Teacher Notes forEvolution of Fur Color in Mice – Mutation, Environment and Natural Selection1

Students view a brief video that presents research findings concerning the roles of mutation and natural selection in the evolution of fur color in rock pocket mice. Multiple-choice questions in the video and analysis and discussion questions in the Student Handout guide students to a better understanding of mutation, environment, and natural selection. Students learn how the same trait can evolve independently in different populations and how analysis at multiple levels from the molecular to the ecological contributes to a better understanding of evolution by natural selection.

Learning GoalsIn accord with the Next Generation Science Standards2: Students will gain understanding of two Disciplinary Core Ideas:

o LS4.B Natural Selection. "Natural selection occurs only if there is both (1) variation in the genetic information between organisms in the population and (2) variation in the expression of that genetic information – that is, trait variation – that leads to differences in performance among individuals. The traits that positively affect survival are more likely to be reproduced, and thus are more common in the population."

o LS4.C Adaptation. "Natural selection leads to adaptation, that is, to a population dominated by organisms that are anatomically, behaviorally, and physiologically well suited to survive and reproduce in a specific environment. That is, the differential survival and reproduction of organisms in a population that have an advantageous heritable trait leads to an increase in the proportion of individuals in future generations that have the trait and to a decrease in the proportion of individuals that do not. Adaptation also means that the distribution of traits in a population can change when conditions change."

Students will engage in the Scientific Practice: Constructing Explanations. “Apply scientific ideas, principles, and/or evidence to provide

an explanation of phenomena… Apply scientific reasoning, theory, and/or models to link evidence to the claims to assess the extent to which the reasoning and data support the explanation or conclusion.”

This activity provides the opportunity to discuss the Crosscutting Concepts:o "Cause and effect: Mechanism and explanation": Students “suggest cause and effect

relationships to explain and predict behaviors in complex natural and designed systems. They also propose causal relationships by examining what is known about smaller scale mechanisms within the system.”

o "Stability and change": “Students understand much of science deals with constructing explanations of how things change and how they remain stable.”

This activity helps to prepare students for the Performance Expectations:o HS-LS4-2, "Construct an explanation based on evidence that the process of evolution

primarily results from four factors: (1) the potential for a species to increase in number, (2) the heritable genetic variation of individuals in a species due to mutation and sexual

1 By Ingrid Waldron, Department of Biology, University of Pennsylvania, © 2017, with helpful input from Ray Howanski, Ridley School District. Teachers are encouraged to copy this Student Handout for classroom use. This Student Handout and Teacher Notes with instructional suggestions and background information are available at http://serendipstudio.org/exchange/bioactivities/NaturalSelectionMice.

2 Next Generation Science Standards (http://www.nextgenscience.org/sites/default/files/HS%20LS%20topics%20combined%206.13.13.pdf)

1

reproduction, (3) competition for limited resources, and (4) the proliferation of those organisms that are better able to survive and reproduce in the environment."

o HS-LS4-4, "Construct an explanation based on evidence for how natural selection leads to adaptation of populations."3

Suggestions for Implementation and Background BiologyAs background for this activity, I recommend that you have your students complete "Evolution by Natural Selection" (http://serendipstudio.org/sci_edu/waldron/#evolution), so they will understand the following: An adaptation is any characteristic or trait that increases fitness; fitness is the ability to

survive and reproduce. A characteristic which is influenced by genes and can be inherited by a parent’s offspring is

called a heritable trait. An adaptive heritable trait tends to become more common in a population. Because the trait

is adaptive, individuals with this trait generally produce more offspring. Because the trait is heritable, offspring generally have the same trait as their parents. Therefore, the adaptive heritable trait tends to become more common in the population. This process is called natural selection.

The PDF of the Student Handout shows the correct format. The Word document is provided in case you want to revise the Student Handout for your students.

If you plan to photocopy the Student Handout for your students, please make a test copy first to make sure the black mice in the figures for questions 4 and 6 don’t totally disappear in the photocopies. You may want to use a somewhat lighter setting for photocopying or insert in the Student Handout the modified version of the pages with these figures provided on pages 7-9 of these Teacher Notes.

A key is available upon request to Ingrid Waldron (iwaldron@sas.upenn.edu). The following paragraphs provide additional instructional suggestions and background information – some for inclusion in your class discussions and some to provide you with relevant background that may be useful for your understanding and/or for responding to student questions.

The introduction through question 1 in the Student Handout is designed to get students thinking about the relevant concepts before they view the video. In discussing question 1, you should be aware that, although most of the rock pocket mice on dark rocks have dark fur, some of them have light fur, partly because mice move between the light-colored and dark rocks and partly because populations of mice with light fur can survive on dark rocks in locations where they are not competing with mice with dark fur (e.g. because a mutation for dark fur has not occurred yet). Similarly, on light-colored rocks located near dark rocks, some of the mice will have dark fur because of movement of mice between the two types of rocks. If your students don’t bring up these complications, you will want to save discussion of these points until your discussion of pages 2 and 3 of the Student Handout.

3 Although this activity is designed for high school students, it can also be used to help prepare middle school students for the performance expectation MS-LS4-4, "Construct an explanation based on evidence that describes how genetic variations of traits in a population increase some individuals' probability of surviving and reproducing in a specific environment."

2

After question 1, students view Pocket Mouse Film with Quiz (available at http://www.hhmi.org/biointeractive/pocket-mouse-film-quiz).4 This video is a little over 10 minutes long, and you will need additional time for students to answer and discuss the six embedded multiple-choice questions. You may want to assign this video as homework; when you click on an answer for a multiple-choice question in the video, you are given an explanation of why the answer is correct or incorrect.

If you show the video in class, I recommend that you have your students vote on the answer to each question at the appropriate place in the video. One effective way to encourage each student to choose his/her answer independently is to have each student hold up a piece of paper with their answer choice for each question. The last page of these Teacher Notes provides a template which you can reproduce for your students to tear into four pieces with a large dark A, B, C or D and two pieces with True or False. (Both of the last two multiple-choice questions have more than one correct answer, so I suggest that you have your students vote true or false on each possible answer for those two questions.)

For each question, after the students have voted, if most of the students voted for the correct answer, you can have one of them explain their reasoning and have a brief discussion. If many students voted for incorrect answers to a particular question, have students discuss the question with each other. They should analyze why each of the answer options is correct or incorrect and try to figure out the right answer together. Then, have the class re-vote and discuss their reasoning.

You may want to emphasize the point mentioned at ~3 minutes in the video that “the mice show no preference for dark or light rocks”, so the differential distribution of fur color on different color rocks is not due to the behavior of the mice, but rather results from natural selection due to the effects of differential predation. “Field studies show that mice move freely between light and dark rocks”.5 Analysis of mitochondrial genes also indicates substantial migration between regions with light rocks and regions with dark rocks.6 The movement of rock pocket mice between habitats is the major reason why not all of the mice have fur color that matches the color of the rocks where they are found.

At ~5 minutes the narrator refers to changes in four chemical letters. It may be helpful to know that the four nucleotides that are changed in the mutated Mc1r gene are separated by many nucleotides and result in changes in four different amino acids at crucial locations in the melanocortin-1-receptor protein. This melanic allele is partially dominant.

4 Although the video shows daytime scenes, rock pocket mice are nocturnal; even at night, visual camouflage is important for protection against owls, the primary predator of rock pocket mice. Rock pocket mice live on rocks and do not live on the nearby sandy areas, although they do sometimes migrate across them.

5 Personal communication, Michael Nachman, November, 2016

6 Evolution (2004) 58:1329-1341

3

The first batch of three multiple-choice questions in the video (just after 5 minutes) is as follows:

Why did dark-colored rock pocket mice first appear in a population of light-colored rock pocket mice?

A. They have a genetic mutation that affects their fur color.B. There is dark lava rock in the area where they live.C. Individuals change color to blend in with the environment.D. Predators eat light-colored rock pocket mice.

This question provides the opportunity to reinforce student understanding of mutation as a major source of the heritable variation that must be present for natural selection to occur. Although some animals (e.g. octopus) can change color to blend in with their environment,7 this is not the case for the rock pocket mice.

Why do dark-colored rock pocket mice on dark lava flows have white bellies?A. There is no selection for dark bellies by visual predators.B. Mutations causing dark bellies do not occur.C. There is a reproductive advantage to having a dark belly.D. White bellies are an important part of camouflage.

Mutations are alwaysA. good.B. bad.C. neutral.D. a change in an organism’s DNA.

The fourth multiple-choice question (shortly after ~7 minutes in the video) is:

When dark-colored fur gives mice a 1% competitive advantage and 1% of the population begins with dark fur, in about 1000 years, 95% of the population will have dark fur. Which of the following statements is true?

A. Dark-colored rock pocket mice, in this population, have fewer offspring than light-colored rock pocket mice.

B. If dark-colored rock pocket mice had a competitive advantage of 0.1% it would take more than 1000 years for 95% of the population to have dark fur.

C. If dark-colored mice had a competitive advantage of 5%, it would take more than 1000 years for 95% of the population to have black fur.

D. If dark-colored mice had a competitive advantage of 10%, it would take more than 1000 years for 95% of the population to have black fur.

This question can be quite challenging so, before you have your students vote, you may want to provide an explanation something like:

Competitive advantage measures how big the difference in survival and reproduction is, comparing mice with dark fur vs. mice with light fur.

Answer B means that if the competitive advantage for dark fur were smaller, then it would take longer before most of the mice in the population would have dark fur.

Answers C and D mean that if the competitive advantage for dark fur were larger, then it would take longer before most of the mice in the population would have dark fur.

You might want to ask your students why the mice with dark fur would have a competitive advantage, and you might want to show your students the “Pocket Mouse and Predation” animation available at http://www.hhmi.org/biointeractive/pocket-mouse-and-predation. This

7 See "Evolution and Adaptations" (http://serendipstudio.org/exchange/bioactivities/evoadapt).

4

animation illustrates how, on dark rocks, light mice are more subject to predation than dark mice. This gives the dark mice a competitive advantage, and the size of this competitive advantage determines how fast natural selection increases the proportion of mice with dark fur (for a population of mice on dark rock). The figure below illustrates how the size of a competitive advantage is associated with the rate of change in the percent of mice that have a new allele (e.g. the allele for dark fur).

(http://content.csbs.utah.edu/~rogers/ant5221/lecture/selection.pdf)

You may also want to mention that evolution by natural selection can occur relatively rapidly in rock pocket mice populations because of their high reproductive rates and short lifecycles.

The last two multiple-choice questions (~9 minutes and 45 seconds in the video) have more than one correct answer, so your students should vote true or false on each of the suggested answers.

What does Dr. Carroll mean when he says “while mutation is random, natural selection is not”? 8

A. Mutations are caused by changes in the environment.B. Natural selection can favor some mutations and not others.C. Selection can change depending on the environment.D. Mutations for advantageous traits are more likely to be passed on to the next generation.

Nachman examined dark-colored mice from two different populations living hundreds of miles apart. The mice looked nearly identical. Their dark color was caused by two different genes. What does this tell you?

A. Dark-colored fur evolved only once in rock pocket mice.B. There are at least two genes involved in creating dark-colored mouse fur.C. Dark fur color evolved independently on each of these lava flows.D. Different mutations in two different genes cannot generate the same phenotype.E. Under similar conditions, natural selection can favor similar adaptations.

8 Mutation is a random process in the sense that, as far as scientists have been able to determine, there is no tendency for genes to mutate to a useful allele in a particular environment. This is probably all that your students need to know, but you may want to know that mutations are not entirely random; for example, rates of mutation differ in different parts of the genome in ways that appear to be adaptive.

5

Competitive Advantage

~10-25%

~2-4%

~1-2%

~0.5-1.0%

~0.2-0.4%

After the video and embedded multiple-choice questions, have your students complete questions 2-7 and the Challenge Question in the Student Handout. Obviously, the figures on pages 2-3 of the Student Handout show only a small subset of the thousands of mice in a population of rock pocket mice.9 (See page 2 of these Teacher Notes for suggestions to successfully photocopy the figures on these pages.)

In the figures on page 2 of the Student Handout, the population of mice with light fur on the light-colored rocks is assumed to be at carrying capacity; since the light-colored rocks provide a stable environment the number of mice on the light-colored rocks also remains stable. The top three figures on page 2 illustrate the potential for population growth when a new environment opens up. For the bottom four figures on page 2, you may find it helpful to have your students count the number of mice with dark fur in each figure and use that information to determine the correct time sequence for the bottom three figures. You could have your students make a histogram to show the change over time in the percent or numbers of mice with dark fur.

After your class discussion of question 4, I recommend that you discuss the Crosscutting Concept of Stability and change. You may want to begin by asking students to compare and contrast the change or lack of change for the populations of rock pocket mice on the light-colored rock vs. the dark rock that results when a lava flow cools. Students should notice that, on the stable light-colored rock, the population of mice remains stable in size and fur color. In contrast, the dark rock changed as cooling lava became a newly available habitat; correspondingly, there was considerable change in the population size and fur color of the mice on this new habitat.

Question 5 introduces the possibility of a mutation that results in white fur; this reinforces student understanding of the random nature of mutation and the importance of non-random natural selection. (You may also want to have your students consider what would happen if a mutation for dark fur occurred in a mouse on the light-colored rock; this mouse may well be killed by a predator or it might possibly wander onto the dark rock where it might survive.) Student understanding of the roles of random mutation and non-random natural selection will be further enhanced by the analysis in question 6.

Question 7 refers to a finding that, on the dark rock of one lava flow in northern Arizona, the mice have light-colored fur. To explain this finding, your students need to know that mutations are rare events. The researchers suggest that the light-colored fur of the mice on this lava flow “likely reflects the fact that [this] lava flow is an extension of the relatively young Sunset Crater (<800 years old); consequently there may have been insufficient time for melanic mice to evolve in this population. In addition, there are no populations of melanic mice nearby from which melanic migrants may invade.”10 This illustrates that natural selection can only operate when one or more alleles for an adaptive trait are present in the population. This is one reason why evolution is a slow process, and the slow pace of evolution is one reason why not all characteristics observed in living organisms are adaptive. This example provides the opportunity to discuss the Crosscutting Concept, Cause and effect: Mechanism and prediction. Students

9 In the figures, the light-colored mice may appear darker on the light background and lighter on the dark background, but they are the exact same shade of gray. This illustrates a common visual illusion where objects appear darker when contrasted with a lighter background.

10 Heredity (2006) 94:217-228. These researchers estimate that “the overall mutation rate to dark color is about 10-6 per genome per generation”.

6

should notice how understanding the mechanism of evolution by natural selection helps us to understand a seeming exception to the predictions of the theory of evolution.

As described on page 4 of the Student Handout, I recommend that students first answer the Challenge Question based on their memory, then view the video again, and then revise their answers to be more complete and accurate. For this viewing, I recommend the video without questions, available at http://www.hhmi.org/biointeractive/making-fittest-natural-selection-and-adaptation. The Challenge Question:

illustrates how analyzing a phenomenon at different levels (molecular, organism, population, and ecosystem) contributes to better understanding of the phenomenon

engages students in evaluating the evidence that supports scientific conclusions reinforces student learning about mutation and natural selection.

With regard to Challenge Question B, the following bullet points summarize the indirect evidence for greater vulnerability to predation for individual mice whose fur color does not match the color of the rocks where they live. (Source = Evolution 58:1329, 2004.)

Experimental evidence for a different type of mouse living on a dark substrate showed that dark coloration decreased the risk of predation by owls.

Genetic evidence indicates that mice move freely between light and dark habitats. Therefore, the correlation of fur color alleles with habitat color is most likely due to differential mortality.

Additional Recommended Activities are described in “Resources for Teaching and Learning about Evolution” (http://serendipstudio.org/exchange/bioactivities/evolrec).

Source for the first figure in Student Handout:http://images.slideplayer.com/14/4440398/slides/slide_19.jpg

Possible Substitute Version of Pages 2-3 of the Student HandoutPages 8-9 of these Teacher Notes provide a version of pages 2-3 of the Student Handout with modified figures. Although the camouflage for the dark mice isn’t as good as the Student Handout figures when viewed online or when printed initially, these figures may be better for photocopying to ensure that the dark mice don’t disappear into the background.

7

This figure shows the rock pocket mice that lived on one little part of a much larger area with light-colored rocks next to a new lava flow that was too hot for mice to live on.

This figure shows the mice in the same area a year after the dark volcanic rock had cooled enough for mice to live there.

3. What changes occurred between the first and second figures? What caused these changes? Include “migrated” and “total number of mice” in your answer.

In the following years the numbers of mice on the dark rock increased. Even though predators are more likely to catch mice with light fur on the dark rock, the mice had so many babies that there was a large population.

4a. Figures A-C show what the population looked like at different times over the next ten centuries. Indicate the actual time sequence for these figures.

____ would be observed first;____ would be observed next; and____ would be observed last.

4b. Explain what is happening. Include “mutation”, “natural selection” and “predator” in your answer.

8

A

B

C

5. Suppose that, properties instead of a mouse with dark fur, a mouse with white fur was born in the population of mice on the dark rock. What do you think would happen during the years after the white mouse appeared? Would you expect that white mice would become increasinglycommon in the population of mice on the dark rock? Explain why or why not.

6. There are multiple different lava flows of dark rock, due to the eruptions of multiple different volcanos. Researchers have found that:

Most of the rock pocket mice that live on these different lava flows have a similar phenotype (dark fur).

However, different mutations are responsible for the dark fur in two populations of mice that live on two different lava flows.

Explain why the mice on these two different lava flows have similar phenotypes but different genotypes.

lava flow 1 light-colored rocks

light-colored rocks lava flow 2 lava flow 1 light-colored rocks

light-colored rocks lava flow 2

7. Researchers have found an exception to the observation that populations of rock pocket mice that live on dark rocks generally have dark fur. On one lava flow of dark rock, all the mice have light fur. This lava flow of dark rock is relatively young (<800 years old) and is far from any population of rock pocket mice that have dark fur. What is a likely reason why dark fur has not evolved in the population of mice that live on this lava flow?

9

A B True

C D FalsePieces for Voting on the Multiple-Choice Questions in the Video

10