OVERVIEW

In the lesson entitled "Comparing MtDNA Sequences to Learn about Human Variation," students compared the mitochondrial DNA (mtDNA) sequences of people from around the world to investigate our species' shared ancestry - and what it tells us about the concept of "race." What they hopefully discovered is that the so-called races of humans are not genetically distinct. In other words, race is not a useful concept for describing genetic relatedness among human populations. So when is race a useful category for describing genetic relatedness?

In this lesson, students will analyze mtDNA sequences from three subspecies of chimpanzees to gain a better understanding of what genetic "race" actually is by measuring the genetic diversity within and between subspecies. (Subspecies is the taxonomic category that corresponds to race.) Sequence comparisons will be used as evidence for or against the idea that different chimpanzee races are genetically distinct. After analyzing chimpanzee genetic diversity, students will use this information to explore race as a genetic concept.

The three subspecies analyzed are Pan troglodytes troglodytes (Central Common Chimpanzee), Pan troglodytes verus (Western Common Chimpanzee) and Pan troglodytes schweinfurthii (Eastern Common Chimpanzee).

Genetic differentiation results when different populations of a species are isolated from one another for a long period of evolutionary time. Over generations, populations naturally accumulate their own set of DNA mutations. When no intermixing occurs between two animal populations, they eventually become so genetically different from one another that they evolve into distinct subspecies or races. (Further isolation and differentiation lead to a new species altogether.)

If chimpanzee populations have evolved into distinct races, there would be more genetic diversity between two races of chimpanzees than within a single race of chimpanzees. In simple terms, one would expect to find more mtDNA sequence differences between Western Chimpanzees and Eastern Chimpanzees than between two Eastern Chimpanzees or between two Western Chimpanzees.

On the other hand, if chimpanzee populations have not been isolated long enough to evolve into distinct subspecies, there would be as much, if not more, genetic diversity within a so-called race than between two separate races. In other words, one would expect to find more mtDNA base sequence differences between two random Western Chimpanzees than between a Western Chimpanzee and an Eastern Chimpanzee.

Remember, unlike DNA that is inherited from both parents, mtDNA is passed exclusively from mother to child. Over generations, mutations occur and accumulate in mtDNA as it is passed from mother to daughter to daughter to daughter, etc. These accumulated mutations have left their mark (or genetic "footprints") as different mtDNA sequences found in chimpanzees throughout Africa. By assuming a regular rate of mutation, scientists can estimate the evolutionary time back to the common ancestor of two different mtDNA sequences.

GLOSSARY

• phylogenetic tree
• subspecies
• genetic diversity
• between- vs. within-group variation
• mitochondrial DNA
• polymorphism

OBJECTIVES

The objectives of these exercises are for students to be able to:

• Explore the genetic diversity of chimpanzees from around the world
• Learn how to compare mtDNA sequence data from different races of chimpanzees
• Understand how mtDNA sequence information can be used to trace genetic diversity
• Learn about the evolution of subspecies and understand when race is a valid biological concept
• Contribute information to a group project about global genetic variation

MATERIALS

• RACE - The Power of an Illusion, Episode 1: The Differences Between Us (video or DVD)
• World map in the classroom
• Computers (1 computer for every 2-3 students) with Internet access
• Computer Projector
• Calculators (optional)

LESSONS

ACTIVITY #1: Learning about Genetic Diversity

Screen or refer students to two video clips from Episode 1: The Difference Between Us (7 minutes):

CLIP A (4 minutes) - explores genetic diversity within other species, compared with our own. Begins with the narrator saying, "We can see differences among populations" and ends with historian Evelyn Hammond explaining that scientists' work is influenced by their social context. (7:50 - 11:56; DVD Scene #4)

CLIP B (3.5 minutes) - describes mtDNA and shows the students using computers to compare their mtDNA sequences, one to another. Begins with Alan Goodman talking about non-concordance and ends with the narrator concluding that the students didn't find racial matches in their mtDNA (32:20 - 35:56; DVD Scene #10)

To view a complete transcript of Episode 1, click here.

ACTIVITY #2: Comparing Chimp mtDNA Sequences Online

Explain to the students that they will now perform similar comparisons of mtDNA using chimpanzee sequences located on the Sequence Server Web site of Cold Spring Harbor's Dolan DNA Learning Center.

First, demonstrate use of the site to the students. At this point, it is very helpful to have the ability to project the computer screen in the classroom.

1. Navigate to the Sequence Server Web site located at http://www.bioservers.org/html/sequences/sequences.html.
2. Click the button to LOGIN AS GUEST. This will give you access to the server.
3. Click "Manage Groups" near the top of the screen. This will open a new window with folders of mtDNA sequences from high school classes around the country.
4. Click on the tool bar labeled "Sequence Sources." You should see other types of DNA sequence groups in the pull-down menu.
5. Find "Public" on the list. Highlight this group with your mouse to select it. This will pull up a list of groups.
6. Scroll down until you find the group "Race- Chimps." To select this group of mtDNA sequences, check the box to the left of the group, and click the "OK" button. This will bring sequences from this group into your workspace in the main window.
7. By clicking on the tool bar, you should see 14 mtDNA sequence files from different chimpanzees. At the top of the list is the mtDNA sequence file called "chimp 10s." (The letter "s" refers to "schweinfurthii," the Eastern Common Chimpanzee. Chimpanzee samples ending in "v" refer to the "verus" race of Chimpanzee, and sequences ending in "t" refer to the "troglodytes" race of Chimpanzee.) Notice that the computer automatically checks the box to the left of the sequence file. The box must be checked to select this sequence for comparison.
8. After you choose the DNA sequence file chimp10s, a second pull-down menu will appear underneath. Scroll down until you see a sequence file called "chimp11s." Click on it to choose it. Notice that the computer automatically checks the box to the left. This box must be checked to select this sequence for comparison.
9. Compare the two sequences by clicking the "Compare" button in the upper left corner of the page. (When you click on "COMPARE", the computer uses a program called CLUSTALW to align the samples where they have similarities.) Differences will appear highlighted in yellow.

Now that the students understand how the computer tool works, give pairs of students specific assignments. Assign pairs of students one of the following comparisons to perform:

• schweinfurthii mtDNA with schweinfurthii mtDNA
• verus mtDNA with verus mtDNA
• troglodytes mtDNA with troglodytes mtDNA
• verus mtDNA with troglodytes mtDNA
• schweinfurthii mtDNA with verus mtDNA
• troglodytes mtDNA with schweinfurthii mtDNA

Have the students perform all of the possible sequence comparisons for their assigned populations. It is critical that students only compare 2 samples at a time, as multiple sequence alignments take longer to analyze.

Students should count the number of base differences between their assigned samples and calculate the result in terms of a percentage. The students should write their results next to an assigned comparison chart on the board for the class to see. This will allow students to observe variation within and between the races.

SAMPLE CHART

Ask students to work with their partner to discuss what these results have to do with the concept of race. Did the students find more differences within, or between races of chimpanzees? Do the results from the mtDNA comparisons support the idea that different races are genetically distinct? How do their chimp results compare to patterns of human genetic diversity? Why have chimpanzees subspeciated while humans have not?

Ask the students to discuss in groups of four how they would define race as a genetic concept. The students should explore the strengths and weaknesses of their scientific approach to this question.

EXTENSION: Exploring Phylogenetic Trees

In addition to performing sequence alignments, students can compare multiple sequences at one time to explore the genetic relationship of populations. Instead of performing CLUSTALW sequence alignments, pull the tool bar down next to the word CLUSTALW. You should see an option for 'phylogenetic tree.'

1. After selecting a group of sequences to work with (see instructions above for bringing the "Race-chimps" group into your workspace), set the sequence comparison tool to the phylogenetic tree function. This analysis arranges the sequences so you can view the genetic relationships in a different way. The closer two sequences are to each other, the closer they will appear on the tree.
2. Select 3 schweinfurthii mtDNA sequences, 3 verus mtDNA sequences, and 3 troglodytes mtDNA sequences.
3. With all nine chosen, click the compare button. It will take a few seconds to perform the multiple sequence analysis. Do not use more than nine sequences.

Questions for discussion: Does the branching pattern of this phylogenetic tree follow what you would expect from the sequence alignment analysis? How many major branches are there in your tree? Are the three races of Chimpanzee genetically distinct in this tree?

Now repeat the phylogentic tree analysis with the human samples from lesson # 2. Choose all of the human samples in the "Race-lesson 2" folder (3 African mtDNA sequences, 3 Asian mtDNA sequences, and 3 European mtDNA sequences) and perform the phylogentic tree analysis. How does this tree pattern compare with that of the chimpanzees? According to the human tree, are different so-called races genetically distinct? How does this change your view of race as a biological concept?

ASSESSMENT

Student assessment can be done through a writing exercise. Using the mtDNA results provided by the exercise, have students report on how their view of the "race" concept has or has not changed. When do mtDNA comparisons support the idea that race is a genetic concept? When do they not? What are the strengths and weaknesses of using mtDNA to understand genetic relationships with regard to race? Did the genetic relations of chimpanzees surprise you? If so, how? Why did chimps evolve into subspecies while modern humans did not? What conditions lead species to sub-speciate over time? This paper can be evaluated on how well students use the mtDNA evidence to support their positions.

RESOURCES

Wisconsin Primate Research Center National Primate Research Centers Program University of Wisconsin-Madison: https://www.primate.wisc.edu/

RELEVANT STANDARDS

From Mid-Continent Research for Learning and Education at http://www.mcrel.org/

Life Sciences Standard 4 Level IV (Grades 9-12):

1. Knows ways in which genes (segments of DNA molecules) may be altered and combined to create genetic variation within a species (e.g., recombination of genetic material; mutations; errors in copying genetic material during cell division)

Life Sciences Standard 7 Level IV (Grade 9-12):

2. Knows how organisms are classified into a hierarchy of groups and subgroups based on similarities that reflect their evolutionary relationships (e.g., shared derived characteristics inherited from a common ancestor; degree of kinship estimated from the similarity of DNA sequences)

Life Sciences Standard 12 Level IV (Grades 9-12):

3. Uses technology (e.g., hand tools, measuring instruments, calculators, computers) and mathematics (e.g., measurement, formulas, charts, graphs) to perform accurate scientific investigations and communications