Adapting
to the
Environment
Helping students
understand
natural selection
Amy L. Kovach
M
ost students enter my biology class with preconceived notions about evolution. Because evolution is a “theory,” some
students feel they should not have to study it, others feel it
contradicts what they have been taught at home, and a few usually are
curious to know if Charles Darwin really stated that humans came
from monkeys. However, because evolution and in particular natural
selection are unifying themes of biology and basic tenets of national
education and New York State curriculum standards, students must be
familiar with the concepts to truly understand biology.
The National Research Council (NRC) indicated, “Students often
do not understand natural selection because they fail to make conceptual connections between the occurrence of new variations in a population and the potential effect of those variations on the long-term
survival of the species” (NRC 1996, p. 52). To dispel some of these
misunderstandings, the following activity about natural selection
demonstrates H.D.B. Kettlewell’s studies of how the peppered moths’
(Biston betularia) adaptive values for their colors changed during the
Industrial Revolution in Manchester, England, influencing their survival and ultimately affecting the survival of their offspring. Most are
probably familiar with the common biology textbook story of the
peppered moths and variation and camouflage, but if the textbook
does not include it, the sidebar “Story of the peppered moth” is a
description to share with students before beginning this activity.
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T h e S c i e n c e Te a c h e r
In recent years Kettlewell’s famous
peppered moth studies have been
challenged and largely discredited
(see sidebar, last paragraph). However, the peppered moth studies still
provide a good model for studying
variation and camouflage.
Activity objectives
Because I teach the evolution unit toward the end of the school year, student anticipation has built up
throughout the year. From this activity, students should learn that “the
variation of organisms within a species increases the likelihood that at
least some members of the species will
survive under changed environmental
conditions” (NYSED 2001). Students
should also “observe the phenotypic
expressions of a mutation and determine their evolutionary significance
in terms of fitness” (Schrank 1996).
After completing the activity,
students should also be able to:
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explain the predator/prey relationship within a food chain of
moths and birds (for example,
birds get their energy from consuming moths, their prey);
explain the importance and
adaptive value of variations
within a species (for example,
how genetics influence evolution); and
explain how variation and
camouflage influences natural
Story of the peppered moth (Biston betularia).
Observations made by everyday citizens during the 1840s through 1890s in Victorian England of the peppered moth (Biston betularia) helped to influence Darwin’s
theory of evolution. Typically when people looked at the peppered moth they
saw a paper white moth with grayish black flecks covering its wings and body.
This color pattern was thought to be used as camouflage against birds, their
predators, in the daytime as the moths lay flat and rest against the lichens growing on trees (Raven and Johnson 1999). However, in the 1840s a few dark, grayishblack moths called melanics began to appear in urban Manchester, England. Soon
the number of dark moths outnumbered the white moths by over 90% (Keeton,
Gould, and Gould 1993). This was thought to be caused by the many factories of
the industrial revolution spewing out smoke and soot that darkened the moths’
hiding spaces (Bartsch and Colvard 2002). The smoke did not darken the moths
themselves. So if the melanics were not affected directly by the smoke, why did
they become so populous during the industrial revolution?
This noticeable phenotypic (physical) color change influenced a scientific study
in the 1950s by H.D.B. Kettlewell to try to understand this curious event. He conducted an experiment in an undamaged forest and a heavily darkened forest by
releasing equal numbers of both variants of moths and examining their survival rate
(Keeton, Gould, and Gould 1993). Kettlewell found that, as expected, the cryptic
appearance of the dark moth was better in the dark forest habitat and the typical
light moth was better in the less polluted forest. His results revealed that in the
polluted area 19% light moths and 40% melanics were recaptured; while in the light
forest, 12% light and 6% dark were recaptured (Raven and Johnson 1999). His associate Niko Tinbergen enhanced this study by filming birds choosing the opposite
colored moths from the lichens (Schraer and Stoltze 1995). Therefore, Kettlewell and
Tinbergen came to the conclusion that there were two variations in the genes of
the moths causing the color of the moths to be expressed differently. When the
environment changed, the peppered moths with the best cryptic coloration survived allowing their offspring to have a better chance at survival. At the same time,
a similar study that obtained comparable results was occurring at a rural Detroit,
Michigan field station (Keeton, Gould, and Gould 1993).
Today, Kettlewell’s peppered moth studies have been challenged and largely
discredited. In nature, Biston betularia seldom seeks refuge on tree trunks, and the
distribution of melanism in England (beyond the two areas Kettlewell studied)
does not correspond to observed pollution. However, the peppered moth studies
still provide a good model for studying variation and camouflage.
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FIGURE 1
Peppered moth templates.
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blend with its environment, but relied on camouflage for protection?
Why were the scientific studies by Kettlewell
and Tinbergen important?
How do the moths phenotypically show variation?
Birds and moths
selection (for example, dark moths are better
suited to the dark environment).
Preparation
The teacher completes the following steps prior to the
activity. Using the three peppered moth templates in Figure 1, the teacher photocopies nine moths for every student in the class onto white cardstock. All of the photocopied moths are not always needed, but are used as
replacements for damaged or missing moths. Using
black and gray crayons or colored pencils, the teacher
colors two-thirds of the moths white with some gray and
black specks to represent the typical peppered moths.
The remaining one-third are colored gray and black
with very little white showing to represent the melanic
peppered moths. Consulting a field guide for color patterns is helpful. The teacher can cut out each moth template and arrange them into two piles: typical and melanic. Once the moths have been cut out and colored, they
can be laminated and used from year to year.
During class time students read the story about the
peppered moths and discuss key concepts, such as:
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How does camouflage help organisms?
What would occur to an organism if it couldn’t
The teacher explains how the class will act out the story of
the peppered moth with six rounds representing the years
1860, 1880, 1900, 1920, 1940, and 1960. Two to four students are selected to act as birds. Their role is to find as
many of the hidden peppered moths as possible. Students
chosen as birds should have a lot of energy—the birds do a
lot of walking back and forth during the activity. The
remaining members of the class serve as individual peppered moths representing 1000 true moths.
Each student chooses one moth from the pile. The
piles for 1860, 1880, 1940 and 1960 should consist of
90% typical white moths with gray and black specks
and 10% melanics. The piles for 1900 and 1920 represent the Industrial Revolution’s impact, so the moths
should be 90% melanic and 10% typical. To keep the
flow of the activity moving, the teacher should create
the piles ahead of time. Some of the extra typical and
melanic moths will be used.
The “birds” are sent out of the room or to a location
where they cannot see what is occurring within the
room. The “moths” are then given a piece of already torn
masking tape, which they adhere to the back of the moth
in a loop. The teacher then tells the students to hide the
moth on something light colored or white because it is
Manchester, England, before the Industrial Revolution.
Students should leave 75% of the moth exposed because
real moths do not always hide under an object. Students
then have 30 s to tape the moth to an object anywhere in
the room. Some have been surprisingly creative and have
used the back of their shirts or another student’s shoes as the
environment. Others have even taken the time to create
a decoy of a chalk drawing on the board. After 30 s
students return to their seats and the birds are called into
the room. The birds now have 1–2 min, depending upon
age level, to find as many moths as possible.
FIGURE 2
Typical (T) and melanic (M) peppered moths (Biston betularia) captured.
1860
Name
T
1880
M
Bird #1
Bird #2
Bird #3
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T h e S c i e n c e Te a c h e r
T
1900
M
T
1920
M
T
1940
M
T
1960
M
T
M
Once a moth is found, it is considered eaten by the
bird. The moths that survive discovery get to reproduce
for the next round, and the student can hide two moths,
the original one and its offspring. This process occurs
during for every round. For instance, by year 1940
(round 5) some of my students have been able to hide
over eight moths. These additional moths need to be
added to the next round’s pile based on the survival rate.
The students play six rounds with a phenotypic frequency change in the rounds as mentioned above. For the
years 1900 and 1920 (rounds 3 and 4) the students must hide
their moths on something dark colored in the room representing the environmental change to the lichens caused by
the industrial revolution’s pollution. The birds remain birds
throughout the activity. The moths that have been found
are eaten and recycled back into the pile while the students
who hid the moths become part of the environment.
Analyzing the data
After each round, students record their data on a table
they have created or one that we created together, depending upon ability level, as shown by Figure 2. Upon
completion of the activity, students graph the data represented in the figure. Many other biological concepts may
be incorporated into this activity as a new idea or as
reinforcement. For instance, I have related this activity to
exponential growth and survival rate equations with
honors students, along with bacterial resistance to antibiotics. We also discuss how humans affect the environment. This activity is also great for meeting various
learning styles by addressing kinesthetic, visual, and auditory learners.
The teacher can pose two thought-provoking questions to ensure students have the most recent accurate
information about peppered moths: “If you were told
that some recent scientists have disagreed with the experimental design used by Kettlewell and Tinbergen because they observed moths during the day and not at
night when the moths are most common, what would be
your argument?” and “What would your argument be if
you were told that the peppered moths do not normally
hide on the lichens on the side of trees, and the photos in
the textbook are staged as scientists have revealed?”
FIGURE 3
Population size of two phenotypes of
lizards in the Verde Forest.
1985
1990
1995
2000
Green Lizards
1500
750
600
425
Brown Lizards
100
250
450
650
Because these two questions may challenge both those
who accept evolution and those who do not, I explain how
the genetic mutation—a micro-evolutionary level
change—enabled a phenotypic change in the moth regardless of time of day or their location. Because the
changing environment is the impetus to creating better
adapted organisms, peppered moths’ appearance changed
to dark and then back to light based upon the darkening
and lightening of their environment. The compilation of
the micro-evolution genetic changes over thousands of
years occurred in the moth population prior to the 1860s.
To assess their understanding of the material, I have
students complete the following tasks. Students should be
able to graph invented data from a similar situation I have
created between brown and green lizards (Figure 3). Students should answer the following question related to the
situation: “What may have influenced the population of
lizards to shift from green to brown and what type of
genetic change had to occur to enable the shift?” Finally,
students should be able to write a response—using the
terms mutation, reproduction, natural selection, environment,
and adaptation—on how a scientist might explain why
giraffes have long necks. Students could also use the terms
in a dialogue from the point of view of melanic or typical
moths and what occurred to them over time.
While the topic of evolution is often a difficult subject to approach in the high school biology classroom,
this activity captures student attention and helps them
learn about the concepts behind evolution; in particular, natural selection. n
Amy L. Kovach is a graduate research assistant in entomology and plant pathology at the University of Tennessee, Knoxville, 205 Ellington Hall, Knoxville, TN 37996;
e-mail: [email protected].
References
Bartsch, J. and M. Colvard. 2002. Brief Review in the Living Environment. Glenview, IL: Prentice Hall.
Keeton, W.T., J.L. Gould and C.G. Gould. 1993. Biological Science.
New York: W.W. Norton and Company, Inc.
McGavin, G. C. 1992. Insects of the Northern Hemisphere. New
York: Smithmark Publishers.
National Research Council (NRC). 1996. National Science Education Standards. Washington, D.C.: National Academy Press.
New York State Department of Education (NYSED). 2001. The
Living Environment Core Curriculum. Albany, N.Y.: The University of the State of New York.
Raven, P.H., and G.B. Johnson. 1999. Biology. 5th ed. Boston:
McGraw-Hill Higher Education.
Schraer, W.D., and H.J. Stoltze. 1995. Biology: The Study of Life.
6th ed. Upper Saddle River, N.J.: Prentice Hall.
Schrank, D. 1996. Scope, Sequence, and Coordination: A National Curriculum Project for High School Education. Arlington, Va.: NSTA.
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