Natural Selection - A Human Example

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Objective

SWBAT explain the role of natural selection as it relates to the disease sickle-cell anemia.

Big Idea

Students often think that natural selection is somehow chosen by the organism in response environmental pressures. This lesson helps students understand the randomness of natural selection.

Warm-up

5 minutes

As students enter the room I have them respond to the following prompts in their science journal:

What is sickle-cell anemia?  

How does it impact people who have this condition?

How does a person come to have this condition?

Students can work on their own or in small groups to answer this question.  If you have access to computers, you might allow some time for research (I sometimes make this a race to encourage students to complete this task quickly).

Activity

20 minutes

Students seem to think of natural selection and adaptation as processes that affect plants and animals but don't see the role they play within the human population. This video from HHMI Biointeractive is excellent for showing a real world example of natural selection within humans and how scientists use the scientific method to answer questions, both important for students to understand.

I show the video and stop at key points to ensure students understand and are following along. Additionally, this provides students with the opportunity to ask questions and helps them stay on task. I have found this process of pausing, and talking, a strategy that helps students make stronger connections to the material and as the year progresses they are better able to pull important information out of videos shown during class.

The following video points out what I believe to be an important part of the HHMI video that should be pointed out to students to ensure their understanding.

The content referred to in this part of the video relates to what my students were introduced to during their 7th grade year (we follow the integrated NGSS model). If this is new information for your students, you may need to take some time to more fully explain this material.

Wrap-up

20 minutes
To conclude this lesson, I have students work with a partner to answer the following questions in their science journal:
  1. In the beginning of the video the narrator states, "Before the advent of modern medicine, sickle-cell anemia almost certainly meant death before adulthood." What do you suppose this meant for the amount of people who had this disease within the population (was it high or low)? Explain your answer.
  2. What do you think is true about the number of people who have this disease today (is it higher or lower than in the past)? Explain your answer
  3. Natural selection is defined as the process in which organisms better adapted to their environment tend to survive and produce more offspring. How does the video you just watch provide an example of natural selection? Be specific in your answer.
  4. Will natural selection always help an organism? Explain.