 12 may have been due to a release from methane contained gas hydrates. This release consequently caused a runaway greenhouse gas reaction. Run-away greenhouse gases are certainly relevant to current environmental issues, and if a tipping point can be identified in the end-Permian extinction event, it would be interesting to compare current levels and rates of greenhouse emissions. There isn’t a direct comparison possible when considering Earth’s Permian climate versus Earth’s current climate, but it would still be relevant and perhaps cautionary to investigate where the tipping point was that caused the largest recorded mass extinction event. Evaluating the survivors of mass extinction events may provide some information relevant to the event itself, but I also believe it is limited and subjective to the person performing the evaluation. There are near limitless traits that can be assigned to a group of individuals, and with the event so far in the past I believe it would be extremely difficult to determine with any certainty which traits were significantly influential for survivorship.
Raup 1994
It seems that the mass extinction events we have studied fit more neatly into Simpson’s passive progression of biodiversity than Darwin’s active and competitive progression. Environmental and physical influences are clearly primary causes for several mass extinctions, such as then end Permian which was most likely caused by run-away greenhouse gases. Darwin perhaps could not realistically envision environmental causes of extinction when he had not observed either current day or historic evidence of such events. I would argue that mass extinction events and natural selection both contribute largely to biodiversity in different ways, though mass extinction events result in more pure adaptive radiation. Adaptive radiation necessitates a new niche to be explored by a species or genera, and this would certainly be more likely to occur following a mass extinction than by the slow progression of natural selection.
Benton and Twitchett 2003
ReplyDeleteI think that the most plausible cause for the end Permian mass extinction is the Siberian traps erupting because none of the other extinction causes make any reasonable sense to me that is. I think that the most likely cause of this mass extinction is a combination of a few events traps for sure as well as massive releases of carbon dioxide and methane in to the atmosphere thus causing a global anoxic event with in the oceans and so on. The runaway greenhouse model is something that should definitely further looked at in order to gather a better understanding of how it works as well as understanding if this model can help us scientist understand how global warming can possibly be sustained. In my personal opinion I think it’s good to analyze both taxa that survived and those that died in this catastrophic event to better extrapolate the mechanisms if any that played a role in survival and death.
Raup
I personally think that biodiversity has radiated from both mass extinction and natural selection because once a mass extinction happens then it opens the niches for other biota to fill those niches and expand on current populations of those that survived. More successful species may be affected more than less successful due to the fact that the successful ones are more wide spread whereas those less successful are localized.
I think that a runaway greenhouse effect is most likely. This would suit the volcanic eruptions from the Siberian traps. Such a huge release of methane would detrimentally affect ecological structures by creating an uninhabitably hot environment for most species.
ReplyDeleteWe should further explore runaway greenhouse systems because it is relevant to climate change today. That is, if the Earth doesn’t cease to increasingly warm, frozen methane and CO2 will melt in a similar way and cause exponential warming in a positive feedback loop. This would mean huge consequences for all life on Earth – if a runaway effect did in fact cause the Permian extinctions, then we should take global warming very seriously and try to avoid such a catastrophe if we can. If warming and CO2 release continues as it is, then we will start to see an exaggerated version of our current extinction crisis within a century.
Surveying surviving taxa would give us some idea as to what did NOT cause the mass extinction – for example, if ocean life wasn’t nearly as affected as terrestrial life (which isn’t the case), we could say that perhaps ocean warming or ocean acidification were not likely involved in the extinction process.
I think the amount of radiation is the same, only it is magnified after a mass extinction because there are that much more niches to fill. Natural selection can only work at as fast of a pace as the environment provides new niches for species to fill, but after a mass extinction, there is an incredible amount of opportunity and not many other competing species to limit this growth.
More successful species tend to be dominant and often specialized to their environments. By becoming “really good” at one or a few roles, a species is less generalized, meaning if a mass extinction were to occur, then they may not be as flexible to such drastic change because they are evolutionarily “situated.”