A lesson on evolution & natural selection.
I describe three activities that allow students to explore the
ideas of evolution, natural selection, extinction, mass extinction, and
rates of evolutionary change by engaging a simple model using paper,
pens, chalk, and a chalkboard. As a culminating activity that supports
expository writing in the sciences, the students write an essay on mass
extinction. All activities are geared for high school biology and
perhaps introductory college biology classes. With little modification,
activities 1 and 2 can be used successfully in middle school and perhaps
in the higher elementary grade levels.
Key Words: Active learning; evolution; extinction; natural selection; sustainability.
(Study and teaching)
Mass extinction theory (Study and teaching)
Evolution (Study and teaching)
|Author:||Curtis, Anthony D.|
|Publication:||Name: The American Biology Teacher Publisher: National Association of Biology Teachers Audience: Academic; Professional Format: Magazine/Journal Subject: Biological sciences; Education Copyright: COPYRIGHT 2010 National Association of Biology Teachers ISSN: 0002-7685|
|Issue:||Date: Feb, 2010 Source Volume: 72 Source Issue: 2|
|Geographic:||Geographic Scope: United States Geographic Code: 1USA United States|
Evolution by means of natural selection is regarded as one of the
most significant scientific theories ever elucidated. In his book On the
Origin of Species, Charles Darwin (1859) proposed this mechanism to
explain how life has changed over geological time and how new species
emerge from common ancestors. The following activities model evolution
and natural selection using simple materials available to most
Scientists routinely use models to simulate or approximate complex systems, evaluate hypotheses, and explore theories. Models are invaluable when controlled experiments cannot be done. For example, mathematical models are used to predict complex global and local weather patterns. Without such models, weather prediction would be less reliable. The model used here is intended to introduce and facilitate understanding of evolution and natural selection.
Extinction is an important component of evolution. Over 99.9% of all species that have existed are now extinct. At times, many species have gone extinct over a very short period; this is called a "mass extinction." There have been at least five mass extinctions, and global climate change has been indicated as a component of each. Because of the continued use of fossil fuels, humans are putting carbon dioxide into the atmosphere at an unprecedented rate. Carbon dioxide is a greenhouse gas, and it traps the sun's energy near the earth's surface. Our world is warming, and global weather patterns are changing in sometimes unpredictable ways. The consensus among the world's scientists is that we are experiencing a sixth mass extinction as a result of global climate change and habitat destruction caused by humans.
* To foster interactive discussion about evolution by natural selection
* To become familiar with terms related to evolution by natural selection
* To model how populations change through time
* To model gradualism and punctuated equilibrium
* To gain perspective on extinction and mass extinctions
* To gain awareness of the current rate of extinction caused by humans
* To develop and improve writing skills through expository writing in science
** Natural Selection
Darwin proposed natural selection as a mechanism of how populations change through time, or evolve. The five principal components of natural selection are as follows:
(1) Organisms produce more offspring than will actually survive to reproduce.
(2) Every organism struggles to survive.
(3) There is variation within species.
(4) Some variations among members of a species allow their bearers to survive and reproduce better than others.
(5) Organisms that survive and reproduce pass their traits to their offspring, and the helpful traits gradually appear in more and more of the population.
** Activity 1: Assemble an Animal
"Assemble an Animal" handout (see Appendix) One six-sided die
Chalk and chalkboard (or markers and white board)
Instructions to Students
Obtain the "Assemble an Animal" handout and a single die from your instructor. For each of the five categories, roll the die to determine which of the six characteristics will be used in determining your animal's environment. Discuss with your group how you will incorporate adaptations to these characteristics into your animal. You have 5 minutes to discuss, plan, and draw your animal on the board. Please send only one person from your group to draw your animal on the board. Note: The time starts when your instructor gives the signal.
The "Assemble an Animal" handout is adapted from Shlomiya Bar-Yam's "Build-a-Beast" evolution Web page at New England Complex Systems Institute (http://necsi.org), which presents a few activities regarding evolution and other topics worth exploring with your students.
For this activity, I have found it useful to put the students in groups of two to three. In the first round, be firm about the time requirement of 5 minutes. Begin with students who have completed the drawing on the board. Ask them how they have adapted their animal to accommodate the environment in which it lives. You will notice that students bring a variety of solutions based on living things they have seen before. Ask them to expand on these ideas in the next round, and encourage them to think of adaptations they may not have seen before. Motivate them to be creative in adapting their animal to its environment. In doing so, you will see more outrageous combinations to accommodate adaptations necessary for their animal's environment.
Tell students who do not complete their drawing within 5 minutes that their creature does not survive. Explain to them that in this case, the individual organism dies. Then explain to the class that in nature, individuals do not evolve--populations do. An individual either survives to pass on genes to offspring or it does not. Introduce the terms "phylogeny" and "ontogeny" at this point and talk about the concept of the gene pool. Explain how gene frequencies may change in successive generations on the basis of their relative merits. Genes that help an organism to survive are passed on and accumulate in successive generations, while those that are not so helpful tend to be eliminated from the gene pool. However, make it clear that not all genes have come under selective pressure. Have them brainstorm to identify morphological and behavioral traits that have no known adaptive function in the human population. Also, make sure your students understand that natural selection is not evolution and that other mechanisms, such as genetic drift and sexual selection (which are not part of this activity), are major evolutionary forces as well. (An extension of this activity at some point could be to tie in your genetics module with a Hardy-Weinberg experiment measuring allele frequencies and how changes occur in successive generations. For a handy example, see Lauer, 2000.)
In the next round, all the students will likely draw their creatures within the 5 minutes allotted. This time, prepare a key with six different "selection" events that correspond to each of the numbers on a die (e.g., 1 = organisms with scales die, 2 = only organisms with wings survive, 3 = organisms with claws perish, etc.). After they have completed their drawings, ask one of the students to roll the die to pick a number. Tell the class that an extinction-level event has just occurred, and wipe out (i.e., erase) their creatures accordingly. This is the time to introduce the concepts of extinction and mass extinction. Having a geological time-scale map with extinction-level events noted at the five major mass extinctions will provide further material for discussion. Emphasize that most of the five previous mass extinctions are characterized by global climate change. Bring this point home by discussing the current global warming trend and the implications of changing weather patterns for life adapted to certain climates. Finally, inform the students that the present human-forced extinction of organisms rivals the rates seen in the five previous mass extinctions. Professional biologists and other scientists regard our era as a sixth period of mass extinction. (For a peer-reviewed article by Niles Eldredge  on this topic and a follow-up activity, see http://www.actionbioscience.org/newfrontiers/ eldredge2.html.)
** Activity 2: Evolution "Telephone" Game Instructions to Students
Please arrange the desks closely in a circle facing each other. Your instructor will begin by whispering a phrase into someone's ear. This person will whisper the phrase into the ear of the person directly to the left. Immediately, that person is to write down the phrase, then whisper it to the next person. (Try to be quiet during this part.) Remember, write first, and then whisper. Repeat this until everyone has written the phrase. Your instructor will now compare the written phrases of various people in the circle and discuss the outcome. Repeat the exercise and change an aspect (e.g., length of the phrase, its familiarity, length of words, etc.).
This is a modification of a game most of us remember from childhood, and most of your students will be familiar with how to play. It is used as an example of mutational meltdown in several texts, most notably Mark Ridley's (2001) The Cooperative Gene: How Mendel's Demon Explains the Evolution of Complex Beings. As you know, it is likely that the phrase you told the first student will be very different from the phrase the last student wrote. This can be a fun game on its own, and it will generate more than a few giggles for sure. However, reel them back in by explaining that each person represents a population that is passing along genes to the next generation each time the phrase is written. Emphasize again that populations evolve, not individuals. Here you can easily demonstrate that evolution is change through time by letting the written phrases represent fossils buried in different strata in the sediment of a lake, ocean, or stream. Perhaps you could explain that the phrases represent highly conserved DNA fragment samples obtained from many taxa collected over time. Another discussion point could ask whether conserved DNA sequences are more or less likely to be subject to natural selection. The game itself has evolved in my classes over the years, and it will likely reveal other teachable moments for you, too.
At this point, you can track the emergence of words in the changing phrase and explain that inherited characteristics are tracked in similar fashion using real fossils (or DNA mutations). A mathematical extension to this activity would be to calculate the percentage of the original message that made it to the last phrase. This would be analogous to comparing the degree of similarity between taxa (e.g., the genomes of chimps and humans are more than 98% identical).
Even if one of your students totally disregards the phrase they heard and opts to generate a new phrase midstream, it could be an excellent opportunity to begin a discussion on the rate of evolutionary change.
In fact, this is exactly what happened when I first performed the activity. Trying to be cute, one of my students introduced a completely different phrase halfway through. When we read the phrases at the end of the round, I was a little put off at first. However, I quickly realized that this was a perfect time to talk about Eldredge and Gould's (1972) landmark paper on punctuated equilibrium. If all your students follow the rules, perhaps you could covertly or overtly introduce another phrase with the help of one of the students. Assuming that all students before and after are following the rules, you would be able to easily detect abrupt change in the "fossil" record of the phrases compared with the more gradual change in the rest of the "lineage."
** Activity 3: Essay
Instructions to Students
Leading scientists (Leakey & Lewin, 1996; Eldredge, 2001; Wilson, 2002) say that we are now experiencing a sixth mass extinction. Using the Internet and printed sources, find out the supporting evidence for this claim and write a three- to five-page essay to compare and contrast two of the mass extinctions listed below. Describe the implications of a sixth extinction for humans and other species. Be sure to identify and include a summary of the two previous extinction events you choose (e.g., types of organisms, climatic conditions, typical habitats, etc.). Double space your paper using a plain font, size 12. The style of the paper must follow Scientific Style and Format: The CSE Manual for Authors, Editors, and Publishers (7th ed., Council of Science Editors, 2006; for an excellent online resource for CSE style, see http://www.dianahacker. com/resdoc/p04_c11_o.html).
Previous Major Mass Extinctions
* Between Ordovician and Silurian periods (438 mya) - 75% of all species
* Between Devonian and Carboniferous periods (360 mya) - 70% of all species
* Between Permian and Triassic periods (245 mya) - 90% of all species
* Between Triassic and Jurassic periods (208 mya) - 60% of all species
* Between Cretaceous and Tertiary periods (66 mya) - 75% of all species
Writing is a very important aspect of a student's overall education that is becoming less prevalent in standard curricula in American schools, and in science classrooms. Through the written lab report and other forms of scientific expository writing, students must organize their thoughts and relay them in a logical and intelligible way. This process reinforces communication skills and brings together big-picture ideas as well as the finer details of a scientific topic. Along the way, reinforcement is given to writing mechanics, acquisition of vocabulary, sentence structure, and spelling. Students can be asked to create a story with the material to make their writing interesting and engaging. This twist of originality may also limit the opportunity for them to merely cut and paste from the Internet.
I have found it effective to stick to a due date. I critically mark up the manuscripts and hand them back for revision at least once. Over time, and with other writing assignments in other disciplines, the students will begin to improve their ability to write and edit their work. It takes time for students and instructors to commit fully to this process, but it is an activity that is worthy of the time. Further, it complements the teaching of writing skills by your colleagues in other departments. Stress that the single most important thing students can do to improve their writing is to allow themselves the luxury of time between finishing the first draft and turning it in. Ask them to set it down for at least 2 days (the more drafts the better) before it is due. Last-minute writing is not effective in learning this process. In the words of Nathaniel Hawthorne, "Easy reading is damn hard writing." It is hard, at least at first, but it will become easier with experience. By the way, teachers also improve this valuable skill set when they require it of their students.
** Glossary of Terms
Evolution: The change in the inherited traits of a population from generation to generation.
Extinction: The cessation of existence of a species or group of taxa.
Gene pool: The complete set of unique alleles in a species or population.
Mass extinction: A sharp decrease in the number of species in a relatively short period of time. Evidence supports the idea of five previous mass extinctions on earth.
Model: A pattern, plan, representation, or description designed to show the structure or workings of an object, system, or concept.
Natural selection: The process in nature by which only the organisms best adapted to their environment tend to survive and transmit their genetic characters in increasing numbers to succeeding generations while those less adapted tend to be eliminated.
Ontogeny: Development of an individual from the moment the egg is fertilized until adulthood.
Phylogeny: The history of organismal lineages as they change through time.
Population: A group of interbreeding individuals of the same species.
Punctuated equilibrium: An idea developed by Stephen J. Gould and Niles Eldredge that the fossil record shows long periods of stasis between abrupt changes in the lineages of organisms.
** Questions for Students & Sample Answers
(1) In activity 1, how did you determine the five characteristics of your creature's environment? Rolled the die.
(2) Is natural selection random? No. Sometimes random events occur in the environment, but natural selection is not random.
(3) What do scientists believe was the underlying cause of most of the mass extinctions that have occurred? Global climate change.
(4) Why do scientists, particularly professional biologists, believe that the earth is now experiencing a sixth mass extinction? Because human-caused extinction rates are higher than the extinction rates for the last five mass extinctions.
(5) Offer some ideas for mitigating the sixth mass extinction. Reduce habitat destruction. Reduce or abandon the practice of clearcutting tropical forests. Get off of fossil fuels and reduce other greenhouse gasses, reduce invasive species, etc.
(6) What is a model and why do scientists use them sometimes? It is a representation of a system and is used by scientists when controlled experiments are unsuitable because of cost or time restraints, or because system is too complex, etc.
(7) Describe how activity 2 was used to model evolution. The written phrases were the fossils, and the change in the phrases demonstrated how populations change over time.
(8) What is punctuated equilibrium? The reevaluation of the fossil record by Niles Eldredge and Stephen Gould, it supports the notion that evolutionary change can occur quickly at times.
(9) Characterize how the message changed in the first round of activity 2. [Students' answers will vary.]
(10) How did the second round of activity 2 compare with the first round? [Students' answers will vary.]
APPENDIX. "Assemble an Animal" handout (adapted from Bar-Yam, 2001).
(A) What is the terrain like?
(4) heavily forested
(5) shrubs and few trees
(B) How much water is available?
(1) almost none, dry and barren
(2) water part of the year, drought the rest
(3) lots of precipitation all year
(4) a river
(5) a swamp
(6) a lake
(C) What is the climate like?
(1) hot and humid
(2) hot and dry
(4) cold, rainy, and windy
(5) mild seasonal changes from hot to cold
(6) freezing temperatures most of the time
(D) What does it eat?
(1) leaves from tall plants
(2) fungus growing just under the soil surface
(3) berries, plants, and small animals
(4) water-dwelling animals
(5) swiftly running deer-like animals
(6) flying insects
(E) What eats it?
(1) stompsuckers step on it and eat its guts
(2) vampire butterflies land on it and suck it dry
(3) buzzbugs lay eggs that burrow into its skin and consume it inside out
(4) webweevils set gooey traps to catch it
(5) archebarks are wolf-like and chase it down
(6) megaworms leap out of the soil and swallow it
I thank Dr. Linda Eyster of Milton Academy for her input during the development of the activities. I thank two anonymous reviewers for offering their helpful comments to strengthen the manuscript.
Bar-Yam, S. (2001). Build-a-Beast. [Online.] New England Complex Systems Institute. Available at http://www.necsi.org/projects/evolution/activities/ build-a-beast/activities_beast.html.
Darwin, C. (1859). On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life. London: John Murray.
Eldredge, N. (2001). The Sixth Extinction. [Online.] Available at http://www. actionbioscience.org/newfrontiers/eldredge2.html.
Eldredge, N. & Gould, S.J. (1972). Punctuated equilibria: an alternative to phyletic gradualism. In Schopf, T.J.M. (Ed.), Models in Paleobiology (pp. 82-115). San Francisco: Freeman, Cooper.
Lauer, T.E. (2000). Jelly Belly* jelly beans & evolutionary principles in the classroom: appealing to the students' stomachs. American Biology Teacher, 62, 42-5.
Leakey, R. & Lewin, R. (1996). The Sixth Extinction: Patterns of of Life and the Future of Humankind. NY: Doubleday.
Miller, G.T., Jr. (2007). Living in the Environment, 15th Ed. Belmont, CA: Wadsworth. Ridley, M. (2001). The Cooperative Gene: How Mendel's Demon Explains the Evolution of Complex Beings. NY: Free Press.
Wilson, E.O. (2002). The Future of Life. NY: Knopf.
ANTHONY CURTIS is a biology teacher at The Roanoke Valley Governor's School, 2104 Grandin Road, SW, Roanoke, Virginia 24015; e-mail: firstname.lastname@example.org.
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