This document provides instruction on natural selection and different modes of selection. It begins with an overview of natural selection, noting it requires variation in a population, struggle for survival/competition, and a changing environment. It then defines and provides examples of three modes of selection: stabilizing selection, which favors the average phenotype; directional selection, which favors one end of the phenotypic range; and disruptive selection, which favors phenotypes at both ends of the range. Students are asked questions to analyze examples and identify the mode of selection occurring. The objective is for students to differentiate selection modes and use shifts in phenotypic distributions as evidence of evolution by natural selection.
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Modes of selection lesson
1. Warm up question
What are the 5 forces that drive the evolution
of populations?
What is the only way that is adaptive?
prior knowledge
3. Objective
Students will differentiate between various
modes of selection, analyze shifts in
numerical distribution of traits, and use
these shifts as evidence that evolution by
natural selection is occurring in a
population, citing specific evidence from
the data to justify their conclusions
4. With your partner – analyze this graph
investigation
At least 3
observations
http://www.radiolab.org
/story/288733-23-
weeks-6-days/
5. With your partner – analyze this case study
https://www.youtube.com/watch?v=LyRA807djLc
How did the phenotypes of the
moths change?
What factors drove those
changes?
How is this case
similar/different from the birth
weight example
investigation
9. Explain how this picture shows the evolution
of a population by natural selection
Include the terms:
• Variation
• Struggle
• Changing environment
• How is new population
better adapted?
prior knowledge
10. Modes of Natural Selection
Natural selection can occur 3 main ways –
depending on what phenotype in a population is
favored
concept
12. Stabilizing Selection example
Robins tend to lay 4 eggs each season. When they lay more eggs – there
may be too many offspring for the parents to provide food for all of them
When they lay fewer eggs, there may not be any healthy offspring that
hatch
concept
14. Directional Selection example
During dry years, there are mostly only larger, tougher seeds
available as food on the Galapagos islands. Birds with the
largest beaks are favored during these times.
concept
16. What type of natural selection is shown in
the graph? How do you know?
concept check – level 1
This example shows
________ selection
because ______________
17. What type of natural selection is shown in
the graph? How do you know?
concept check – level 1
This example shows
________ selection
because ____________
18. What type of natural selection is shown in
the graph? How do you know?
This example shows
________ selection
because
______________
concept check – level 1
19. What type of selection is at work? How do
you know? Cite specific evidence
skill – level 1
20. What type of selection is at work? How do you know?
What would a graph of this population look like?
skill – level 1
21. In wild populations of lilies,
short stemmed plants do not
receive enough sunlight to
grow, while long stemmed
flowers often suffer wind
damage
skill – level 1
What type of selection is at work? How do you know?
What would a graph of this population look like?
22. What type of selection is at work? How do you know?
What would a graph of the original and new populations
look like?
In a population of petunias, flowers with
the brightest color attract the most
pollinators whereas plants of moderate
color or dull color are less likely to be
visited by a pollinator and thus less likely to
reproduce. Data of a population of petunias
first grown in a greenhouse and then
planted outdoors is given below
Year – 2004
Grown in
greenhouse
Year – 2014
Growing in the
wild
Brightly colored 40 80
Moderate color 85 15
Dull color 25 5
skill – level 2
23. What type of selection is at work? How do you know?
What would a graph of the original and new populations
look like?
A desert population of spiny cacti are being
eaten by an invasive species – the peccary.
Peccaries favor cacti with few spines.
skill – level 2
Number of cacti
before introduction
of peccaries
Number of cacti
after introduction
of peccaries
Many spines 407 524
Moderate spines 388 435
Few spines 324 36
25. What type of selection is at work? How do you know?
What would a graph of the original and new populations
look like?
Just as the peccary and cacti populations
are starting to adjust to each other, another
invasive species arrives – a parasitic wasp.
The wasp lays her eggs on the cacti, and
the grubs eat the soft inner flesh of the
leaves, killing the plant. Wasps prefect
plants with many spines to protect their
grubs.
How would this new species affect the
distribution of phenotypes in the cacti
population?
skill – level 2
26. Graph this data and determine the mode of
selection at work on the desert cacti population
29. Objective
Students will differentiate between various
modes of selection, analyze shifts in
numerical distribution of traits, and use
these shifts as evidence that evolution by
natural selection is occurring in a
population, citing specific evidence from
the data to justify their conclusions
closure
30. Independent Practice
Types of Sel ect ion
A group of snakes were all placed into environments that had differing selective pressures. Read the
data tables below and graph each snake in its environment. From the graph, determine what type of
selection is being portrayed: directional, stabilizing or disruptive.
1. Snakes placed on an island where there are two predators that eat snakes.
Number of Snakes at Each Length
2 cm 4cm 6 cm 8 cm 10 cm
Year 1980 2 14 18 12 4
1984 4 12 8 9 8
1988 10 6 3 6 14
Graph
Type of Selection:
Summarize what
happened to the snakes.
31. What type of selection is at work? How do
you know?
32. What type of selection is at work? How do
you know?
33. Draw a new curve on the graph below to show how the distribution
of beak sizes might change as a result of selection in this new
environment.
A population of birds, with various size beaks, eats seeds. Small
seeds can be eaten by birds with small beaks. Larger, thicker
seeds can only be eaten by birds with larger, thicker beaks.
Suppose there is a shortage of small seeds but that there are
still many large seeds.
Editor's Notes
Prior Knowledge
CFU: the 5 fingers of evolution – could be whiteboards, TPS with random selection, PearDeck, etc
After students describe graph – this 5 minute video from RadioLab is fun
What is the most common birth weight in pounds?
Why is infant mortality higher for very large or very small babies?
How might technology and medical advancements influence this graph going into the future?
This is called STABILIZING SELECTION – it is just one way that natural selection influences the distribution of phenotypes in a population.
Today we are going to learn about the 3 major modes of selection and how data can help us determine what mode of selection is at work.
2 minute video showing peppered moths
Prior Knowledge:
In order for natural selection to occur, there must be differences between individuals in a population – some will have traits that help them survive and reproduce better than others (remind students that DNA (genes) give us traits and variation)
variation from random mutations, also sexual reproduction, crossing over, etc
Prior Knowledge:
There must be competition and other limiting factors that lead to a struggle for existence (survival of the fittest)
Prior Knowledge:
There must be environmental resistance (pressure) natural or man-made. When environment is changing especially quickly, some populations adapt quickly, while others may die out.
Can give examples like ice ages and interglacials, climate change, deforestation, etc.
CFU on prior knowledge: longer answers required here so perhaps think-write-pair-share?
New Concept
These are pocket mice found in the deserts of New Mexico
Sand colored mice hide from predators best when on the sand
Black mice hide from predators best on the exposed lava rock
Have students draw a basic copy of this graph.
Pushes population in a certain direction with regard to phenotype
Have students make a basic copy of this graph
Here, it pays to be lightest and blend in with the sandy substrate OR darkest and blend in with the black lava rock. Intermediate rock pocket mice have decreased viability on both substrates
https://www.hhmi.org/biointeractive/making-fittest-natural-selection-and-adaptation I just show the first 2 minutes or so of this video
Could add a sentence frame here if needed: “This graph shows _______ selection because….” Could do this as a TWPS or TPS. I might Pear Deck or whiteboard this because it is super basic everyone should be able to do it stuff
Could add a sentence frame here if needed: “This graph shows _______ selection because….” Could do this as a TWPS or TPS. I might Pear Deck or whiteboard this because it is super basic everyone should be able to do it stuff
Could add a sentence frame here if needed: “This graph shows _______ selection because….” Could do this as a TWPS or TPS. I might Pear Deck or whiteboard this because it is super basic everyone should be able to do it stuff
Ask students to cite specific evidence from the “text” to support their conclusions. Start video at 6 minutes and end at 9:15, or play all 15 minutes if you like
https://www.hhmi.org/biointeractive/origin-species-beak-finch
Hand pollination could occur in the greenhouse population – thus eliminating the need for wild pollinators. Ask students to cite specific data/evidence from the prompt in their answer. You might stress to students that in both cases (pollinators and finches), it was a fairly rapid change in the environment that led to directional selection. This is when we see directional selection most often.