Investigating Natural Selection: Observing Stabilizing and Directional Selection through Graphical Analysis of Population Samples.

John Haug
Stewartville High School
Stewartville, MN
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Summary

In this biology lab students will compare two sample populations of sunflower seeds. Comparison of the two populations will be done through the generation of a histogram for each population. One population will be black oil sunflower seeds used as bird food. These companies don't select seed size so the histogram is a natural bell curve showing stabilizing selection. The other population will be striped sunflower seeds used for human consumption. These seed companies benefit from choosing larger seeds because they are selling to a human consumer, the histogram will show a skew towards larger seed size and represent a directional selection event. In completing this activity students will understand that scientists can demonstrate a selection event by data collection and graphical analysis of a population.

Learning Goals

1.Illustrate how variation of a trait in a population is graphically distributed.
2.Compare and contrast histograms of populations undergoing stabilizing and directional selection.
3.Propose an explanation why black oil sunflowers are exhibiting stabilizing selection while striped sunflowers are exhibiting directional selection
Key terms: natural selection, stabilizing selection, directional selection, disruptive selection

Context for Use

This lab activity is intended for general biology students who have been introduced to the idea of Darwin's theory of natural selection. It can be used as an introduction to the concept of variation within a species or as an activity that bridges the idea of natural selection and how changes in phenotype frequencies will affect the distribution of traits in a population
Modification 1: If students are not experienced making histograms use this lab as an introduction to the idea of trait variation within a population of a species and have students make their own histograms.
Modification 2: If students are experienced in graph making then you can post data to a class excel sheet and generate graphs instantly. This will focus the conversation to the analysis of the graphs and allow students to think beyond the activity; making connections to other concepts like divergent evolution and genetic drift.
I used to use modification 1 and now use modification 2. I also use this lab after the students have become familiar with the work of Peter and Rosemary Grant on the Galapagos Islands and their observation of selection events during wet and dry seasons on the islands. ("What Darwin Never Saw" video of Grants' finch research in Galapagos Kurtis Productions, 1-312-951-5700)

Description and Teaching Materials

1.Pick up a sample of either black oil sunflowers or striped sunflowers from your teacher.
2.Make a data table with distances in the left margin ranging starting at 1mm and continuing down the page to 25mm. You will record seed length as a tally mark next to the appropriate length.
3.With a partner measure the length of whole sunflowers in the hull. Record the length of each seed on your data table and discard the seed.
4.Combine your data along with the rest of the class either on the classroom board of computer. Make sure you post data to the correct population, either black oil or striped sunflower.
5.Generate a histogram of both populations.
Materials: supply of black oil and striped sunflower seeds, metric rulers
Time: Modification 1-two 50 minute periods; Modification 2-one 50 minute period

Teaching Notes and Tips

It works better to have the students measure as many seeds as they can in an amount of time than to have them measure a predetermined number of seeds. A class of 30 students can measure about 1500 seeds in twenty minutes.
It doesn't matter where you get the black oil seeds but you should buy a name brand of the striped seeds. I have used both David® and Fisher® brand seeds with good results because both companies are choosing larger seeds and this shows up as a directional selection event. A cheaper brand may not select seed size and would make a good example of a stabilizing selection event.

Assessment

Assessment of learning goals is achieved through classroom discussion, lab quizzes and unit tests. When students generate graphs from the data that is also included in the assessment.

Standards

Evolution in Living Systems (9-12)
3.3.3 Recognize that artificial selection has led to offspring through successive generations that can be very different in appearance and behavior from their distant ancestors.
3.3.4 Explain why genetic variation within a population is essential for evolution to occur.

References and Resources