Joshua Evans
To what extent do mass extinctions prevent evolution?
Abstract
My final product will be a report on mass extinctions and their effects on evolution, and argues to what extent
mass extinctions prevent evolution. I discuss what a mass extinction is and how many mass extinctions have
occurred, and the time periods in which they occurred. I move on to discuss the factors which affect evolution
in order to give the reader an idea of why certain species go extinct, and why others survive. I then draw
attention to case studies which consist of taxa to have gone extinct during the Ordovician, Permian-Triassic
and Cretaceous-Palaeogene mass extinctions and why the taxa went extinct, and taxa to have evolved during
Ordovician, Permian-Triassic and Cretaceous-Palaeogene mass extinctions and why the taxa survived. I then
conclude the final product by arguing that overall, the extent to which mass extinctions prevented evolution
was slightly higher than the extent to which mass extinctions caused evolution.
Introduction
Over 99% of species that have ever lived are now extinct (Cornell, E., 2012) and over 95% of those
species died as a result of competition with other organisms, or a lack of food (Natural History
Museum, 2013) but for the remaining 4%, mass extinctions were responsible for their demise.
The first topic to discuss is what a mass extinction is. Whilst there is no precise definition, there are a
few general rules that must be satisfied. The first of these rules is that the rate at which species are
becoming extinct must be at least 75% higher than the rate of speciation, traditionally over a period
of approximately 2 million years (Tennant, J., 2013). The second of these rules is as follows: more
than half of the Earth’s surface needs to be environmentally affected (Hallam, A. and Wignall, P.B,
1997). However, due to the imperfection of the fossil record and the inability to explore exactly how
widespread the extinction was, an approximation must be made as to whether such a catastrophic
event in Earth’s history fits the criteria to be classified as a mass extinction.
Therefore, my general definition is as follows:
‘A mass extinction is the rapid death of a major number of species over a geologically insignificant
period of time, during which the rate of extinction is at least 75% higher than the rate of speciation
and more than half of the Earth’s surface is environmentally affected.’
The use of the terms ‘major’ and ‘insignificant’ seems vague but there are no measurements to go
by, since biodiversity varies over time. This is why the number of mass extinctions to have occurred
is widely debated, but for convenience-sake I shall use the generally accepted number of five.
These five extinction events have occurred in the following time periods (Natural History Museum,
2013):
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The Late-Ordovician event happened 445-440 million years ago.
The Devonian-Carboniferous event happened 375-359 million years ago.
The Permian-Triassic event occurred c252 million years ago.
The Triassic-Jurassic event occurred c201 million years ago.
The Cretaceous-Palaeogene event happened c66 million years ago.
Joshua Evans
When I come to discuss to what extent mass extinctions prevent evolution, the effects will take
place from when the climate starts changing to when the environmental change is negligible, usually
5-10 million years after the start of the mass extinction (Wikipedia, 2013). The key point in the title is
the use of ‘extent’ which means that I will be making judgements on the degree to which species
recovered, mainly by comparing the amount of evolution in the given time period, previously
mentioned, against the degree of extinction for the discussed mass extinctions.
Another fundamental question when talking of mass extinctions is how do we come to know they
have occurred? The answer lies in biostratigraphy, which correlates and assesses relative ages of
rocks. This in turn gives us a fossil record that can be dated by comparing the known approximate
ages of the rocks where they are found. When organisms die, occasionally their remains are
preserved as fossils and determining the ages of these fossils provides a record of the organisms that
lived during different periods of Earth’s history. Evidence for mass extinction comes from comparing
the fossils found at one period, for example the Devonian, to the fossils in the next period, in this
case it would be the Carboniferous (Exploring Earth, 2013). Marine fossils are mainly used because
of their superior fossil record and greater stratigraphic range compared to land organisms. However,
even using marine fossils is not entirely reliable due to the Signor-Lipps effect which states that the
fossil record is never complete (Wikipedia, 2013).
The second element of the essay title involves evolution, so the next question to ask is what is
essential for species to evolve? Greg Laden, in 2009, called the following points ‘the three necessary
and sufficient conditions of natural selection.’ First of all there is variation in trait. This can be a
mutation, which alters DNA sequence of a gene which in turn changes the feature that the gene
codes for. Secondly, these variations must inheritable, in other words, the mutated gene must be
able to be passed down through generations. Lastly, there must be differential fitness conferred by
the trait- the trait must have ‘good’ and ‘bad’ effects, only the organisms with the ‘good’ traits will
survive to pass their genes on to the next generation. I am going to add two more essentials for
species to evolve. The first of these is the isolation of populations and migration of individuals, to
bring new genes to the area and to avoid the genes becoming lost amongst often vast gene pools.
The differential fitness could be due to competition, therefore the last point is: a basic principle of
evolution is a selective pressure (Berg, M.J., et al, 2002) - an agent of differential mortality or fertility
that tends to make a population change genetically (Simpson, J. and Weiner, E. 2013).
Inevitably, mass extinctions will have effects on evolution; from changing climates to the point they
become unbearable to certain organisms, to decreasing competition for other organisms by making
species go extinct. Mass extinctions have changed the course of evolution (Douglas, H. D., 2001): the
end-Permian extinction resulted in the death of c96% of marine species (Wikipedia, 2013). However,
the change need not necessarily be extinction, because where certain organisms die-off, others can
prosper in the numerous new niches created. Therefore, whilst mass extinctions have produced a
huge dying of a large number of species: 93-97% of all species in the case of the largest extinction
event, the end-Permian, new species are constantly arising. This is achieved by filling the gaps left by
the organisms which could not evolve to adapt to the changing environment.
Joshua Evans
The Late-Ordovician mass extinction
The Late-Ordovician mass extinction is regarded as the second largest extinction event of all time;
26% of all marine families, 60% of all genera and 82-88% of all species went extinct as a result of its
devastation (Natural History Museum, 2013). But this doesn’t mean that there was complete
devastation, for new species could evolve, as shall be discussed.
Organisms to have gone extinct during the Late-Ordovician mass extinction:
It cannot be doubted that many species suffered the effects of rapidly changing and unbearable
climates over a prolonged period of time. One class to have largely become extinct were the
trilobites: 113 genera were reduced to 45, 10 or 11 families (33%) were lost (Hallam, A. and Wignall
P.B., 1997) and over 90% of species were lost (Natural History Museum, 2013). However, shallow
water taxa were relatively unscathed, representing an opportunity for other species of trilobite to
evolve to exploit the now available niches once the myriad of other species had died-off.
Returning back to the widespread die-off of most trilobites, it was only trilobites in carbonate mud
mounds that escaped any generic-level extinction, representing an indisputably large loss of many
species. Those trilobites that went extinct had a bewildering array of morphologies and, presumably,
life-styles (Hallam, A. and Wignall P.B., 1997). Asaphina, one of the most diverse and numerous
trilobite orders disappeared, and an examination of the protapsis (the larval stage of trilobites)
provides a clue to the demise. Some protapses appear to have been adapted to a planktonic
existence and all taxa possessing this larval morphology, together with all the trilobites that pursued
a pelagic (open water) existence in the adult stage became extinct. Widespread forms were wiped
out whilst more endemic taxa such as Phacopida largely survived. The majority of extinct taxa had a
planktonic larval stage that ensures widespread dispersal (found in a wide range of facies) whilst the
survivors had a benthic (ocean-bottom dwelling) protapsis which limited dispersal ability. Once many
species had gone extinct, trilobites never again evolved pelagic adult life-styles (Hallam, A. and
Wignall P.B., 1997).
The crisis is thought to have been caused by deleterious changes in the water column, and this is
why: after c1 megaannum into the Late-Ordovician mass extinction, glacial conditions ended rapidly
(Natural History Museum, 2013), meaning that deep ocean waters stagnated, thus the pelagic
species which spread due to the planktonic protapsis widely suffered in the stagnated deep water.
There was less chance of having a prolonged isolation of populations with a planktonic protapsis,
which, as aforementioned, is required for evolution to occur. Another requirement for evolution was
a selective pressure such as competition, but in the case of pelagic trilobites, there was too much
competition from the trilobites inhabiting shallow waters, so they could not find any niches to fill in
these less affected areas.
The demise of the trilobites highlights and supports the view that mass extinctions do indeed
prevent evolution, and based on the proportion of taxa that went extinct- 60% of genera and 82-88%
of all species- they prevent evolution to a large extent.
Organisms to have evolved as a result of the Late Ordovician mass extinction:
However, despite the fact that 82-88% of all species went extinct, during the Late-Ordovician mass
extinction many species evolved to exploit gaps in the ecosystem, and not least of these were
cyanobacteria. These organisms create biofilms, trapping rocks, forming an accretionary disk:
Joshua Evans
microbialites - structures which date back to c3.5 billion years (Wikipedia, 2013). Mass extinctions
are normally assessed on extinction of macro-organisms, not microorganisms; however,
microbialites are an exception, because they are large structures - roughly 20cm in length - so are
clearly evident in the fossil record (Wikipedia, 2013). During the Late-Ordovician, CO2 was c12 times
higher than today’s levels, suggesting that this is time when the cyanobacteria were in the process of
developing an extremely effective CO2 concentrating mechanism (CCM) which operates to elevate
CO2 around Rubisco during photosynthesis (Badger, R.M. and Price, D. G., 2002) meaning that the
Late-Ordovician mass extinction provided cyanobacteria with the ability to produce more sugars for
energy which further increased reproductive success. This certainly disagrees with the view that
mass extinctions are evolutionary dead-ends and adds weight to the extent to which mass
extinctions cause evolution.
A microbialite resurgence of abundance and size occurred in Western North America. The
resurgences were associated with the loss of mat-inhabiting animals, providing insights into shallowwater community structures after mass extinction events (Sheehan, M.P. and Harris, T.M., 2004).
This was after a decline occurred during the Ordovician radiation of marine animals (Sheehan, M.P.
and Harris, T.M., 2004), but evolution of microorganisms is far quicker than in macro-organisms due
to their asexual reproduction entailing superior reproductive success, therefore their genes are
distributed amongst the population in a shorter period of time.
I have been talking of microbialites going extinct, but it is the cyanobacteria which are the living
organisms, so when discussing why they evolved it is necessary to use the cyanobacteria as a talking
point. Reverting back to the paragraph on what is essential for species to evolve; one component
was variation in trait. In the case of cyanobacteria this was a series of gradual variations (far less
gradual than in macro-organisms) which probably led to (at least a developing) CCM which got
inherited by subsequent generations. The selection pressure for this being competition by other
cyanobacteria for space on the limited ocean floor. Therefore, this underlines how fallacious is the
view that mass extinctions prevent evolution, and Douglas H. Erwin, in 2001, rightly pointed out that
there is no apparent relationship between the magnitude of an extinction and its evolutionary
impact which shows that regardless of the effects of a mass extinction, evolution is an inevitabilitywhere there is extinction there is evolution.
To what extent did the Late-Ordovician mass extinction prevent evolution?
But this brings me to the key point of the argument; to what extent do mass extinctions prevent
evolution? The degree of extinction is based on the percentage of species to have diminished, for
convenience sake. I have already mentioned that 82-88% of species went extinct during the LateOrdovician mass extinction. However, to determine the extent of prevention towards evolution, the
rate of recovery must be taken into account and as aforementioned, the recovery will be discussed
up to the point where the effects on the environment are negligible. In the Late-Ordovician mass
extinction, extinction rates were high for approximately 20 million years after the beginning of the
event (Wikipedia, 2013), however another source suggests that they were high for up to 8 million
years (Palaeobiology and Biodiversity Research Group, 2012), so I shall take the average of
approximately 14 million years. Therefore to answer the root of the question, the extent to which
life recovered in the 14 million years must be known; diversity was slow to rebound, achieving preextinction levels in approximately 15 million years (Krug, A.Z. and Patzkowsky, M.E., 2004). This
Joshua Evans
period of recovery extends just outside the point at which effects on the environment were
negligible; therefore the extent to which the mass extinction prevented evolution was slightly higher
than the extent to which it caused evolution. Once I come to the conclusion, I will take the mean
average recovery period in relation to the average period up to which extinction rates were high for
the three mass extinctions to be discussed however, so the extra 1 million years needed for recovery
may become almost negligible.
The Permian-Triassic mass extinction
Now let’s turn to the Permian-Triassic mass extinction, an event which occurred approximately 252
million years ago (Natural History Museum, 2013). This is claimed to be the largest mass extinction
of all time: 96% of all species marine species went extinct along with 70% of all terrestrial vertebrate
species; 57% of all families; 83% of all genera (Wikipedia, 2013) and 93-97% of all species (Natural
History Museum, 2013).
Organisms to have gone extinct during the Permian-Tertiary mass extinction:
Tetrapods are a superclass to have largely gone extinct during the Permian-Triassic mass extinction:
there was a loss of 21 terrestrial tetrapod families (63% of all families) (Hallam, A. and Wignall, P.B.,
1997). I will discuss the widespread extinction of two orders: herbivores (for example pareiasaurs,
one of the most dominant tetrapods at the time) (Hallam, A. and Wignall, P.B., 1997) and the
extinction pulse caused the loss of small piscivores (fish-feeders). The loss of herbivores is linked to a
loss of vegetation (Palaeobiology and Biodiversity Research Group, 2012) and the loss of piscivores is
linked to a loss of fish, but why did plants and fish become affected? This wide-scale extinction can
be linked by the following events: massive eruptions pumped out carbon dioxide and caused acid
rain and global warming which killed the plants (Palaeobiology and Biodiversity Research Group,
2012), this is where the herbivores begin to diminish. As the roots shrivelled, the soil was exposed,
and normal rainfall caused massive erosion of soil and plants into the rivers and seas (Palaeobiology
and Biodiversity Research Group, 2012). This led to algal blooms which naturally died in huge
numbers, leading to widespread decomposition, carried out by decomposing bacteria. When these
bacteria respire aerobically they use up oxygen in the water, leading to fish suffocating. The
piscivores extinction begins.
The selection pressure was food availability, however no favoured variation amongst an isolated
population which could be inherited, therefore no evolution could occur. The herbivores and
piscivores are two taxa of many to have gone extinct during the Permian-Triassic mass extinction,
and it represents how interdependent species are on one another, and as a result, evolution is
prevented from occurring in many cases. Most species simply cannot evolve in time to cope with the
rapidly changing environment. The extent of prevention of evolution was the largest of all mass
extinctions on the one hand, as 93-97% species went extinct, but as I discussed with the LateOrdovician mass extinction, the extent must also take into account the degree of recovery of these
species up to the time when the effects on the environment from the catastrophic events were
negligible.
Joshua Evans
Organisms to have evolved as a result of the Permian-Triassic mass extinction:
To continue with the tetrapods, I have mentioned that they largely went extinct, that is to say there
wasn’t a complete extinction. Therefore, there was less competition and newly available niches in
the environment which spells one thing- evolution. Permian tetrapods were sprawlers, and Triassic
tetrapods were upright in posture and gait (Palaeobiology and Biodiversity Research Group, 2012). It
seems that the mass extinction had an astonishing effect on both the diapsid (possessing two holes
in the skull, e.g. crocodilians) (Wikipedia, 2013) and synapsid (possessing a fused skull, including all
mammals) lines (Wikipedia, 2013). This is natural selection at work, as the diapsids were predators
of synapsids, thus exerting a selective pressure on them making synapsids evolve to become better
adapted at escaping from predators. Conversely, the synapsids in turn, forced the diapsids to evolve
towards becoming efficient and successful predators by changing their body structure to make them
faster at catching prey. I am not saying that either subclass consciously changed the other, but
rather indirectly caused the evolution of one another by acting as a selective pressure.
Therefore, even though in the Permian-Triassic mass extinction, tetrapods became extinct, many
tetrapods could evolve, partly by sheer chance that their life-styles weren’t affected to the point of
extinction, as in the case of diapsids and synapsids. However, going back to the necessities for
evolution to occur, there must also have been isolated populations, which had a favoured variability
(in this instance, the variability being the change of bone structure to be able to stand upright and
gait) that was inherited.
To what extent did the Permian-Triassic mass extinction prevent evolution?
This certainly adds weight to the view that mass extinctions don’t prevent evolution, but I am
arguing the extent to which they do or don’t prevent evolution. I have mentioned that 93-97% of all
species went extinct during and after the Permian-Triassic extinction event. The extinction rates
were high for approximately 27 million years: the recovery of vertebrates took 30 million years
(Wikipedia, 2013) and as an approximation, it is justified to assume that as vertebrates are top of the
food chains, they were amongst the last organisms to recover. It generally takes 5-10 million years
for biodiversity to recover (Wikipedia, 2013) so when we are talking about a class recovering, it
could be said that it would take approximately 3 million years for vertebrates to recover. This time
period for recovery has been worked out by deduction but remember that due to the vast timescale, the deductions are based on approximations. In the c27 million years after the start of the
Permian-Triassic mass extinction, it is estimated that recovery took up to 10 million years
(Wikipedia, 2013), so for arguments sake, say the recovery took 10 million years; I am assuming that
in the spare c17 million years, because biodiversity was at pre-extinction levels, it didn’t increase
enough to add any weight to the biodiversity value. This means that of the 93-97% of all species to
have gone extinct, they recovered in 10 million years of the 27 million years it took for extinction
rates to cease being high, so the extent to which the Permian-Triassic mass extinction prevented
evolution must be said to be approximately equal to the extent to which is caused evolution.
The Cretaceous-Palaeogene mass extinction
The Cretaceous-Palaeogene mass extinction occurred 66 million years ago and caused the extinction
of 16% of all marine families, 47% of all genera and an estimated 71-81% of all species (Natural
History Museum, 2013).
Joshua Evans
Organisms to have gone extinct during the Cretaceous-Palaeogene mass extinction:
Dinosaurs (e.g. Tyrannosaurus rex and Velociraptor) are undoubtedly the most famous group of
organisms to have gone extinct in Cretaceous-Palaeogene mass extinction and appear to have
become a symbol for extinction; this could be partly down to the fact that they went fully extinct
unlike other taxa mentioned such as the trilobites in the Late-Ordovician mass extinction. However,
less well known is exactly why they went extinct. Recent studies suggest that they declined slowly
over 10 million years as a result of cooling climates (Benton, M.J., 2013), so the impact of the
extinction of the dinosaurs may by less severe than was first thought, but as the study hasn’t been
fully accepted in the scientific community, I can’t use it when I discuss the extent to which mass
extinctions prevent evolution. Most people know the immediate cause of this mass extinction, and
that is asteroid impact. The most widely accepted theory is that a large asteroid or comet collided
with Earth, this is known as Asteroid Theory (lifestudiesonline, 2013). It has been contrived that the
supposed asteroid was 10 km in diameter based on the size of the crater made (Benton, M.J., 2013)
and within the first 24 hours, mega tsunamis crashed coastlines and the surface of the Earth was
pummelled by debris (Animal Discovery Channel, 2004).
These were direct effects on the environment and wiped out many species. Throughout the essay I
have discussed more indirect effects on the environment causing widespread extinction. This is
exactly what occurred with dinosaurs. The asteroid kicked up enough dust and debris to block out
sunlight for a long time (Sachatello-SawyerDon, B. and Charlesworth, L., 2013). Without the sunlight,
plants could not photosynthesise (Benton, M.J., 2013) meaning that most plants died, therefore
plant-eaters such as the Sauropods died which led to the demise of the meat-eaters, such as the
famous Tyrannosaurus rex and Velociraptor. This represents the interdependence of species and it
prevented the evolution of dinosaurs. Even with an inheritable favoured variability, the conditions
were too extreme to survive on the surface of the land. Any isolated populations would have
sufficed, meaning that evolution could not occur; it could be said that the selective pressure was too
great. The entire extinction of this great group of organisms certainly adds to the extent to which
the Cretaceous-Palaeogene mass extinction prevented evolution.
Organisms to have evolved as a result of the Cretaceous-Palaeogene mass extinction:
However, the extinction of dinosaurs left copious niches to be filled, and c60 million years ago, the
mammals diversified (Gore, R., 2013) and they became the dominant landform, taking over from the
dinosaurs; this is of particular interest to us as mammals. I previously mentioned that conditions
from the Cretaceous-Palaeogene mass extinction were too extreme to survive on the surface of the
land, so the animals which could go beneath the surface of the Earth could survive and the primitive
mammals did just that (Attenborough, D., 2011). In just 10 million years since the dinosaurs went
extinct, about 130 genera had evolved, containing c4000 species (Public Broadcasting Service, 2001).
This sudden expansion of species diversity into new ways of life is known as adaptive radiation
(Public Broadcasting Service, 2001) and this most certainly occurred with the mammals.
Marsupials such as kangaroos were some of the earliest mammals to have evolved, alongside the
dinosaurs, but never rising to prominence due to the domination of dinosaurs on land. However, the
dinosaur extinction opened new gaps in the ecosystem for the marsupials to diverge and exploit. In
South America, they evolved to become carnivores, and one of the carnivores had an astonishingly
Joshua Evans
close resemblance to the independently evolved sabre-toothed cat, a placental mammal (Byrant,
P.J., 2002), developing elongated canine teeth and starting to walk on four legs for speed to catch
prey more successfully. After the Cretaceous-Palaeogene mass extinction, plants diversified,
becoming widespread once more under the increasingly stable conditions and in South America
again, placental mammals (giving birth to well-developed young) evolved as herbivores, such as the
ground sloth (Byrant, P.J., 2002). They evolved to become massive in size- 20 feet when stood
upright Wikipedia, 2013) in order to be able to reach the tops of trees to eat the leaves.
The rapid and widespread evolution of mammals must approximately cancel out the massive
extinction of the dinosaurs- one huge group was effectively replaced by another huge group- thus
adding weight to the extent to which the Cretaceous-Palaeogene mass extinction caused evolution.
To what extent did the Cretaceous-Palaeogene mass extinction prevent evolution?
I have established that 71-81% of all species went extinct during the Cretaceous-Palaeogene mass
extinction, but as has become a routine method throughout this essay, I must establish the period of
time up to which the effects on the environment were negligible. This time value varies a lot
depending on the source; Wikipedia states it as a few million years or much longer (Wikipedia, 2013)
and Mr Feneru of the Natural History Museum has told me in an email on 14th August, 2013 that this
mass extinction may have lasted on a scale of thousands of years. I will take the approximate
average from all sources and say that the environment was affected up to approximately 6 million
years. The time of recovery however, is estimated to be much longer: nearly 25 million years for
diversity to fully recover (Hallam, A. and Wignall, P.B., 1997). This unequivocally suggests that the
extent to which the late Cretaceous mass extinction prevented evolution was far greater than the
extent to which it caused evolution, as life only roughly 25% recovered up to the point where
extinction rates were no longer high.
Conclusion
To come to a judgement as to the extent to which mass extinctions have prevented evolution, I must
first come to averaging the recovery period in relation to the average period up to which extinction
rates were high for the three mass extinctions discussed. The period of time up to which the effects
on the environment were negligible in the Late-Ordovician mass extinction was approximately 14
million years (Palaeobiology and Biodiversity Research Group, 2012 and Wikipedia, 2013)
approximately 27 million years in the Permian-Tertiary mass extinction (Wikipedia, 2013) and
approximately 6 million years in the Cretaceous-Palaeogene mass extinction event (Wikipedia,
2013).. This gives a mean average of 15,700,000 years which I shall round up to 16 million years as I
am working on approximations. The next quantity to be taken into account is the time period for
recovery. Biodiversity took 15 million years to recover after the Late-Ordovician mass extinction
(Krug, A.Z. and Patzkowsky, M.E., 2004), approximately the same period of time in which extinction
rates were high; biodiversity took c10 million years to fully recover after the Permian-Tertiary mass
extinction (Wikipedia, 2013), well within the time up to which extinction rates were high and it took
c25 million years for biodiversity to fully recover after the Cretaceous-Palaeogene mass extinction
(Hallam, A. and Wignall, P.B., 1997)., which was substantially outside the time up to which extinction
rates were high. The mean average time of recovery is therefore 16,700,000 years which, again, I
shall round up to 17 million years.
Joshua Evans
The mean average period of time up to which effects on the environment were negligible is c16
million years, whereas the mean average period of time for biodiversity to recover was c17 million
years. Therefore, the two values are close, but it must be said that overall, the extent to which the
three discussed mass extinctions prevented evolution was slightly higher than the extent to which
mass extinctions caused evolution.
Evidently, mass extinctions have changed the course of evolution (Douglas, E. H., 2001); however,
the change doesn’t have to be total extinction as in the case of the dinosaurs. Instead, it can cause
evolution as in the case of the mammals; this is slightly out of favour to the extent to which mass
extinctions prevent evolution. Nevertheless the two always occur simultaneously.
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