The Story of the Peppermint Worms
A Lab on Variation and Natural Selection
Objectives
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Identify variations in a population of a ficticious animal species
Observe how some variations may improve the ability of a species to
survive
Calculate and graph changes over time in the size of a population
Materials
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Calculator
Colored Pencils (red, green and blue)
Pen or Pencil
Ruler
Graph paper
Background
Not all members of a population of living things are alike. The differences in
traits between one member of a population and another are called variations.
Variations in traits may not affect an organism’s ability to survive in its
environment. Some variations, however, can be helpful while other variations
can be harmful. Variations in traits that help the organism survive better in its
environment are called adaptations and typically become common in a
population due to natural selection, (survival of the fittest). Charles Darwin
suggested that organisms that are best adapted to their environment are most
likely to survive and reproduce. Parents with helpful adaptations can pass them
on to their offspring. Organisms with harmful traits are not as likely to survive
long enough to reproduce, or will likely reproduce less often. Therefore,
harmful traits eventually disappear from a population. This idea is the bases of
Darwin’s theory of natural selection.
Read the following ficticious scenario about peppermint worms. These
carnivorous worms vary in their ability to see prey.
The peppermint worm, Candy crawlis, is found only in the grasslands of Africa.
The green and white striped worms crawl among the tall grasses in search of
prey. They dine exclusively on the African grass bug – a tiny, green, beetle-like
insect that clings to the stems of grasses.
You are a member of a team of two (or three) zoologists, scientists who study
animals. Your team established a work station on an African grassland to gather
data on peppermint worms. Your entire team wanted to study the worms for
several years, but since there wasn’t enough funding, it was decided that all
three team members would study the peppermint worms for two years and that
one team member would remain for six more years to gather additional data.
Your team counted the population of worms only twice. In the first count, your
team found there were three interesting variations of peppermint worms:
1. Normal-vision peppermint worms are the most common group. They are able to
see great distances. Upon spotting an African grass bug, a normal-vision
peppermint worm maneuvers through the grass, catches its prey, and consumes
the little bug. A normal-vision female wormo gives birth to an average of SIX
baby worms every year. She cares for her babies until the youngsters reach
adolescence and move out.
2. Droopy-eyed peppermint worms do not have the accurate eyesight found in
normal-vision worms. A droopy-eyed worm must be close to an African grass
bug before it can see its prey. Therefore, a droopy-eyed peppermint worm is
unable to obtain as much food as a normal-vision peppermint worm. A droopyeyed peppermint worm raises only FOUR baby worms to adolescence each year.
3. X-Ray vision peppeprmint worms are rare among peppermint worms, and your
team was surprised to find them at all. These worms can see through objects.
However, X-Ray vision is of no real advantage to the worms in their search for
food because the African bugs live on the outside of grass blades. These worms
can see as well as the normal-vision worms. They successfully raise SIX
offspring each year .
The life expectancy of all peppermint worms is 12 months. The continuation of
the species depends on the number of offspring produced each year. The firstand second-year population counts made by your team of zoologists are shown
in Data Table 1. After the second year, the other two team members left the
workstation, but you stayed an additional six years. During the first two years of
the study, your team developed the following methods for calculating expected
peppermint worm populations:
a. To calculate the expected populations of normal-vision worms AND XRay vision worms, multiply the total number of females from the
previous year by SIX, (both normal-vision and X-Ray vision female
peppermint worms produce SIX offspring each year). Divide this
number by two to determine how many of the new worms would be
expected to be females. Round off all results to the closest whole
number.
b. To calculate the expected populations of droopy-eyed worms, multiply
the total number of females from the previous year by FOUR, (droopyeyed female peppermint worms produce FOUR offspring each year).
Divide this number by two to determine how many of the new worms
would be expected to be females. Round off all results to the closest
whole number.
Procedures
I.
Meet the Worms:
II.
Data Table Calculation Procedures:
Draw illustrations of each variety of worm in their natural environment.
Include the grass blades and the grass bugs the worms prey upon in
your drawings.
1. The first and second year calculations in Data Table 1 have been
completed for you. Use the numbers at the end of the second year
to complete the calculations for the third year. For each type of
worm, calculate both the expected total population and the number
of females expected. Place your answers in Data Table 1 in the
appropriate space of the row labeled “Year 3”. (Use the methods
described on page 2 for your calculation).
2.
After the third year, a flock of birds migrated through the
grasslands where the peppermint worms lived. These birds
swarmed down on the peppermint worms and devoured them in
great numbers. You estimated each type of worm had only half the
usual numbers of offspring survive that year. Using this
information, calculate the populations for the FOURTH year. Enter
this information in the row labeled “Year 4” of Data Table 1.
3. After the fourth year, the birdsleft the grasslands.
However, they
left behind a deadly virus that spread to some of the surviving
worms. Once again, many worms died. Again, you estimated that
only half the usual number of offspring survived that year. Using
this information, calculate the populations for the fifth year. Enter
this information in the row labeled “Year 5” of Data Table 1.
4. After the fifth year, severe climatic changes occurred in the oceans
surrounding Africa, bringing strong winds to the grasslands. The
grass bugs instinctively moved inside the grass blades to get out of
the strong winds. This move helped the bugs because it protected
them from the winds and also from the view of most peppermint
worms. You noted that the X-Ray vision worms could still see the
bugs inside the blades of grass. The X-Ray vision females continued
to produce SIX offspring each year. The normal-vision worms could
not see the bugs located inside the grass blades. They could only
dine on the few bugs they caught outside the grasses. Due to this
food shortage, each normal-vision female worms produced only
ONE surviving offspring. The droopy-eyed worms also had fewer
bugs to eat. They too had only ONE young survive per female.
Using this information, calculate the populations for the SIXTH year.
Enter this information in the row labeled “Year 6” of Data Table 1.
5. During the seventh and eighth years, no unusual events occurred.
The winds died down, but most of the grass bugs continued to
burrow inside the grass blades for protection. You noted the
reproductive rates of normal- and droopy-eyed worms continued to
remain low, and each female continued to raise only ONE offspring
to adolescence in each of these years. The X-Ray vision worms
continued to produce SIX offspring per female each year. Using this
information, calculate the total populations for the seventh and
eighth years. Enter this information in the rows labeled “Year 7”
and “Year 8” of Data Table 1.
6. Complete the Data Table, graph the results in a line graph and
answer the conclusion questions. (See instructions attached).
III. DATA TABLE 1:
Start
NORMAL VISION
Number of
Total Population
Females
DROOPY EYED
Number of
Total Population
Females
X-RAY VISION
Number of
Total Population
Females
Year 1
40
20
20
10
40
20
Year 2
120
60
40
20
120
60
Year 3
Year 4
Year 5
Year 6
Year 7
Year 8
IV. GRAPH INSTRUCTIONS
1. Using the attached graph paper, create a line graph showing the
changes in the TOTAL POPULATION for each variety of worm from Year
1 through Year 8. Let the horizontal (X-) axis represent the years, and
the vertical (Y-) axis represent the total population of worms. Graph the
population of normal-vision worms, droopy-eyed worms and X-Ray vision
worms in three different colors on the graph.
2. Develop a legend for the graph to indicate which color represents each
type of worm.
3. Use your graphed results to help answer the conclusion questions
below.
V. CONCLUSION QUESTIONS:
1. Which type of worm was dominant in the population for the first few
years of the study?
2. Which type of worm was dominant in the population at the end of the
study?
3. In what years did the population of normal-vision worms greatly decline?
4. If their environment remains the same, what might happen to the
populations of normal-vision and droopy-eyed worms?
5. Why didn’t the population of X-Ray vision worms decline during the
study?
6. Which trait or variation changed from one that was neither harmful nor
helpful to one that was helpful? Explain your response.
7. Explain how this lab represents the process of Natural Selection.
8. Explain how a variation in a population of organisms, such as the
peppermint worms, influences that population’s chance of survival.
9. Some variations in the population are due to mutations within an
individual. Explain the role of mutation in natural selection.