Evolution by Natural Selection
Natural Selection on Mussel Shell Thickness
Students will explore the effects of natural selection on populations with trait variation and
without trait variation.
Students should have some understanding of evolution due to previous classes and/or the big
bang theory (this will be discussed later in a following unit).
Each student should have some prior knowledge of species interactions. (Predator and prey,
omnivore, herbivore, etc.)
NGSS Standard: HS-LS2-6:
“Evaluate the claims, evidence, and reasoning that the complex interactions in
ecosystems maintain relatively consistent numbers and types of organisms in stable
conditions, but changing conditions may result in a new ecosystem”
Colorado Standard: 2.2 High School Life Science
“The size and persistence of populations depend on their interactions with each other and
the abiotic factors in an ecosystem.”
Inquiry Question: “How do keystone species maintain balance in ecosystems?”
Learning objectives: SWBAT
Recall the three requirements for evolution to occur by means of natural selection.
1. Phenotypic variation in a trait is necessary within the population
2. Phenotypes must be heritable within the population
3. Some phenotypes are more fit than others
Determine whether evolution occurred through natural selection on a given species or by
a different mechanism of evolution.
Explain how evolution by natural selection could lead to biodiversity in an environment.
Materials
Peanuts and almonds: have a 9:1 ration of peanuts to almonds. Students will start with 9 peanuts
and 1 almond each but these numbers will double each reproductive cycle of the activity
Student worksheet (attached)
Paper bag for nuts and plastic bins to put nuts in so they are not on the tables
Graph paper
Computer lab access to make video project
Camera (students will check out a camera from the school and be responsible to return it)
Instructional planning
Video: misconception https://www.youtube.com/watch?v=S1t5_iGN0xE
Worksheet: https://www.kbs.msu.edu/images/stories/docs/nuts_student_worksheet.pdf
Background: Read aloud to students before the activity or have them popcorn read
Background
Asian shore crabs (Hemigrapsus sanguineus) are native to the western Pacific Ocean, but since
the late 1980s they have been spreading throughout the northern Atlantic coast of the United
States. These crabs are tiny, about the size of a penny, but populations of this predator are
growing very quickly. Blue mussels (Mytilus edulis) in the northeast US are becoming more
resistant to predation by crabs. How is this happening?
Within populations of this single species of mussel, some individual mussels have thicker shells
than others, so there is phenotypic variation in the trait. This trait, shell thickness, is heritable,
meaning that thick-shelled crabs have thick-shelled offspring and thin-shelled crabs have thinshelled offspring. Crabs are exerting selection pressure on this trait, creating a relationship
between the trait and their fitness by eating only the thin-shelled mussels.
The three requirements for evolution are heritable trait, phenotypic variation in the trait,
relationship between trait and fitness. These requirements are all met, so the mussel population is
able to evolve in response to predation by the crabs.
Evolution is a change in the average trait value in a population over time. Evolution does not
happen to an individual; it happens to a population. It cannot happen in less than a single
generation, because traits must be passed on to the offspring. Evolution can happen rapidly;
“rapid evolution” happens in tens or hundreds of years instead of thousands of years or longer.
There are many different examples of rapid evolution, with the most famous being the peppered
moth in England during the Industrial Revolution. The moth was originally light colored with
dark speckles, but after the heavy coal burning dirtied the buildings and trees, predators could
easily pick out light colored moths and populations of the moths shifted to being dark colored
with light speckles.
Procedure/activity
Student Activity
Engage
Students make a KWL in their
scientific notebook about evolution
and natural selection.
Students share their responses with a
partner and write one of their
thoughts on the board
Explore
Students complete lab with almond
and peanuts.
Students who are allergic to nuts
should observe only, but still fill in
the data sheet.
Students will not be eating the nuts,
but placing them in a paper bag to
simulate that they have been eaten.
Teacher Activity
Ask students to make a KWL about
evolution and natural selection.
Teacher walks around and asks students
what they know and want to know about
the topic.
Asks students to put a statement on the
board about evolution or natural selection
then reads them allowed for the class to
hear. Use this to gauge the lesson (If
students have misconception use this
information to incorporate into a future
lesson.)
Ask if students have ever heard of Asian
shore crabs or blue mussels? Explain
background on the situation. (Read the
background information to the class.)
Explain lab and hand out papers.
Students will work in groups of 4. Split
them up into groups through counting off
1-4.
Tell students they will not be eating the
nuts, but placing them in a paper bag to
simulate that they have been eaten. Also
tell them to keep the nuts in the plastic bins
provided.
https://www.kbs.msu.edu/images/stories/do
cs/nuts_student_worksheet.pdf
Explain
Elaborate
Students go over worksheet as a
class and fill in the correct answers
if needed.
Go over work sheet as a class and give
correct answers.
Answer any questions that come up.
Have students grab the classroom set Ask students about their prior knowledge
of science books and look up these
topics
o Other mechanisms of
evolution (genetic drift,
bottle necking, etc)
o Evolutionary or Selective
Pressure
Evaluate
of these topics and ask them to go deeper
by looking up information in their book
and writing down their findings in their
scientific notebook.
o Other mechanisms of evolution
(genetic drift, bottle necking, etc)
o Evolutionary or Selective Pressure
o How evolution is measured
and what factors account for
evolutionary change
o How evolution is measured and
what factors account for
evolutionary change
o Dynamic Environment
o Dynamic Environment
o Teleological change does not
occur
o Teleological change does not occur
Students will research another
example of natural selection in the
world.
As a group they will then create a
video explaining the natural
selection they found.
The video must include the scientific
name of the species involved, cited
pictures, the sources they used and
how they know this is an example of
natural selection.
They should research sites like
National geographic, Natural
History, and Discover magazines.
Explain the video project about natural
selection and what must be included.
The video must include the scientific name
of the species involved, cited pictures, the
sources they used and how they know this
is an example of natural selection.
This can be made like a business
proposition with a PowerPoint, like a
commercial for a news show, or like a
music video. Allow students to choose
which they would like to make, but they
are to stay in their lab group.
Rubric attached
Assessment
Formative: KWL, statements written on the board,
Summative: Asks students the difference between natural selection and evolution, Video
project
Evaluation Rubric for Video Project
Performance Level
Group Cooperation
Documentation
Subject Content
Video content and
organization
Introduction
Needs Improvement
Students needed to
be reminded to stay
on task frequently.
One or two people
did all the work.
Students argued
with one another or
left some students
out of the process.
0 points
There is no
documentation.
0 points
Subject knowledge
is not evident.
Information is
confusing, incorrect,
or flawed.
Satisfactory
All students
contributed a fair
share to the project,
though some
workloads varied.
Students had to be
reminded
occasionally to stay
on task. Students
made an effort to
include all group
members in the
process.
3 points
Most of the
elements taken from
other sources are
documented;
however, some
documentation may
be inaccurate or
missing.
3 points
Subject knowledge
is evident in much
of the video. Most
information is clear,
appropriate, and
correct.
0 points
The video lacks a
central theme. Much
of the information is
irrelevant to the
overall message
5 points
Information is
connected to a
theme. Details are
logical and
information is
relevant throughout
most of the video
0 points
The introduction
does not orient the
3 points
The introduction is
clear and coherent
Excellent
All students
contributed equally
to the video.
Students stayed on
task at all times.
Students worked
with each other in a
friendly manner.
5 points
All elements from
other sources are
accurately
documented and
cited properly
(MLA format).
5 points
Subject knowledge
is evident
throughout the
video. All
information is clear,
appropriate and
correct.
10 points
Video includes a
clear statement of
purpose. Events and
messages are
presented in a
logical order, with
relevant information
that supports the
video’s main ideas.
5 points
The introduction is
motivating, and
viewer to what will
follow.
Mechanics
Production
0 points
The text and audio
have 4 or more
grammar or spelling
errors.
0 points
Video is of poor
quality and is
unedited. There are
no transitions added
or transitions are
used so frequently
that they detract
from the video.
There are no
graphics.
0 points
and evokes
moderate
interest/response
from the viewer.
3 points
The text and audio
have 1-2 grammar
or spelling errors.
hooks the viewer
from the beginning.
5 points
The text and audio
have no grammar or
spelling errors
3 points
5 points
Video is edited. A
Video is edited.
variety of transitions
Video runs
are used and most
smoothly from shot
transitions help tell to shot. A variety of
the story. Most of
transitions are used
video has good
to assist in
pacing and timing.
communicating the
Graphics are used
main idea. Shots
appropriately.
and scenes work
well together.
Graphics explain
and reinforce key
points in the video.
5 points
10 points
Total
Project Grade
/50
Anticipated misconceptions/alternative conceptions
Evolution is not the same as natural selection. Evolution means change and change can happen
through the process of natural selection, but they are not the same.
Evolution does not happen to an individual; it happens to a population.
Accommodations/modifications
(Ask ahead of time) Some students may be allergic to nuts so they could complete the task with
another sort of food like fruit snacks or candy that is individually wrapped. If any student is so
allergic to nuts that being in the same room as them agitates the allergy then do the lab another
way. For example, have students use different utensils to simulate beaks of a bird to pick up
objects.
Vocabulary:
Phenotype: Observable or measureable traits that result from a combination of genes and
environment (G x E)
Genotype: Genetic makeup of an organism
Inherited: The genotype is passed on from parent to offspring
Fitness: Survival and reproductive success
Evolution: The change in allele frequency over at least one generation
Natural selection: is the gradual process by which heritable biological traits become either more
or less common in a population as a function of the effect of inherited traits on the
differential reproductive success of organisms interacting with their environment
Names:
Date:
Don’t Shell Yourself Short
Asian Shore Crab: flickr Bas Kers (NL)
Mussels: flickr Ingo Meironke
Introduction
Asian shore crabs (Hemigrapsus sanguineus) are native to the western Pacific Ocean.
Since the late 1980s, they have been spreading throughout the northeast coast of the US. Crabs
are tiny (about the size of a penny), but populations are growing very quickly. They are
generalist predators (eat lots of different prey). Blue mussels (Mytilus edulis) in the northeast US
are becoming more resistant to predation by crabs. How is this happening?
In this session, we will play a game with nuts simulating crab-mussel interactions. We
will gather data from the game, analyze it, and draw conclusions about how an introduced
predator could cause quick evolutionary change in only a few generations of the prey species.
Definitions
Evolution:
Natural selection:
Phenotype:
Genotype:
Inherited:
Fitness:
Methods
1. Divide into groups of 4. Each person in the group gets 9 peanuts in the shell and 1almond.
Everybody dumps their nuts into a pile in the middle of the table. This pile is your group’s
population. All the mussels (nuts) are the same species, but some have a different phenotype.
2. Data collection: fill out the table below for your population.
3. Generation 1:
a. Selection: Everybody removes one nut, shells and eats it, and goes back for more nuts,
one at a time. If you can’t shell the nut, return it to the table and take another. Selection
continues for 60 seconds.
b. Count the number of each type of nut left and write to the left of the x2 in the
appropriate box in the data table below.
c. Reproduction: Next, add more nuts to double the number of each type. (Example: 5
peanuts are left and 2 almonds are left. After reproduction, you will have 10 peanuts and
4 almonds.)
d. Fill out the data table to the right of the x2 with the number of each type of nut after
reproduction.
4. Repeat Selection and Reproduction (steps a-d) for generations 2 and 3, filling out the data
table for each generation.
Results
5. Compile all the groups’ data after 3 generations
6. Line graphs: Plot the class data over time, using the graph paper provided by your teacher.
Discussion
7. Did the percent almonds (hard shells) change over time? Why?
8. Which phenotype had a higher fitness? Why?
9. What three things are necessary for evolution to occur? What part of the game each?
10. Did evolution occur? How do you know?
11. How quickly did evolution occur?
12. Did population sizes respond to predation by the crab?
13. Did any of the populations go extinct? Why?
14. The introduction of a new species (in this example, the crabs) can cause new selection
pressures. What else could cause new selection pressures? Give an example of each.
a.
b.
c.
d.
15. In our game, what would have happened if natural selection was different in some groups? If
some populations could ONLY eat almonds and not peanuts, what would have happened?
16. In real life, do you think natural selection is the same in all environments? Why?
17. Explain how natural selection can lead to biodiversity
Names:
Date:
Don’t Shell Yourself Short
Asian Shore Crab: flickr Bas Kers (NL)
Mussels: flickr Ingo Meironke
Introduction
Asian shore crabs (Hemigrapsus sanguineus) are native to the western Pacific Ocean.
Since the late 1980s, they have been spreading throughout the northeast coast of the US. Crabs
are tiny (about the size of a penny), but populations are growing very quickly. They are
generalist predators (eat lots of different prey). Blue mussels (Mytilus edulis) in the northeast US
are becoming more resistant to predation by crabs. How is this happening?
In this session, we will play a game with nuts simulating crab-mussel interactions. We
will gather data from the game, analyze it, and draw conclusions about how an introduced
predator could cause quick evolutionary change in only a few generations of the prey species.
Definitions
Evolution:
Natural selection:
Phenotype:
Genotype:
Inherited:
Fitness:
Methods
1. Divide into groups of 4. Each person in the group gets 9 peanuts in the shell and 1almond.
Everybody dumps their nuts into a pile in the middle of the table. This pile is your group’s
population. All the mussels (nuts) are the same species, but some have a different phenotype.
2. Data collection: fill out the table below for your population.
3. Generation 1:
a. Selection: Everybody removes one nut, shells and places the nut in a paper bag this
simulates eating the nut for this activity, and goes back for more nuts, one at a time. If
you can’t shell the nut, return it to the table and take another. Selection continues for 60
seconds.
b. Count the number of each type of nut left and write to the left of the x2 in the
appropriate box in the data table below.
c. Reproduction: Next, add more nuts to double the number of each type. (Example: 5
peanuts are left and 2 almonds are left. After reproduction, you will have 10 peanuts and
4 almonds.)
d. Fill out the data table to the right of the x2 with the number of each type of nut after
reproduction.
4. Repeat Selection and Reproduction (steps a-d) for generations 2 and 3, filling out the data
table for each generation.
Results
5. Compile all the groups’ data after 3 generations
6. Line graphs: Plot the class data over time, using the graph paper provided by your teacher.
Discussion
7. Did the percent almonds (hard shells) change over time? Why?
8. Which phenotype had a higher fitness? Why?
9. What three things are necessary for evolution to occur? What part of the game each?
10. Did evolution occur? How do you know?
11. How quickly did evolution occur?
12. Did population sizes respond to predation by the crab?
13. Did any of the populations go extinct? Why?
14. The introduction of a new species (in this example, the crabs) can cause new selection
pressures. What else could cause new selection pressures? Give an example of each.
a.
b.
c.
d.
15. In our game, what would have happened if natural selection was different in some groups? If
some populations could ONLY eat almonds and not peanuts, what would have happened?
16. In real life, do you think natural selection is the same in all environments? Why?
17. Explain how natural selection can lead to biodiversity