To develop an understanding of natural selection, specifically, how it unfolds from generation to generation.
With over a million living species on earth, students cannot ignore the vast diversity of living things. In elementary and middle school, they can enjoy their curiosity about nature and explore the many different organisms in many different capacities. It is in high school that an explanation for the diversity in living things itself must come to light.
Understanding evolution is the key to understanding diversity. And natural selection, being the mechanism (or cause) of evolution must also be understood. Though students may understand evolutionary change and how it works in a general sense, they need to broaden their thinking from the specifics of individuals with certain traits to how the traits in a whole population can change.
This lesson is an introduction to natural selection and may be more suited for a 9th grader, or any student who is just beginning his or her discovery of evolution.
The Motivation introduces a species of bird that became (over millions of years) numerous species, through adaptation. The Development is a hands-on activity that demonstrates how populations change little by little, generation by generation, due to survival of species that have traits that are beneficial in an environment. Students will learn why organisms evolve over time, how natural selection works, and how certain factors determine survival and differences in organisms. This hands-on activity is probably more effective than any reading students will do on the subject, particularly if the subject is new.
Also, the lesson deals with natural selection itself. It does not bring up the discovery of natural selection or its discoverer, Charles Darwin. You may want to plan this lesson, however, in coincidence with such information.
According to research, high-school students have the following misconceptions about natural selection: High-school and college students, even after some years of biology instruction, have difficulties understanding the notion of natural selection. A major hindrance to understanding natural selection appears to be students' inability to integrate two distinct processes in evolution, the occurrence of new traits in a population and their effect on long-term survival. Many students believe that environmental conditions are responsible for changes in traits, or that organisms develop new traits because they need them to survive, or that they overuse or under-use certain bodily organs or abilities. (Benchmarks for Science Literacy, p. 343.)
It is recommended that you read in advance the article: Of Hawaii, Birds, Evolution and Poetry in Science. Students will read this article in the Motivation section of the lesson.
Have students read the article: Of Hawaii, Birds, Evolution and Poetry in Science, found at the UniSci website. The article, without much detail, tells the story of one species of finch that came to the island of Kaua'i several million years ago. It describes how the one population adapted over millions of years to the food and habitat.
The following questions will help you determine what your students know about adaptation. It is a commonly misunderstood concept (see above). If students cannot answer the questions, that is fine. This situation could even prove to be better because it means you are starting with a clean slate. If they answer them incorrectly, the development should address any misunderstandings.
Ask the following:
- What were some characteristics the finches developed to give them an advantage in surviving?
- How do you think the one species of finch evolved into many different species, each with its own advantages?
- Do you think these advantages helped them survive and reproduce?
- What might have happened if they didn't evolve into many different species?
- What were the environmental changes that led to the demise of many species of finch?
- Do you think that environmental changes are always bad for a species or do you think such changes can actually help certain species?
- Do you think that extinction can occur because of environmental changes? Why? Why not?
- What do you think will happen to future generations of finches? Why?
Divide students into groups of four, or whatever works best for your class. Distribute a copy of the student sheet from the Natural Selection activity to each group. Set the stage for the lesson by briefly discussing the questions posed in the first paragraph of the student sheet. Then have the groups work on their own to conduct the activity.
In this activity, students poke six different-sized holes in the bottom of the bowl. They create a generation by choosing five of each kind of bean to put in the bowl. Students shake the bowl, and the beans that fall through are considered "dead" organisms. The population that is left in the bowl is the first generation of parents. Students then add beans only of the survivors and shake again. This process is repeated and students watch the population change.
Although students work in groups, each student will create a lab report as they do the activity. You may want students to include the following sections:
- A hypothesis (as discussed in "Let's make a hypothesis" on page 2 of the facilitator sheet)
- A table of generations (step four of the student sheet)
- Answers to the questions found on the student sheet under "What Did You Find Out By Doing the Activity?"
Note: The activity is set up to teach students about the evolution of "microbes," but the beans can represent any species. There are good questions within the activity, but even after you have gone over these with students, you may want to focus on the part of the benchmark that is crucial: chance alone can result in heritable characteristics. By focusing on this part of the benchmark, you can be sure that students won't misconstrue the "why" of survival value. Be sure students do not think that the environment dictated the changes, rather the environment dictated who would survive.
To broaden the discussion from beans and microbes to other organisms, discuss the following:
- What do you think caused some heritable characteristics in the finches?
- Even though the beans that "survived" the activity depended on the size of the holes, can you think of other environmental factors that could change the survival value of a species (both for the good or the bad)?
- What could have changed the whole outcome of the bean activity?
- What could have changed the whole progression of the finches?
The lab reports will serve as an assessment, but to confirm that students can apply the concept to all living things, assign one of the following:
1. Keep students in their same groups and instruct each group to create their own experiments that demonstrate natural selection. Basically, they will need some items to represent the generations as well as a tool to weed out the "fittest." Encourage students to be creative. They should be able to write directions for their experiment as well as demonstrate it if time allows. As they are demonstrating, they should be able to verbally explain how natural selection works.
2. Assign a short writing exercise. Tell students to write one to two pages on a species. They can fictionalize the species, describe the species' advantages, and how it may have changed over millions of years to adapt to the environment. Students can be as creative as they wish. For instance, if they want to create a deer with extra long legs, the situation could be that food became higher, or that they had to run faster to get away from predators. Instruct students to do the following:
- Describe the species in the beginning.
- Explain how it changed from generation to generation. Describe in detail the variation in heritable characteristics; how some characteristics gave individuals an advantage over others, and how this affected reproduction and future populations.
Papers should show how the proportion of individuals that have advantageous characteristics would increase.
Students can go to the UniSci Weekday Archives and do an archive search using the word "adaptation." If you want to further the class discussion on adaptation, below is a recommended article from the UniSci site that focuses on another aspect of adaptation. The article not only covers how animals have adapted to a habitat, but how the successful survival of the animals can in turn impact the habitat: