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Natural Selection

Module Overview

How do species change over time?

Can traits of populations change over time?

Rock Pocket Mice

This video introduces students to the rock pocket mouse, whose coat color tends to match its habitat. It sets up the next activity, a simulation that explores how this pattern might come to be over many generations.

  • If an individual has an advantageous trait, then it is more likely to reproduce.
  • Since traits pass from parents to offspring, then offspring are likely to inherit advantageous traits.
  • Over time, advantageous traits become more common in a population.

5 minutes

Reproductive Advantage Simulation

This guided simulation connects the concepts of mutation and variation with the idea that through reproductive advantage an advantageous trait can become the most prevalent in a population over many generations. It is intended to set up the question, HOW do species change over time?

Features of the simulation include the following:
  • A dark coat color variant occasionally appears at random in the population.
  • Some but not all of the offspring of the dark mice inherit the dark coat allele.
  • Predators are more likely to take mice whose fur color does not match their habitat.
  • The frequency of the fur color in the population changes over time because mice whose fur matches their habitat are more likely to survive and reproduce.

Project the simulation to the whole class, following the procedure and covering the talking points listed in the Teacher Guide.

  • If an individual has an advantageous trait, then it is more likely to reproduce.
  • Since traits pass from parents to offspring, then offspring are likely to inherit advantageous traits.
  • Over time, advantageous traits become more common in a population.
Developing and/or Using Models

This model simulates the reproductive rate of a variant trait in differing circumstances.

25 minutes

What is natural selection?

Recipe for Natural Selection

This video introduces natural selection and the three key ingredients through which it effects change over time: the trait must vary in the population, it must be heritable, and individuals with a certain variation must have a reproductive advantage over those that do not.

  • Natural selection can cause a trait variation to become more or less common in a population over time.
  • Natural selection requires variability, heritability, and reproductive advantage.
  • Natural selection acts on random variations.
  • Variations must pass from parent to offspring.

5 minutes

Natural Selection Checklist

This checklist will serve as a scaffold to help students organize and write an argument about natural selection in the threespine stickleback. They will use it after the next few activities and fill in what they learned:

  • Loberg Lake Stickleback Data Collection (change over time and variability)
  • Stickleback Crosses and The Candidate Gene Approach (heritability)
  • Reproductive Advantage in Sticklebacks: Plausible Arguments (reproductive advantage)

Give each student a copy of the checklist. Review the definition of Natural Selection at the top of the checklist. Instruct students to set it aside until the conclusion of the next activity. Make sure they keep it in a safe place.

  • Natural selection can cause a trait variation to become more or less common in a population over time.
  • Natural selection requires variability, heritability, and reproductive advantage.

2 minutes

Is natural selection changing the traits in a population of fish in Loberg Lake?

Meet the Threespine Stickleback

This short video introduces a contemporary example of evolution, setting up the next activity in the sequence (Loberg Lake Stickleback Data Collection).

Lake-dwelling stickleback fish have fewer lateral plates than ocean-dwelling fish. When a population of completely plated ocean-dwelling fish colonize Loberg lake, will the low-plated variant become more frequent over time?

Use immediately prior to the Loberg Lake Stickleback Data Collection simulation.

  • Individuals in a population typically vary in their traits.
  • Opportunities exist to observe natural selection happen in real time.

5 minutes

Meet the Threespine Stickleback (video)

Loberg Lake Stickleback Data Collection

Does the lateral plate trait in a population of stickleback fish change over time? Is the trait variable among individuals in the population? In this simulation, students collect a random sample of stickleback fish from a virtual Loberg Lake and arrange them by lateral plate number on a labeled bar graph. They repeat the process for three sampling periods.

In tandem, teachers use a separate website that distributes data, controls student progression through the simulation, and aggregates the data from all students to generate a class bar graph for each sampling period.

A teacher guide provides step by step instructions for using the teacher website, leading discussions, and helping students begin to fill in the Natural Selection Checklist.

  • Individuals in a population typically vary in their traits.
  • The frequencies of trait variations in a population can change over time.
Patterns

Through aggregate graphs, students can see a shift in lateral plate trait over time.

Analyzing and Interpreting Data

Students collect data, create graphs, and interpret them to decide if a trait is changing over time in a population.

30 minutes

Computer lab with internet and projection. *Teacher will need to project a separate internet site during the activity.

Note: This activity will NOT work on tablets.

Natural Selection checklists

Student Site (interactive)
Teacher Site (web page)
Teacher Guide (pdf)

Alternate paper version:
Teacher Guide (pdf)
Fish Cards (pdf)

(Optional) Use with either version:
Student Organizer (pdf)
Make one copy per student or pair

Stickleback Crosses

Is the lateral plate trait heritable? Students analyze a scatterplot showing data from different stickleback crosses. The plot shows a positive correlation between lateral plate number in parents and offspring. Give each student or pair of students one of the 16 data cards, which they will use to complete the worksheet.

Upon conclusion of this activity, revisit the Natural Selection Checklist:

  • Have students fill in item #2 (Heritability), by checking the appropriate box and creating an evidence statement that sums up the data analyzed in this activity.
  • Heritable traits pass by way of genes from parents to offspring.
  • Heritability can be determined by comparing traits in parents and offspring. If a trait is heritable, offspring tend to resemble parents.
Patterns

Students analyze a scatterplot looking for a trend in the data.

Analyzing and Interpreting Data

Students analyze a scatterplot of actual data from stickleback crosses, looking for a correlation between a trait in parents and offspring.

20 minutes

Natural Selection checklists

Teacher Guide and Key (pdf)

Data Cards (pdf)
Includes 16 cards. Print enough copies to distribute one card per student or pair.

Worksheet (pdf)
Make one copy per student

The Candidate Gene Approach

Is the lateral plate trait heritable? Working in pairs, students sort cards representing stickleback genotypes to find the gene that correlates with lateral plate phenotype.

After identifying the gene, students receive a short reading about the Eda gene.

Note: As the reading states, Eda accounts for about 75% of plate number variability. A number of minor genes contribute the other 25%. This mechanism makes lateral plate number a continuous trait.

Upon conclusion of this activity, revisit the Natural Selection Checklist:

  • Have students fill in item #2 (Heritability), by checking the appropriate box and creating an evidence statement that sums up the data analyzed in this activity.
  • Heritable traits pass by way of genes from parent to offspring.
  • One way to determine which gene controls a trait is to compare DNA sequences of alleles from individuals with different phenotypes.
Analyzing and Interpreting Data

Students compare gene sequences to find a gene correlated with a specific trait.

10 minutes

Natural Selection checklists

Student Instructions (pdf)
Make one copy per student or pair

Gene Cards (pdf)
Make one copy per student or pair. Cut into cards (cards may be re-used).

Eda Information (pdf)
Make one copy per student or pair (copies may be re-used). Distribute after they have identified the candidate gene.

Reproductive Advantage in Sticklebacks: Plausible Arguments

Is there a reproductive advantage to having a low number of lateral plates in Loberg Lake? Students gain more practice with the components of an argument. First, they match “evidence cards” to reasoning, then they identify claims about who has the reproductive advantage in Loberg Lake. Students then choose which of the arguments are plausible (2 out of the 5). A teacher guide offers more information about logistics and discussion items.

Note: As in reality, in this activity the same pieces of evidence might be used to support multiple arguments; therefore, multiple copies are provided. Also, not all copies of the evidence cards can be matched an argument; hence, some cards will be left over.

Upon conclusion of this activity, revisit the Natural Selection Checklist:

  • Have students fill in item #3 (Reproductive Advantage) by checking the appropriate box and writing an evidence statement that summarizes the plausible arguments. Note that even though it is not attached to a plausible argument, evidence card 4 can also be used to write a supporting statement.
  • Low-plated sticklebacks have a reproductive advantage in lakes; there are a number of possible explanations as to why.
  • Organisms with an advantageous heritable trait tend to increase in proportion to organisms lacking this trait.
  • (Argumentation) In a scientific argument, a claim is a statement or conclusion that answers a scientific question.
  • (Argumentation) In a scientific argument, evidence is data or observations that support a claim.
  • (Argumentation Practice) Identify the claim that is consistent with a given line of reasoning.
  • (Argumentation Practice) Match evidence to reasoning and identify plausible arguments.
Cause and Effect

Students consider how several different factors (cause) might affect stickleback survival and reproduction.

Argumentation

Students match appropriate evidence to support a claim and reasoning for several arguments.

20 minutes

Teacher Guide and Key (pdf)

Student Pages (pdf)
Make one copy per student or pair

Organizer (pdf)
(Optional) Make one copy per student or pair

Natural Selection in Loberg Lake?

Students write, peer review, then revise an argument about natural selection in Loberg Lake.

1. Have students use their Natural Selection Checklists to write an argument using this prompt:

Is natural selection causing the lateral plate number in the population of sticklebacks in Loberg Lake to change over time?

Their argument must include:

  • A claim: An answer to the prompt
  • Evidence: As listed on the checklist
  • Reasoning: Ideas about Natural Selection that are stated on the checklist (e.g., Natural Selection acts on traits that are variable, heritable, and confer a reproductive advantage, causing a trait to become more or less common over time.)
  • Give students as much or as little support in writing the components of the argument as you see fit.

    2. Have students peer review the arguments using the Argumentation Checklist. For any item checked "no," ask reviewers to provide a specific suggestion for improvement.

    3. Give students the opportunity to revise their arguments based on peer review(s).

    This assignment is a good assessment of students' ability to write an argument and their understanding of natural selection. Be on the lookout for common misconceptions:

    • Individuals adapt to their environment.
    • Natural selection is driven only by environmental change.
    • Genetic variations arise in response to selective pressure (rather than pre-existing variations being selected).
    • Changes in populations usually happen within one or a few generations.
    • Natural selection can cause a trait variation to become more or less common in a population over time.
    • Natural selection requires variability, heritability, and reproductive advantage.
    • (Argumentation Practice) Gather evidence and write an argument that includes claim, evidence, and reasoning.
    • (Argumentation Practice) Peer review and revise an argument.
    Argumentation

    Students compile data and concepts from previous activities to write an argument that includes a claim, evidence, and reasoning.

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    Evaluating Arguments Checklist (pdf)
    Make one copy per student

    Is it Natural Selection? Other Examples

    Students apply the criteria from the Natural Selection Checklist to another example. Small groups or individuals complete a short reading and a minor graphing exercise. Then they use the information to fill in the Natural Selection Checklist to determine whether or not the example is indeed the result of natural selection, then report out.

    There are seven different examples, 5 non-human and 2 human. Use the examples you and your students will be comfortable with. Some examples meet all of the criteria for natural selection and others do not. A few are not necessarily straight-forward; one of the criteria must be inferred or may be open to interpretation. These “fuzzy” examples are great discussion-generators and can help tease out misconceptions about natural selection.

    1. Divide the class in to 7 groups, giving each group a different example and a natural selection checklist.
    2. Have each group complete the assigned worksheets and fill out the natural selection checklist.
    3. Have each group report out to the class. They should name the organism and trait, summarize how each checklist criterion was met or not met, and report their conclusions.
    • Natural selection can cause a trait variation to become more or less common in a population over time.
    • Natural selection requires variability, heritability, and reproductive advantage.
    • (Argumentation Practice) Gather evidence and write an argument that includes claim, evidence, and reasoning.
    Analyzing and Interpreting Data

    Students consider and work with data about a given trait in a certain population.

    50 minutes

    Student pages (pdf)
    Includes 7 examples total. Separate and make enough copies to distribute one example per group of 2-3 students.

    Natural Selection Checklist (pdf)

    Key (pdf)

    Adaptation

    This video summarizes important concepts from heredity and natural selection. It emphasizes that mutation and allele shuffling increase variation in a population, and natural selection decreases variation through the propagation of advantageous variations—with multiple possible outcomes.

    • Adaptation is the process by which populations become well-suited to their environments through natural selection acting on many traits over many generations.
    • An adaptive trait evolves when increasingly advantageous variants repeatedly arise and become more common in subsequent generations through natural selection.
    • Environmental changes can lead to adaptation or extinction. New phenotypes may allow populations to survive in new environments.

    10 minutes

    Misconceptions About Adaptation

    This worksheet tasks students with correcting common myths about natural selection and adaptation.

    • Biological adaptation takes place in populations, not individuals.
    • Biological adaptation happens gradually over many generations.
    • Biological adaptation is an increase in the proportion of individuals with an inherited characteristic that is favorable to survival and reproduction.

    20 minutes

    Student Worksheet (pdf)
    Make one copy per student or group

    Key (pdf)

    Formative Assessment

    This quick formative assessment checks to see how well students understand graph interpretation and natural selection (the process by which DNA variations that confer a reproductive advantage become more frequent in a population over time).

    Copies or individual student computers.

    Before moving on...

    Before moving on, make sure your students understand the following:

    • Natural selection can cause a trait variation to become more or less common in a population over time.
    • Individuals in a population typically vary in their traits.
    • Natural selection acts on existing heritable variations that confer a reproductive advantage.
    • Natural selection requires three ingredients: variability, heritability, and reproductive advantage.
    Next Module: Speciation