Performance Benchmark L.12.D.4
Students know the extinction of species can be a natural process. E/S
When teaching all of the L.12.D benchmarks, it is imperative to help students understand the
process of science. Most objections and misconceptions about evolution are directly related to
the misunderstanding of how science works. When students understand the nature of science,
they will understand how scientists have studied the process of evolution. As questions arise
about a “supernatural” creation of Earth and the Universe, students who understand the nature of
science will understand why supernatural forces cannot be studied as part of scientific processes.
Extinctions occur “often” in evolutionary history. Scientists estimate that more than 90% of the
species that have lived on earth are probably extinct. Most extinctions occur due to selective
nature of the species. That is, if an organism does not have or does not develop adaptations that
allow it to flourish in an environment, that species will eventually die out. These are called
background extinctions. The fossil record provides evidence that background extinctions
regularly occur throughout time. However, the number of species which have become extinct at
any one point in time is relatively low. Scientific studies show another type of extinction in
which massive numbers of species become extinct in a short period of time. These are called
mass extinctions. There are 5 recognizable periods of mass extinction in the earth’s history.
There are several theories as to the cause of the mass extinctions, most of which hypothesize that
major climatic and environmental changes occurred globally. These changes were most likely
due to impact of extraterrestrial object or extreme geological activity such as many active
volcanoes and crustal movements. Many species that were not adapted to the resulting changes,
died out. The following sites provide more information about both mass and background
extinctions:
http://www.pbs.org/wgbh/evolution/library/03/2/l_032_01.html
http://www.earth.rochester.edu/ees207/Mass_Ext/higgins_mass2.html
http://sciencebulletins.amnh.org/biobulletin/biobulletin/story985.html
One of the largest mass extinctions was the Permo-Triassic (PT) extinction which occurred
between the Permian Period and the Triassic Period about 248 million years ago. Based upon
fossil records and other evidence, it is estimated, that up to 90 percent of all existing species
were lost during the P-T extinction. To learn more about the P-T extinction, see
http://www.earth.rochester.edu/ees207/Mass_Ext/higgins_mass4.html
The most familiar mass extinction is associated with the demise of the dinosaurs. This
Cretacious-Tertiary (KT) extinction occurred about 65 million years ago, between the Cretaceous
Period of the Mesozoic Era and the Tertiary Period of the Cenozoic Era. To learn more about
the KT extinction, see http://www.earth.rochester.edu/ees207/Mass_Ext/higgins_mass3.html
Most scientists believe that the KT extinction was most likely caused by the impact of a large
asteroid. Evidence of this asteroid includes deep sea core samples taken near the suspected
impact area. The core samples provide evidence of ash and ejecta material as well as distinctive
fossil variants above and below the ash layer.
Figure 1. This is a diagram of a deep
sea core that supports the theory of an
asteroid impact on earth at the KT time
boundary. Diagram is from:
http://www.nmnh.si.edu/paleo/blast/inde
x.html .
Other evidence of asteroid impacts near the K-T time boundary includes the presence of an
unusually high concentration of Iridium in layers of clay at the K-T boundary. Iridium’s sources
are cosmic dust and the earth’s core. The high Iridium concentration therefore indicates that
either an asteroid struck the earth or a massive volcanic eruption occurred at that time. The
iridium layer was first discovered by Luis and Walter Alvarez in Italy in the 1970’s, but has also
been observed in several other sites around the world.
The exact effects of an asteroid impact on life are debated by scientists. To learn more about the
uncertainty of the cause of the KT extinction, see
http://www.ucmp.berkeley.edu/diapsids/extinction.html
The following graph shows peaks of invertebrate extinctions over the last 600 million years. As
the graph shows, there have been 5 major peaks indicating mass extinction episodes.
Figure 2. A graph of
invertebrate extinctions over the
last 600 million years.
The mass extinctions appear as
periodic peaks rising above the
background extinction levels. This
data is from the work of D. M.
Raup and J. J. Sepkoski.
This figure can be seen at http://www.enchantedlearning.com/subjects/dinosaurs/extinction/
Background extinctions can occur as a result of many things. The extinction of a species can
occur through the process of evolution. Extinction by natural causes may be due either to actual
death of a species or due to evolution of the species into one or more different species.
Organisms become new species through modification over time. If the ancestral species
disappears and is replaced by the new species, extinction has occurred. Evidence from the fossil
records substantiates that many species have become extinct and new species have developed
over time.
Many scientists believe that Earth may be in a 6th mass extinction phase, as many species are
going extinct every day. Humans are not the sole cause of extinctions. However, since our
appearance on earth, humans have had an impact on extinction of several species. This has
occurred via hunting, habitat destruction, and other environmental impacts.
For more information on the human impact on extinctions, go to
http://www.nmnh.si.edu/paleo/geotime/main/index.html
Performance Benchmark L.12.D.4
Students know the extinction of species can be a natural process. E/S
Common Misconceptions associated with this benchmark
1. Humans cause all extinction misconceptions
Students incorrectly believe that humans have caused all extinctions. Although humans
do play a role in modern extinctions due to alteration and destruction of habitat, pollution,
overexploitation, and disease there are other causes that contribute to the extinction of
species. Some scientists believe that we may be in the midst of a sixth period of mass
extinction. The rate of extinction is currently higher than the average background
extinction rates, and some of this may be due to human influence. However humans are
not the only factor affecting extinctions today. In fact, most extinctions that we know
about today, occurred prior to the appearance of humans. As extinctions have occurred
naturally throughout the history of life, they continue naturally today.
To learn how several esteemed scientists responded to questions about whether or not we
are in the midst of mass extinction and how humans might be involved, see:
http://www.pbs.org/wgbh/evolution/extinction/massext/index.html
2. Dinosaur extinction represents the failure of an entire branch of life
Although extinct now, dinosaurs represent one of the greatest successes of adaptation and
survival. They existed on Earth for more than 150 million years, which is longer than
any other land animal. Birds evolved from dinosaurs. To learn more about the success of
dinosaurs, see
http://www.nmnh.si.edu/paleo/dinosaurs/index.htm
Performance Benchmark L.12.D.4
Students know the extinction of species can be a natural process. E/S
Sample Test Questions
1. The Permo-Triassic mass extinction which occurred between the Permian and Triassic
Periods
a. affected a large percentage of marine and terrestrial species.
b. most likely was caused by a large volcano that erupted.
c. caused extinction of the dinosaurs
d. occurred about 10 million years ago
2. What percent of species that have ever lived on earth are probably extinct?
a. 28%
b. 52%
c. 77%
d. 90%
3. Which of the following does NOT likely contribute to background extinctions?
a. evolution through natural selection
b. destruction of a habitat due to natural or human cause
c. interbreeding between species
d. changes in local environmental conditions
4. Which of the following has NOT been implicated as contributing to mass extinctions?
a. volcano eruptions
b. changes in ocean water level
c. major hurricanes
d. asteroid impact
5. Based upon fossil evidence, scientists believe that the “lifespan” of a species from its origin to
its extinction is about
a. 1-5 million years
b. 10-50 million years
c. 100-500 million years
d. 1000-5000 million years
6. The following diagram illustrates numbers of extinctions that have occurred over time. What
is the highest rate of background extinction indicated in this diagram?
a.
b.
c.
d.
10 families per million years
7 families per million years
17 families per million years
5 families per million years
Performance Benchmark L.12.D.4
Students know the extinction of species can be a natural process. E/S
Answers to Sample Test Questions
1.
2.
3.
4.
5.
6.
(a)
(d)
(c)
(c)
(a)
(b)
Performance Benchmark L.12.D.4
Students know the extinction of species can be a natural process. E/S
Intervention Strategies and Resources
The following list of intervention strategies and resources will facilitate student understanding of
this benchmark.
1. Hypothesis of Dinosaur Extinction
To study several hypotheses about dinosaur extinction and the evidence used to support the
hypotheses, see this site: http://www.pbs.org/wgbh/evolution/extinction/dinosaurs/index.html
2. Panel discussion of Scientists Addressing Mass Extinction
Students can listen to several scientists as they address questions about whether or not we are
currently in another period of mass extinction.
http://www.pbs.org/wgbh/evolution/extinction/massext/index.html
3. Video clip to stimulate class discussion on human caused extinction.
To stimulate a discussion about human impact on extinction, this short video clip that discusses
how an invasive species causes extinction.
http://www.pbs.org/wgbh/evolution/library/10/3/quicktime/l_103_03.html
4. Ocean Core Evidence of Asteroid Impact
A recently recovered deep sea core contains evidence of an asteroid impact approximately 65
million years ago. Students and teachers can study the evidence in this sample.
http://www.nmnh.si.edu/paleo/blast/index.html
5. Humans Role in Extinction
An activity that causes students to think about the role of humans in extinctions can be accessed
at http://www.nationalgeographic.com/xpeditions/activities/08/biodiversity.html
6. AAAS Recommended Evolution Education
For additional information on what is recommended in evolution education, see: Evolution on
the Front Line: An Abbreviated Guide for Teaching Evolution from Project 2061 at AAAS.
http://www.project2061.org/publications/2061Connections/2006/media/Evo_Handout_Teachers.
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