Evolutionary Medicine

CONTENTS OF CURRICULUM UNIT 10.06.08

  1. Unit Guide
  1. Overview and Rationale
  2. Background Information
  3. Strategies and Lesson Plans
  4. Activities
  5. Notes
  6. Bibliography
  7. Websites for Teachers
  8. Websites for Teachers and Students
  9. Appendix

Survival of the Fittest?—Evolution and Human Health

Connie Scercy Wood

Published September 2010

Tools for this Unit:

Strategies and Lesson Plans

The objectives for this unit are ambitious—teach evolution, microbes, body systems and disease with an integrated approach. If successful, I believe this will not only solve the problem of never having enough time to teach each of these subjects separately, but will also help my students make better connections about how they are all related. During the course of the semester, students will be maintaining a biology journal which will include not only lab work and observations, but also their thoughts, reflections, and responses to pre- and post-instructional questions. The journals will be assessed both formatively and formally. A very good website on keeping science notebooks is at http://www.sciencenotebooks.org/. Also, Google the topic "interactive notebooks" for a variety of sites that show how to organize science notebooks.

Day 1—The students will have just completed a unit on genetics, and will have an understanding of how traits are inherited, different patterns of inheritance, and an introduction into some genetic diseases. I will begin the unit by asking students to reflect on the following question in their journals: "If individuals with unfavorable characteristics, like genetic disorders, would have been unlikely to survive in an environment with no doctors or medicines, then why are people still being born with these disorders today? Why haven't these genes disappeared from the population?" After giving them some time to think and write about this question, I will have them work in pairs to discuss their answers, change partners and share what they heard. I will then call on a few students to share what they heard in either of their groups. Students can make additions to their journals from the discussions.

In order to provide students an example that might help them discover answers to this question, I will then have students work in pairs to complete an activity that models natural selection against a recessive allele. This is another modified version of the popular Teddy Graham lab. A copy can be found in the Appendix. After reading the story of the bears, students will propose a hypothesis, complete the activity, and record a summary of the results in their journals. This activity allows students to observe a population with variation in phenotypes, environmental selection against one phenotype, and an increase in the frequency of one phenotype over time, showing that the population has evolved. After a discussion of each group's results, I will introduce Charles Darwin's theory of natural selection and have students compare what they just observed in the activity to this theory by responding in their journals.

Day 2—A second activity will model the effects of the heterozygote advantage with sickle cell anemia and malaria, but without revealing the name of the disorder. Reading the instructions before coming to class will be part of the previous day's homework assignment. Students are told to record questions about the lab's purpose or procedures in their journals for discussion before the activity begins. A copy of the activity can be found in the appendix. Students will again keep a record of the activity in their journals. What they should observe is that the individuals who survived in this activity were those who were heterozygous for the recessive allele that was lethal when an individual was homozygous for that allele. Following a discussion of the consequences of this type of selection, I will reveal that the disease demonstrated by this activity was sickle cell anemia, and that the selective agent was malaria. Many of my students are familiar with sickle cell anemia, and a few have told me they, or a family member, either have "the trait" or have sickle cell.

Students will learn more about sickle cell anemia, malaria and the circulatory system at rotating stations set up at lab tables for students to explore more about these two diseases. Station 1 will have microscopes set up with slides showing normal red blood cells and sickle cells for students to sketch into their journals. Information in the form of charts and diagrams of the components of the circulatory system will be available for students to complete a foldable to place in their journal. I use Dinah Zike's Foldables frequently instead of normal notes, because students can use them much like flash cards for studying.

Station 2 will have a model of a capillary and normal and sickle cells that will allow students to visualize the problem that the change in red blood cell shape creates. The capillary will be represented by a piece of clear flexible plastic tubing, the normal red blood cell by round fabric red blood cell models and sickle cells will be made of stiffer material. The normal red blood cells should go easily, one by one through the tubing, while the sickle cells should jam up in the tubing, at least some of the time. Students will record in their journals their thoughts on why this might create problems. Also at this station will be diagrams and information on gas exchange between red blood cells, the lungs, and body tissues. Students will complete a worksheet to be folded and taped into their journals. They should be able to make the connection between the information about gas exchange and how a sickle crisis might cause problems.

At Station 3, students will compare the mRNA sequence for normal and sickle cell hemoglobin and translate a portion—the beta-globin gene—into amino acids. This is a good review of protein synthesis, and also shows how mutations can create traits that can be selected for. They will compare structural models of normal hemoglobin and sickle cell hemoglobin that show how the substitution of one amino acid affects the shape of the protein. Reflections will be recorded in their journals.

Station 4 will cover the symptoms and genetics of sickle cell. Students will complete a Punnett square showing the possible offspring of two individuals heterozygous for sickle cell anemia. They will relate the symptoms of the disease to the effects of the faulty hemoglobin using a cause and effect diagram.

At Station 5, students will look at the characteristics of the disease malaria. Here students will analyze a diagram of the Plasmodium and Anopheles mosquito life cycles, investigate the symptoms of malaria, and compare maps showing where malaria and sickle cell anemia occur.

Students will rotate through these stations. I have eight lab tables and will set up duplicates of four of the stations on each side of the room, with, either one of the stations at the rear of the room, or two stations set up on one lab table. All information collected by the students will be entered into their journals. A rubric for the work will be provided to the students before they begin. They will complete a self-assessment column and staple the rubric at the end of their responses.

Day 3,4, 5—This is another "survival of the sickest" day. One of the themes of the books Why We Get Sick and Survival of the Sickest is that harmful traits that have continued to be passed down must have provided some reproductive advantage. Students will by now have come to understand how sickle cell gave an advantage to individuals who were heterozygous for the trait in resisting malaria. The lessons on these days will cover several other diseases whose benefits can only be speculated on—diabetes, PKU, Huntington's and cystic fibrosis. We will begin first with an overview of the digestive system, including a discussion of whether or not the appendix should be considered a vestigial structure in light of recent evidence that it may be reservoir for beneficial bacteria. I will give them a brief overview of what the diet of early humans was like when we were hunter-gatherers, and how that diet changed with the advent of agricultural practices. They will watch excerpts from the movie Super Size Me about our country's addiction to fast food. Students will then record in their journals what they eat for a week on one side of their page and then what healthier choices they could have made beside the first list. Many of the students are familiar with diabetes, but they often do not know the difference between Type 1 and Type 2, so I will give them an overview of the disease. Then I will tell them the story of the ice wine and the wood frog. They will watch a video clip of the wood frog freezing and thawing—available at http://www.pbs.org/wgbh/nova/sciencenow/3209/05.html. I'll tell them the story of the diabetics who may have survived the last ice age because of the anti-freeze effects of their high blood sugar. I will have students discuss why some people with certain ancestry might not be able to digest dairy products very well, while those with different ancestry still can and what the survival advantages might have been at one time to having each kind of digestive capability.

Students will have already heard about PKU, Huntington's and cystic fibrosis briefly in our genetics unit. I will demonstrate the effects of genetic drift and the founder effect on gene frequency in a population and we will discuss and evaluate the hypotheses that the relatively high proportion of these harmful diseases might be the result of genetic drift, or natural selection, and how they are being maintained in the population today. Students will construct a model of a neuron and will do an activity that demonstrates how a signal is passed along the axon of a neuron. They will look at how PKU and Huntington's affect this signaling in the nervous system. After looking at the various structures of the respiratory system, they will learn about the effects of cystic fibrosis on this system and view a video about a family who opted to use in vitro fertilization and genetic testing in an attempt to have a child free from the disease.

Day 6, 7, 8, 9—The Immune system and infectious diseases. There will be several labs for this part of the unit. The first will be on bacterial resistance to various antimicrobial products. Students will design an experiment to test the effects of a product or products on E. coli bacteria. I envision them doing comparison testing on several anti-microbial products—a fairly simple experiment, but the emphasis will be on experimental design and control of variables. I plan to cover bacterial structure in the cell unit, but will review the structure of bacteria and describe viral life cycles. After an introduction to the various components of the immune system, I will have students work in groups to develop skits and act out various things that go on in the immune system—the action of macrophages, the inflammatory response, and how T cells and B cells work. Once students understand about antigens and antibodies, they will do an experiment from Lab-Aids, Inc called "Immunology and Evolution". This simulation lab tests blood proteins of various animals using antibodies to human blood serum produced by rabbits. The idea is that the greater the degree of precipitation, the more closely related the test subject is to humans.

Outbreak!—As a final culminating project, each student will select one infectious disease to research and describe a scenario in which there is an epidemic associated with that disease. Their presentation may take many forms—a PowerPoint, video, newscast, or graphic novel. They must include information on historical outbreaks of the pathogen, its characteristics, how it is transmitted, its effects on the body, what factors might have led to the current outbreak being described, and what possible measures might be used to contain the outbreak today. Students will be required to write a 1000 to 1200 word essay that describes their research findings to be submitted one week prior to their presentation. This will allow me the chance to review their facts to be sure they have the science right before they present to the class.

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