Download The making of the Fittest: Natural Selection and Adaptation

The Origin
of Species
The Making
of the
Fittest:
The Making of the Fittest:
TheSelection
Beak of the
Finch
Natural
and
Adaptation
Natural Selection and Adaptation
TEACHER MATERIALS
BEAKS AS TOOLS: SELECTIVE ADVANTAGE IN CHANGING ENVIRONMENTS
OVERVIEW
This activity requires students to have watched The Origin of Species: The Beak of the Finch, which focuses on Peter and Rosemary
Grant’s study of medium ground finches during two major droughts in the 1970s and 1980s. A brief summary of the study is in
the student handout.
Students will act as the finches and fight for their survival under different environmental conditions by “eating” as many
seeds as possible. Each group of students will use two different types of tools: tweezers and pliers. These tools have been
chosen to exaggerate the mechanical difference that can exist between beaks and to illustrate the advantage an individual may
derive from this difference under certain environmental circumstances. Students will test which of their two tools or “beaks” is
best adapted to collect and “eat” food under three different conditions: “Land of Plenty” (large amounts of both small and large
seeds), “Drought 1” (small amounts of large seeds), and “Drought 2” (small amounts of small seeds). A plastic box with substrate
will serve as their model environment, and two types of seeds (e.g., rice and kidney beans) will be used as food.
LEARNING OBJECTIVES
In their study of the ground finches, the Grants were able to track the evolution of beak size twice in an amazingly short period
of time. This activity demonstrates that the physical characteristics of individuals in a population can change very rapidly when
exposed to sudden and significant environmental changes. Students will be able to explain why even slight mechanical
differences due to variations in beak size can have a huge impact on a bird’s ability to obtain food and survive. While the activity
does not simulate evolution, it emphasizes the role of trait variations and selective pressure as prerequisites for the process of
evolution. It will reinforce students’ understanding of natural selection, adaptations, and fitness. After completing this activity,
students should be able to:
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Explain how changes in environmental conditions can result in strong selective pressures and cause adaptations to
evolve very quickly.
Understand that even slight differences in a physical trait can significantly impact an animal’s ability to survive.
Follow an experimental protocol in collaboration with other students.
Make predictions based on observations and collect quantitative data to test their predictions.
Organize and analyze results by interpreting graphs and performing simple calculations.
Draw conclusions about traits that offer a selective advantage under different environmental conditions.
KEY CONCEPTS
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Traits tend to vary among individuals in a population. Individuals with one form of a trait may have a selective
advantage over individuals with other forms of the trait, if the trait allows individuals to better exploit some aspects of
the environment.
Even slight variations in the physical characteristics of an animal population can make some individuals better suited
to survive under new circumstances than others.
Natural selection acts on variations of a physical trait. It is a process by which, under certain selective pressures, some
individuals are more likely to survive or reproduce than others.
Adaptations are traits that increase an individual’s ability to survive and produce more offspring in a particular
environment.
Evolution by natural selection occurs if, over time, beneficial traits (and their associated alleles) become more common
in the population while unfavorable traits slowly disappear.
Evolutionary change can occur very rapidly, in only a few generations, if there is genetic variation in a population and
if natural selection acting on this variation is strong. However, major change, such as the evolution of new species,
often takes many thousands of generations.
www.BioInteractive.org
Published April 2014
Updated June 2015
Page 1 of 6
The Origin
of Species
The Making
of the
Fittest:
The Making of the Fittest:
TheSelection
Beak of the
Finch
Natural
and
Adaptation
Natural Selection and Adaptation
TEACHER MATRIALS
TIME REQUIREMENT
This activity requires one 50-minute class period; additional time may be needed for the discussion and analysis
questions. Some questions could be assigned as homework or discussed in an additional class period. To save class time,
teachers may want to show the film in class the day before or assign it as homework (see Teaching Tips).
CURRICULUM AND TEXTBOOK CONNECTIONS
Curriculum
NGSS (2014)
Standards
HS-LS2-1, HS-LS2-2, HS-LS4-2, HS-LS4-4, HS-LS4-5
HS-LS2.A, HS-LS2.C, HS-LS4.B, HS-LS4.C
COMMON CORE
CCSS.ELA-LITERACY.RST.9-10.3, CCSS.ELA-LITERACY.RST.9-10.4,
CCSS.ELA-LITERACY.RST.9-10.5, CCSS.ELA-LITERACY.RST.11-12.3,
CCSS.ELA-LITERACY.RST.11-12.4, CCSS.MATH.CONTENT.HSSID.A.2
AP (2012-13)
1.A.1, 1.A.2, 1.A.4
IB (2009)
5.4, D2
Textbook
Chapter Sections
Miller and Levine (2010), Biology
3.1, 4.2, 5.1, 16.1, 16.3, 16.4, 17.1, 17.2
Reece et al. (2011), Campbell Biology (9th ed.)
22.2, 23.1, 23.4, 53.3, 54.4
PRIOR KNOWLEDGE
In the activity, students identify which important steps in the evolutionary process they actually demonstrate in their
experiment. It would be helpful if students had a basic understanding of the process of evolution by natural selection.
They should know that evolution by means of natural selection requires trait variations that are heritable. They should be
familiar with the concept of adaptations, which are selected for by the environment through a process called natural
selection. Students should understand that some variations of a trait increase an individual’s fitness by allowing it to
survive and reproduce, and that evolution can occur if these beneficial traits (and their associated alleles) become more
common in the population while unfavorable traits slowly disappear.
MATERIALS
Each group of students will need:
• 1 cardboard or plastic box (medium sized, for example 13 × 17 × 5 inches; lid is not required)
• Two types of tools: a pair of regular tweezers and pliers
• Astroturf attached to box bottom with glue, tape, or Velcro
• Two types of seeds (4.5 tbsp. of rice; ~1 cup of kidney beans)
• 4 plastic or paper cups of any size for seed collection (2 per tool); dixie cups are large enough
• Timer (students can use phones or watches that show seconds)
• Groups can share tablespoons and measuring cups (use one 1-tbsp. and one 0.5-tbsp., or just one 0.5-tbsp., and one
1/3-cup)
Lab Worksheet
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Page 2 of 6
The Origin
of Species
The Making
of the
Fittest:
The Making of the Fittest:
TheSelection
Beak of the
Finch
Natural
and
Adaptation
Natural Selection and Adaptation
TEACHER MATRIALS
Tools:
Students can bring their own pairs of tweezers and pliers from home.
Make sure that the tools used in the experiment are sufficiently different.
The tweezers should be small enough to get into confined spaces easily
and pick up small seeds. The pliers, on the other hand, should be large,
with a blunt tip and a gripping surface that can hold on to and crush
large seeds (see picture).
Seeds:
For each group, students will need 1 cup of large seeds, such as kidney beans, and 4.5 tbsp. of small seeds, such as rice.
Students can bring their own “seeds” from home. The teacher may choose any other type of grain, as long as the small
seeds fall to the bottom of the substrate and the large seeds rest on top of it. Use lighter-colored seeds that will be more
visible against the dark turf. Many of the small seeds can be reused. The large seeds, however, have to be crushed when
collected, so students will have to use new seeds each time.
Substrate:
The substrate is an important part of the experiment, as it will
make smaller seeds less accessible. Sufficiently thick and rigid
astroturf pieces cut from a large turf mat work well, but you can
use any substrate that fulfills a similar purpose. It is important that
large pliers cannot get to the small seeds simply by pushing the
substrate aside. Large turf mats of approximately 24 × 1 × 36
inches (as the one shown here) cost between $11 and $15 and can
be cut into six to eight pieces large enough to cover the bottom of
the box. It is best to attach the mat piece to the bottom of the box
with Velcro. Otherwise, students may accidentally lift up the entire
mat when trying to pick up seeds. Also, the mat can then be easily
pulled out to remove seeds between food conditions.
TEACHING TIPS
Students work in groups of three to five. The procedure is explained in the student handout.
It may be a good idea to assign students to watch the film as homework prior to class. This will give students more time
to review their results and answer the analysis and discussion questions in class. Ask students to focus especially on the
medium ground finch study that is introduced toward the middle of the film (at 5:55). Encourage them to take notes and
to write down questions they have. The class period could start with students summarizing the film followed by a brief
discussion of the medium ground finch study.
If you have access to a computer and projector, students can enter their group results into the accompanying Excel
spreadsheet. The spreadsheet automatically calculates descriptive statistics (mean and standard deviation) and plots the
results as graphs. Alternatively, you can write each group’s results on the board and let students copy the numbers into
the class table in the student handout. They can then work with the Excel sheet at home by uploading it into Google
Drive, a cloud-based file storage and sharing platform that many students are already familiar with, and sharing it. For
Lab Worksheet
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Page 3 of 6
The Origin
of Species
The Making
of the
Fittest:
The Making of the Fittest:
TheSelection
Beak of the
Finch
Natural
and
Adaptation
Natural Selection and Adaptation
TEACHER MATRIALS
more detailed instructions, go to https://support.google.com/drive/answer/2494822?hl=en or share this link with your
students.
Ask students to discuss the class results and subsequent questions with their group members before writing down any
answers.
Be sure to reinforce the concept that populations evolve over time; individuals do not evolve during their lifetimes.
Address the common misconception that new traits arise “as needed.” Smaller or larger variations of the medium ground
finch’s beak did not arise in response to the two droughts; variations already existed prior to the droughts. When the
food supply changed after the two major droughts, birds with one of these variations (i.e., either a larger or smaller beak)
had an edge over the birds with the other variation of the trait. It is also important that students do not come away with
the idea that the experiment demonstrates evolution. The differential survival rates alone are not sufficient for evolution
to occur. The beneficial trait has to be heritable and then passed on to future generations. Some of the discussion
questions at the end of the student handout should emphasize this point.
ANSWER KEY
Foraging Practice and Preparation of Box Environment (steps 1-4)
Steps 1 and 2: Students set up their box environment and practice picking up and crushing seeds with their tools.
Step 3: Students write down observations of the tools (or “beaks”), the box environment, and the seeds. They should
notice that the pliers are larger, heavier, and much blunter overall than the tweezers. Compared to the tweezers, pliers
also seem to have a sturdier grasp and can hold or even crush larger objects. Tweezers on the other hand seem better
suited for jobs that require greater dexterity. The environment is rather simple, but students should notice the astroturf’s
rough surface and the small spaces into which small food items could fall. Students should note the difference in seed
size).
Step 4: Students make predictions about each beak’s ability to pick up enough food under three different food
conditions. When plenty of both seeds are available, either beak should pick up at least one of the seed types. Large
beaks (pliers) will likely pick up mostly large seeds, while small beaks (tweezers) will probably be best at collecting the
small seeds. When only small seeds are available, the large beak should have difficulty picking up seeds without also
partially grabbing the turf. The small beak, on the other hand, ought to be well-suited to get in between the spaces.
Large seeds have to be crushed, so when only large seeds are available, the small beak won’t be able to effectively crush
the large seeds and collect enough food.
Foraging Experiment (steps 5–12)
The experimental procedure and data collection is described in detail in the student handout. Individual groups collect
their results in the group table on page 4 of the student handout.
After the Experiment (steps 13–16)
Steps 13-14: Collect group totals for the class on the board or in the optional Excel spreadsheet. In either case, students
should copy the class results into the class table on page 5 of the student handout. The spreadsheet can be used if the
teacher wishes to emphasize the quantitative aspect of the activity or to familiarize students with Excel. The formulas for
calculating means and standard deviations as well as the provided plots can be deleted if the teacher wants students to
learn to conduct basic quantitative analyses in Excel. Students are asked to calculate and later compare the total number
of seeds collected across all four trials (instead of an average number per trial). The rationale is that it is less important for
a bird’s survival how many seeds it gets in one feeding than the total amount it manages to eat during a whole day or
week. So the four trials could represent either the times a finch feeds during one day or a day’s worth of food across four
days.
Lab Worksheet
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Page 4 of 6
The Origin
of Species
The Making
of the
Fittest:
The Making of the Fittest:
TheSelection
Beak of the
Finch
Natural
and
Adaptation
Natural Selection and Adaptation
TEACHER MATRIALS
Step 15: The 80% food limit needed for survival that students calculate is based on the total number of seeds that groups
collected for each finch. Students use the average of all the group totals (calculated in bottom row of their class table) to
calculate the least amount of food necessary to survive.
Students may ask why finches don’t need many more small seeds than large seeds to meet their minimum food
requirements. The size of a food item is not necessarily indicative of its nutrient value. Large seeds, like the spiny seeds
mentioned in the short film, may have tough hulls or shells. The seeds in the center can be quite small, and it takes
energy to crack open the shell. So the net caloric intake per large seed may end up being small. Small seeds produced by
grasses, on the other hand, consist mostly of the actual seed core, while the surrounding hulls are soft and easily
removed or even edible. Therefore, the net caloric intake per large seed can equal the net caloric intake per small seed.
Questions
1)
Carefully review the class results.
All answers are data-dependent.
2)
Was there any difference in the two beaks’ ability to pick up small seeds? If so, what features made one beak more
successful than the other?
The students should have had the greatest trouble with pliers and noticed that its blunt “beak” was not wellsuited to get into the small spaces of the astroturf where all the small seeds were hiding. On the other hand,
the more slender, smaller shape of the tweezers made the “beak” better suited for picking out small seeds
from the astroturf.
3)
Was there any difference in the ability of the two beaks to pick up and crush large seeds? If so, what features made
one beak more successful than the other?
The large beak (or pliers) will likely be the only one capable of crushing the seeds. The small beak may be able to
pick up large seeds but cannot crush them.
4)
Did your results support your predictions? Explain your answer.
Answers will depend on the students’ predictions and results. The students should explain why the results did
or did not support their predictions by briefly comparing each prediction with the relevant results they
obtained.
5)
Consider the physical characteristics of the substrate (the astroturf) in your model environment and the effort it took
to collect large and small seeds under any of the food conditions.
a)
What characteristics made seeds less accessible, and which seeds were most affected?
The density and thickness of the turf makes the spaces between the fake grass strands small and
inaccessible. Seeds that are small enough to fall through are much less accessible than seeds that are
large enough to rest on top. Students should point out that the physical characteristics of the
environment mostly affected the accessibility of the small seeds.
b) Would you say that the astroturf made it hard for some finches to find enough food to survive? If so, which finch
had the greatest trouble? You may go back to Question 1e) and 2 to help with your answer.
Drought 2: Finches with large beaks likely did not collect 80% of the total number of seeds they needed,
since large beaks have trouble collecting small seeds that hide within the astroturf. Even if they managed
to get 80% or more, the amount should still be less than what these birds collected in the Land of Plenty
condition.
Lab Worksheet
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Page 5 of 6
The Origin
of Species
The Making
of the
Fittest:
The Making of the Fittest:
TheSelection
Beak of the
Finch
Natural
and
Adaptation
Natural Selection and Adaptation
6)
TEACHER MATRIALS
Based on your results, what (if anything) can you conclude about the ability of each bird to collect food and survive
in your model environment under the three different conditions? Propose an answer for each condition separately
and incorporate the effect of your substrate on food availability.
In this question students are supposed to coherently summarize their step-by-step answers from above. They
should try to independently articulate the big picture and the overall conclusions one can draw from the
results.
7)
If you saw a difference between a bird’s ability to obtain food based on beak shape, explain how this leads to
evolution over time. (Remember that the different beak types you tested in your experiment represent birds of the
same medium ground finch species. They merely show variations in beak size.)
Students should indicate that individual variation in important traits like beak size or depth make it more
likely for one form of the trait to be beneficial under new environmental conditions that affect food supply.
The variation has to be heritable, and individuals with the beneficial, heritable trait have to be more likely to
survive under these new conditions than individuals without the trait. Evolution can occur over time if those
that survive also reproduce and pass on the beneficial trait to future generations.
8)
This activity simulates a variety of concepts that play an important role in the process of evolution. Name a specific
example from this activity simulating each concept below:
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Variation:
Differences in the size of the medium ground finch beaks.
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Adaptation (i.e., a beneficial trait that is heritable and increases an individual’s ability to survive and reproduce):
Differences in beak size are heritable morphological trait variations. Larger beaks are adaptive when only
large seeds are available; small beaks are adaptive when only small seeds are available. These features
are adaptations when they become more common in the population as a result of natural selection.
•
Selective pressure or selecting agent (i.e., what selected for one beak type versus the other?):
Food shortages and changing food supplies exert a selective pressure on finch populations. Students
may answer that the droughts were the selective pressure. The amount of rainfall itself, however, had
only indirect effects. The finches’ survival depended on the food supplies that resulted from changes in
weather patterns.
•
Fitness (i.e., increased likelihood to survive and reproduce):
Fitness refers to an individual’s survival and lifetime reproductive success. Students should point out that
individuals with different beak sizes are more or less likely to survive and eventually reproduce under
drought conditions. Finches with the beneficial trait variation therefore had a fitness advantage over the
finches with the other, less favorable trait variation.
AUTHOR
Written by Jason Crean, MA, MS, Lyons Township High School, La Grange, Illinois
Edited by Sandra Blumenrath, PhD and Laura Bonetta, PhD, HHMI, and Ann Brokaw, Rocky River High School, Ohio; copyedited by Linda Felaco
Scientific review by Jonathan Losos, PhD
Lab Worksheet
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