Using the Soil Nematode Caenorhabditis elegans
To Test Darwin’s Premises About Populations
B
iological evolution is one of the over-arching concepts
recommended for student learning by the National Science
Education Standards (NRC, 1996). As with all such complex
concepts, student understanding of evolution is improved
when instruction includes hands-on, inquiry-based activities
(Layman, 1996). However, even authors writing in strong
support of teaching evolution sometimes offer discouraging
remarks about using inquiry-based learning. “In spite of strong
justification for including evolution-related instruction in
biology curricula, ‘descent with modification’ is a particularly
difficult educational issue, for by its very nature, evolution
is an abstract and generally nonobservable phenomenon”
(McComas, 1994, p.5). “Things in science can be studied
even if they cannot be directly observed or experimented on”
(National Academy of Sciences, 1998).
Certainly some important aspects of the evolutionary
process fit these descriptions: Macroevolution and speciation
are unlikely to be demonstrated in a classroom lab experiment.
As Alberts and Labov (2004) point out, however, “evolutionary theory makes no such distinction [between macro- and
microevolution]; the processes that lead to changes within
species, when accumulated over time, also can give rise to new
species.” That those processes—such as genetic variability, and
differential survival and reproduction within a population—can
sometimes be observed directly in living populations is vividly
described in Weiner (1994).
A number of paper-and-pencil and simulation activities
have been developed to give students hands-on experiences
with evolutionary concepts (for example, National Academy of
Sciences, 1998; Desharnais & Bell, 2000). “Real-time” activities
using live organisms are far fewer. Investigating Evolutionary
Biology in the Laboratory (National Association of Biology
Teachers, 1994) includes six activities using living organisms
(along with 17 simulation or paper-and-pencil activities and
two activities using fossils or preserved specimens). National
Association of Biology Teachers (1994) and National Academy
of Sciences (1998) offer activities or suggestions for using fruit
flies, “red wiggler” worms, bacteria, fungi, plant proteins, and
dihybrid crosses of plants.
The goal of this laboratory activity is to provide students
with an instructive and classroom-friendly living model with
which to test, firsthand, some of Darwin’s premises (influenced by his reading of Thomas Malthus) about populations,
competition, and natural selection. This activity addresses the
following National Science Education Standards for grades 9-12:
• Content Standard A: “As a result of their activities in
grades 9-12, students should develop abilities necessary to do scientific enquiry.”
• Content Standard C: “As a result of their activities
Melinda M. Mueller is on the science faculty, Seattle Academy,
Seattle, WA 98122, e-mail: mmueller@seattleacademy.org.
Melinda M. Mueller
in grades 9-12, students should develop an understanding of … biological evolution.” In particular,
this activity speaks to the following guideline for this
standard: “Evolution is the consequence of the interactions of (1) the potential for a species to increase
its numbers, (2) the genetic variability of offspring
due to mutation and recombination of genes, (3) a
finite supply of the resources required for life, and
(4) the ensuing selection by the environment of
those offspring better able to survive and leave offspring.” (National Research Council, 1996).
For both Charles Darwin and Alfred Russel Wallace, a
key insight leading to their theory of evolution by natural
selection was Thomas Robert Malthus’ Essay on the Principle
of Population (1798). In his essay, Malthus wrote that “the
power of population is indefinitely greater than the power in
the earth to produce subsistence for man … Population, when
unchecked, increases in a geometrical ratio. Subsistence
increases only in an arithmetical ratio. A slight acquaintance
with numbers will show the immensity of the first power in
comparison of the second.” Malthus applied the same principle to other species:
Through the animal and vegetable kingdoms, nature
has scattered the seeds of life abroad with the most
profuse and liberal hand. She has been comparatively
sparing in the room and the nourishment necessary
to rear them. The germs of existence contained
in this spot of earth, with ample food, and ample
room to expand in, would fill millions of worlds in
the course of a few thousand years. Necessity, that
imperious all pervading law of nature, restrains them
within the prescribed bounds. The race of plants and
the race of animals shrink under this great restrictive law. . . Among plants and animals its effects
are waste of seed, sickness, and premature death.
Malthus, 1798
i n q u i ry & i n v es t i gat i o n
Malthus Under a Microscope:
Darwin and Wallace both applied Malthus’ arguments
on populations to their own observations and developed
their theory: Populations of all species increase exponentially
and will outstrip resources, leading to intense competition
for resources. Further, any variations within a population
that confer even a slight advantage to the carriers of these
variations will be favored in the “struggle for existence.”
Thus, we get “natural selection:” i.e., greater reproductive success for advantaged individuals, with a concomitant increase
in the proportion of those in the next generation who possess the favorable variations (Wallace, 1858; Darwin, 1859).
Many of these premises can be tested experimentally
using populations of the soil nematode, Caenorhabditis elegans.
In this lab activity, students place a small number of C.elegans
in culture plates, and record population changes over a period
of two weeks. Included in the students’ experiments are plates
with two strains of C.elegans; students observe whether one
Darwin’s Premises About Populations
219
strain out-competes the other. Students compare their results
with what they would expect from Darwin’s premises. In doing
this activity, students use microscopy, metric measurements, estimation, data-recording, and graphing techniques using Microsoft
ExcelTM.
C.elegans has the advantages of a short life cycle (several
days from egg to sexual maturity) and high reproductive rate.
The nematodes are small enough that thousands can be grown
in a culture plate, but they are easily visualized and counted,
using student-grade dissecting or compound microscopes (See
Figures 1 and 2). Their behavior is lively (therefore interesting to
students) but, unlike fruit-flies, they are readily contained (See
WORM Initiative, 2004 for images and more information about
using C.elegans in the classroom). They are easy and inexpensive
to cultivate and maintain (Kahn-Kirby, 2000). Many different
strains are available, permitting students to test for competition
between strains with differing characteristics.
Figure 1. N2 strain C. elegans at 40X magnification.
This activity was designed for high school students in a
basic biology course. It is described below as taught to high
school juniors.1
Before doing this activity, students should have already had
some practice with basic lab skills and concepts, including use of
microscopes, identification of experimental variables, and data
collection. The activity could be modified or extended in scope
for higher-level courses in high school or college. Alternatively,
it could be simplified for younger students.
Materials
• Stock of C. elegans, N2 (wild type) strain with males (C.
elegans stocks are available to schools at no charge from
the Caenorhabditis Genetics Center [CGC]. See ordering information at http://biosci.umn.edu/CGC/Strains/
request.htm . Allow 10-14 days for delivery.)
• Stock of a strain to contrast with N2. Some suggested
strains (also available from CGC):
– CB 61: a “dumpy” strain (genotype dpy-5 e61 I),
whose adults are noticeably slower moving and
“chubbier” than N2 adults. ES3 (adult).2
– CB 187: a “roller” strain (genotype rol-6 e187 II), whose
spiraling motion is distinct from the wave-like motion
of N2. ES3 (adults and later-stage larvae).
– CB 190: a nearly paralyzed strain (genotype unc-54
e190 I). ES3 (all stages).
• Broth culture of E. coli (to serve as food source for C.
elegans).
• Nematode growth agar (available from Carolina™, catalog # 17-3520, or you can make your own. See KahnKirby, 2000).
• Disposable culture (Petri) plates, 60 x 15 mm. Three
plates are needed for each student lab team, plus some
extras for expanding stocks.
• Permanent markers or labels for culture plates.
• Growth chambers. Plastic storage containers, lined with
moistened paper towels, work well. A shoebox-size con1
Figure 2. CB190 strain of C. elegans at 40X magnification, showing
characteristic “frozen board” behavior.
tainer is large enough to hold all plates for a class of 30
students divided into three-person teams.
• Microscopes. Ideally, each student lab team will have
access to one dissecting scope and one compound scope
with 4X and 10X objectives.
• Small metric rulers, one for each student lab team, for
measuring culture plate diameters and field-of-view
diameters.
• Access to a computer with Excel, or calculators, and
graph paper.3
Advance Preparation by Instructor
1. Order materials and stock organisms, to arrive one week
before activity.
2. Prepare culture plates for student activity (three plates
for each lab team) and stock expansion (two additional
plates for each strain, for each class—see Step 3).
a. Melt agar and pour plates (fill plates about half-full).
b. Spread a few drops of broth-cultured E. coli over
cooled medium in each plate.
c. Wrap plates or place in sealed plastic containers to
prevent medium from desiccating.
d. Refrigerate prepared plates until used.
At Seattle Academy, Biology is a junior-year required course.
Riddle (1997) includes an appendix listing hundreds of C. elegans strains. Included in most strain descriptions is an ES (ease of scoring) code. ES3
indicates a phenotype that is easy to distinguish from other phenotypes. You will only need the strain designation (e.g., CB61) to order from CGC.
3
Students at Seattle Academy all have laptops that they bring to the classroom. I send them the lab handout and lab spreadsheet via the school’s network. For schools with classroom computers, Excel spreadsheets can be set up on these computers. Alternatively, paper and calculators can be used
to record data, carry out calculations, and prepare graphs.
2
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The American Biology Teacher, Volume 69, No. 4, april 2007
3. Prepare additional stocks from original stock plates
(about four days before activity).
a. Use a sterilized scalpel or razorblade to remove “pie
slices” of agar from original stock plates (see Figure
3). The number of nematodes on each slice may vary
from about a dozen to several hundred. However, the
nematode’s reproductive rate when provided with
fresh resources is so rapid that any initial sample
within this range will give a large population within
a few days, sufficient for division among student
groups.
b. Place one of these pie slices upside-down in a prepared plate (from Step 2, above). Do this for as many
stock plates as needed for your classes (two stock
plates for each strain, for each class of 30 students).
c. Place prepared and labeled stock plates in growth
chambers (see Materials list) and leave at room temperature.
d. After 24 hours, the pie slices can be removed from the
new stock plates. The worms that were on the slice of
agar will have crawled off.
e. After four days, the new stock plates should have
sufficient populations for this activity (see Step 3a,
above—whether each student group starts with a few
or a few hundred nematodes, the eventual outcome
will satisfy the needs of the activity).
4. If students have access to computers with Excel, prepare
spreadsheet for data and calculations (see Figure 4).4
Alternatively, students can use calculators and lab journals/and or graph paper for calculations and graphs.
The following data and calculations will be recorded:
a. First-day date and population count for each plate
(three plates per lab team).
b. For each subsequent day
of the activity, the date and
three microscope field-ofview population counts for
each plate.
c. Calculated average field-ofview population for each
plate, for each day.
d. Diameter of field-of-view
used each day (students
may need to switch to
smaller
fields-of-view—
therefore higher magnification—as populations
become more crowded and
difficult to count).
e. Diameter of culture plates
(this value, obviously, is
constant).
f. Calculated estimate of total
population for each plate.
The formula:
i. Use the diameter of the
field-of-view to find the
area of the field-of-view
4
“Pie-slice” cut from stock plate & transferred to fresh plate
Stock plate
Fresh plate with agar & E. coli
Figure 3. Set up of additional stock plates or experimental plates.
(diameter divided by 2 to give the radius, then the
radius squared and multiplied by pi).
ii. Use the diameter of the plate to find the plate’s
total area.
iii.Divide the plate area by the field-of-view area to
find the number of “fields-of-view” needed to
“fill” the entire plate.
iv.Multiply the number found in (iii) by the average
field-of-view population to obtain estimated population for entire plate.
5. Plan the class schedule for this activity. Sixty to ninety
minutes are needed on the first day for students to
observe the nematodes and set up experimental populations. After that, students should census their nematode
populations every several days for at least two weeks.
These subsequent population counts usually take less
than 30 minutes, especially as students gain experience.
6. Prepare students for activity:
a. Assign lab teams (I found three-person teams work
well).
DAY
ONE DATE: _________________________ # WORMS(CB61):_ _____________________
DATE:_ _____________________________DATE:_____________________________
# in SAMPLE 1:
# in SAMPLE 2:
# in SAMPLE 3:
AVERAGE #:
Plate Diameter:
Sample Diameter:
Estimated
Total:
worms
# in SAMPLE 1:
worms
# in SAMPLE 2:
worms
# in SAMPLE 3:
worms/sample
AVERAGE #:
in millimeters
Plate Diameter:
in millimetersSample Diameter:
worms on plateEstimated Total:
worms
worms
worms
worms/sample
in millimeters
in millimeters
worms on plate
DATE:_ _____________________________DATE:_____________________________
# in SAMPLE 1:
# in SAMPLE 2:
# in SAMPLE 3:
AVERAGE #:
Plate Diameter:
Sample Diameter:
Estimated Total:
worms
# in SAMPLE 1:
worms
# in SAMPLE 2:
worms
# in SAMPLE 3:
worms/sample
AVERAGE #:
in millimeters
Plate Diameter:
in millimetersSample Diameter:
worms on plateEstimated Total:
worms
worms
worms
worms/sample
in millimeters
in millimeters
worms on plate
Figure 4. Sample Excel document for recording data and calculations. A separate worksheet is created
for each experimental plate.
In response to e-mail requests, I will e-mail as an attachment the spreadsheet used in this lab activity.
Darwin’s Premises About Populations
221
b. Assign background readings on Darwin’s basic premises (from any biology textbook that includes the
topic of evolution).
purpose and experimental variables of each experiment, the names of their lab partners and a flow chart
of the lab procedures).5
c. Assign students to read the lab handout (I require
students to keep lab journals in which, before each
experiment begins, they record the title, date and
d. Before students begin activity, ask them to predict,
by sketching x-y graphs, what the nematode populations will do over the course of the activity.
Instructions to Students
This subsection is the handout that students receive.
Lab question: Do populations behave as Darwin described?
Purpose: To test for the presence of geometric reproductive rates,
variation, competition, and a “struggle for existence” in a
model organism population.
Experimental variables:
• Independent variable: the nematode strains (two strains
each grown on separate plates, plus the two combined in
a third plate).
• Dependent variable: change in populations over time.
Concepts:
Darwin gained an important insight in his thinking about
how species might change over time when he read Thomas
Malthus’ An Essay on the Principle of Population. Malthus
wrote that living things reproduce at a rate much greater
than any possible increase in resources (such as food), and
that populations are therefore subject to “waste of seed, sickness, and premature death” (Malthus, 1798). Malthus’ particular concern was human populations, but from Malthus’
principles Darwin developed his ideas about a “struggle for
existence” and natural selection in all living species.
The organism you will use for this lab is Caenorhabditis
elegans (C. elegans). This is a tiny worm (nematode) that
normally lives in soil. Here are some facts about C. elegans:
• It feeds on bacteria. You will grow the nematode on
plates of agar with bacteria added. The agar provides
moisture and micro-nutrients.
• Most of the individuals you will see are hermaphrodites,
capable of producing both eggs and sperm and self-fertilizing. Hermaphrodites have thin, thread-like tails.
• A few individuals are male, producing only sperm.
You can recognize the males by the stubby “fishhook” structure of their tails. Males can mate with
hermaphrodites.
• No individuals are female; that is, none produce only
egg cells.
• C. elegans has many of the same cell processes and
proteins (including enzymes) as humans and other
more “familiar” organisms, so it is commonly used as
a research model for biological processes.
• C. elegans has a short life cycle: It reaches maturity
and begins producing offspring in a matter of days.
Over the next two or three weeks, you will be looking for whether Darwin’s premises (influenced by his reading of Malthus’
essay) are borne out in the nematode populations:
• Is the reproductive rate of the nematodes geometric?
5
• Is the nematode population growth eventually
restricted (or reversed) by limits in resources (such
as food or space)?
• Do the nematodes vary in appearance or behavior, as
Darwin suggested would be true in any population?
• Does there seem to be a competitive factor as the
population increases, with some variations out-competing (having more surviving offspring) than other
variations?
Procedures
Day 1: Observing the nematodes
1. Look at the various stock plates of nematodes. Each
plate has a particular “strain” or variety of the nematode
species. Try to figure out and describe/draw the features
that distinguish one strain from another.
2. In the N2 strain, find a male, and draw its appearance
(the other strains do not produce males). If you do not
find a male after a reasonable period of searching (they
are rare), move to the next step.
Day 1: Setting up a population
1. Obtain a culture plate that has been prepared with nematode growth agar and bacteria. Label the underside of
the plate with the date, your team’s name, and the strain
you will work with first. Every team will work with the
N2 strain and one additional strain.
2. Obtain a stock plate of your first strain of nematodes.
Use a razor blade to cut out a pie slice of agar from the
stock-plate. Your piece should be small—only a few millimeters across. Place this agar triangle (right-side up) in
the fresh plate you labeled in Step 1.
3. Using a dissection microscope, focus on the upper
surface of your agar triangle. Count and record the number of nematodes present. Then turn the agar triangle
upside-down, to help the nematodes crawl off and onto
your plate.
4. Repeat Steps 1-3, above, using a stock plate with a different strain of nematode.
5. Obtain a third fresh culture plate of medium & bacteria.
On this plate, you will place a pie slice from both stock
plates of your chosen strains. Prepare and label this plate
accordingly.
6. Place all your plates in the growing chamber (a plastic
tub with a lid, lined with moistened paper towels—this
keeps the nematode medium from drying out). The
growing chamber will be kept at room temperature.
7. Record your Day 1 data in your lab journal and in the
Excel document provided.
continued
In response to e-mail requests, I will e-mail as an attachment the student lab handout.
222
The American Biology Teacher, Volume 69, No. 4, april 2007
Instructions to Students
continued from previous page
Subsequent Observations
1. On the second day of this experiment, remove the pie
slices from each of your plates. By this time, most or all
the nematodes will have crawled off the pie slice onto
the surface of the medium in your plates. Then go on to
the next steps.
2. Each day in class for the next two weeks, as time permits, observe your nematodes, and record the following
information (in your lab journal and in the Excel document provided):
• Focus on three different areas of each plate. Count
the number of nematodes in each field of view (for
the N2 strain, you will need to note the number of
males seen, if any, along with the total number of
worms).
• Measure and record the diameter in mm of your
field-of-view. Your instructor will give directions
for finding this measurement the first time.6 As the
nematodes become more crowded, you may need to
use a smaller field of view in your microscope. Be sure
to record the field-of-view diameter each time you collect
data.
• Measure and record the diameter of your culture
plate, in mm.
3. The Excel spreadsheet provided for this experiment is
set up to calculate the number of nematodes in each
plate. A formula in the spreadsheet will calculate the
average number of nematodes per field-of-view in each
plate. Another formula will use the diameter of the fieldof-view and the plate diameter to calculate an estimated
total population for the plate.
4. Using the calculations in the spreadsheet, you should
record the following in your lab journal:
• How many nematodes are now in each plate, according to the calculated estimates?
• In the plates that include N2 nematodes, how many
are male?
• How many adults of each strain are in your “twostrain” plate?
6
• Other observations that strike you (for example,
are the nematodes on a given plate dispersed more
or less evenly, or do they clump together? Does the
proportion of adult versus young worms appear to
change over time? Etc.).
5. Each time you make observations, comment on the
questions that introduce this lab regarding variation,
population growth, and competition. Are Darwin’s predictions evident in the nematode populations?
6. For additional credit, you may continue your data-gathering until the end of our unit on evolution (or beyond).
Discuss this with your instructor.
Lab Conclusions
To be written in lab journal.
1. Prepare graphs for each of your plates, with estimated
total populations of each strain plotted against time.
2. How did the changes in the populations’ numbers compare to your original predictions? What changes were
unexpected or surprising to you?
3. Are Darwin’s premises, as inspired by Malthus, borne
out by the nematode populations? Support each answer
by referring to your data.
• Do the nematode populations increase geometrically?
• Do resources (food or space) seem to put a limit on
the populations?
• Do the nematodes vary in appearance or behavior, as
Darwin suggested would be true in any population?
• Does there seem to be a competitive factor as the
population increases, with some variations (strains,
genders, behaviors, or ages) out-competing other
variations, as evidenced by their proportion of the
population total?
4. What might be the advantage to a population (such
as the N2 strain) in having many hermaphrodites, few
males, and no females (remember that “advantage”
in evolutionary terms means “produces the most offspring”).
Field-of-view diameters at lower magnifications can be measured directly, by placing a metric ruler on the stage and counting the mm that cross the
field of view. For higher magnifications, the field-of-view diameter can be calculated proportionally. For example, if the field-of-view diameter with
the 4X objective were 4 mm, then the field-of-view diameter for the 10X objective would be 4/10 * 4 mm.
Assessment & Results
“winner” in the competition for resources, and sometimes with
strains cycling back and forth in dominance.
Students who carry out this activity are nearly always astonished at the rapid growth of the C. elegans population—commonly from a few dozen individuals at the outset, to thousands
within two or three days. The population crash that follows
is equally dramatic (see Figure 5, sample of student graph).
Less dramatic, but just as significant, is what happens after the
“crash.” Populations may fluctuate around an apparent “carrying
capacity:” When the nematode population drops, the bacterial
population rebounds, followed by a rebound in the nematode
population … etc. In plates with two strains, the proportion of
each strain may also fluctuate over time, sometimes with a clear
I collect lab journals after the experiment is concluded.
Besides evaluating the work on basic criteria (such as clarity of
recorded data, etc.), I assess the students’ conclusions. Do their
conclusions reflect an understanding of Darwin’s premises?
Have they logically applied their experimental findings to these
premises? In particular, the quality of students’ graphs and their
interpretation of those graphs offers windows into the students’
understanding.
Each student lab team is expected to analyze its findings as
a team. Depending on time available, this is done in one or more
of the following ways:
Darwin’s Premises About Populations
223
• A “symposium” during which
each team formally presents
its findings (including tables
and graphs of data) and
answers questions from other
teams .
• A co-authored typed lab
report, including the lab
question, purpose, basic procedures, data tables, graphs,
conclusions, and short bibliography .
If we place the N2 and CB87 strains of C. elegans in the same environment, who will thrive?
Total population in environment
• A “lab meeting” during which
all teams compare and analyze their results, discuss possible errors and debate conclusions .
At the conclusion of the unit,
students take an exam on principles
Figure 5. Student-produced graph of C.elegans data.
of the Darwin-Wallace theory . Several
questions on the exam require students to apply data and observations
when food became scarce the more sluggish CB190, who
from this experiment in their explanations of Darwin’s premises .
probably used less energy than their fast moving counterFor example, one student analyzed the C. elegans data as follows:
parts, flourished as they were able to survive on less food.
Although there were more CB 190 at first the N2 quickly
overtook them and continued to dominate; however, after
about two weeks as the food supply became scarce the
CB190 seemed like they were coming back. I believe that
the initial success of the N2 was based on our hypothesis
that they are better able to find and acquire food, however,
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The AmericAN Biology TeAcher, Volume 69, No. 4, APril 2007
For more advanced courses, a number of extensions are
possible . For example, rather than give students a method for
estimating the population, the instructor can wait until students
are confronted with uncountable thousands of nematodes .
Then the problem can be posed: How can we estimate these
populations with reasonable accuracy? Other more sophisti-
cated mathematical analyses could be done, such as an analysis
of population age structure over time.
This activity was designed to examine Darwin’s original
ideas about populations, before knowledge of genetics informed
our understanding of evolution. The activity could be extended
to examine natural selection in the light of population genetics.
As the student comment above intimates, different strains may
flourish as the environment changes. Would different strains
(phenotypes) flourish under different conditions of temperature,
moisture, light, bacterial food source, etc.? Likewise, students
could start with small initial populations of mixed strains and
observe genetic drift (founder or bottleneck effects) in action.
Students can also be challenged to develop and carry out
their own lines of inquiry about population dynamics, competition under different experimental conditions, spatial distribution under different conditions, and so on.
Because C. elegans is a widely-used model organism in biological research (It was the first animal species whose entire genome
was sequenced [Pines, 2001]), instructors and interested students
will be able to find a wealth of information about this organism
(for example, Riddle et al., 1997; Brown, 2003; CGC, nd).
Acknowledgments
I wish to thank Nancy Hutchinson and Mary Vail of
the Science Education Partnership (SEP) at Fred Hutchinson
Cancer Research Center in Seattle, Washington, for their assistance in the preparation of this article and for the SEP program’s
twelve-plus years of supporting and spurring my professional
development. Theresa Stiernagle, curator at the Caenorhabditis
Genetics Center, has been an unstintingly generous resource for
C. elegans stocks and information. Thanks also to Mary Margaret
Welch, Katie Morrison and Laurie Greco, who shared their
expertise in the use of C. elegans as a teaching tool, to the great
improvement of this activity. Finally, I am grateful to my students at Seattle Academy for their suggestions, samples of their
work, and the constant reminders they provide of why teaching
is well worth the effort.
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