Another Way to Connect Selection, Phenotype, and Genotype

Now that it’s summer, I’m catching up on my American Biology Teacher backlog. I found this interesting-looking activity by Janina Jördens and three coauthors in the February 2018 issue. The title of the article says it all: “Interrelating Concepts from Genetics and Evolution: Why Are Cod Shrinking?” I haven’t tried it — it is summer, after all — but it looks like it addresses some of the learning objectives that I have previously outlined in the oft-requested reptilobird and antibiotic resistance activities.

The authors point out that traditional activities illustrating natural selection — like picking scattered colored dots off a piece of patterned fabric — emphasize phenotypes while ignoring genotypes. But if evolution means changes in allele frequencies over multiple generations, we should not let students focus purely on changes in phenotypic frequencies. Jördens and her coauthors developed a simulation that could help students understand the connection between phenotype and genotype in the context of cod overfishing.

All of the materials needed for the simulation are described at the evolution-of-life website. As I discovered in writing this blog post, the site is not easy to navigate. Thank goodness for the site map, which led me to all of the site’s teaching materials, which led me to the teaching resources for this activity. Here they are. They include handouts with background information and instructions, the playing board, and teaching advice.

The lesson begins with an 8-minute video explaining how the average body size of cod has declined over time, thanks to decades of selectively harvesting the biggest fish. Students also read about the situation on handouts. Each group of five students then receives a playing board depicting 10 cod fish. They also get a non-transparent bag containing 60 colored plastic disks: 15 red, 15 yellow, 15 black, and 15 blue (or, less durably but more deliciously, chocolate candies with different colored wrappers). Separately, they also receive enough “spare” disks to make up a total of 60 of each color.

Each of the 10 fish on the playing board has room for six alleles, two for each of three genes (labeled as A1, A2, B1, B2, C1, and C2) that control the individual’s body size. The students begin by randomly drawing 6 disks from the bag (no peeking!) and placing them on the first fish on the playing board. They repeat that process for each of the remaining nine fish — if they have done it right, they should have used up all of the disks in the bag.

Screenshot of cod fish game board

The upper third of the game board, showing where to put two alleles for each of the three genes (A, B, and C) conferring fish size. Screenshot taken from game board at

The colors represent different numbers of “size units,” with red conferring the largest size (4 units), followed by yellow (3 units), black (2 units), and blue (1 unit). With this information, students can calculate the size of each fish and compute the average size for the entire population of 10 fish. At this point, the largest five individuals are “caught” and their alleles are removed from the board before they have a chance to “reproduce.” The alleles from the surviving individuals are duplicated (to simulate reproduction) and the surviving alleles plus the duplicates are returned to the bag. At this point, the number of alleles in the bag should be back to 60.

Students are now ready to produce the second generation. They randomly assign each fish on the playing board its 6 alleles, measure the size of each fish, compute the average population size, and tally the frequency of each allele. Then the five largest individuals are caught, and the entire process is repeated to produce and measure generations 3 and 4. Afterward, students graph the individual and/or average body sizes for each generation; given the strong selection pressure against large body size, the trend should point downward. Likewise, the frequency of the red and yellow alleles should decline. The authors suggest several thought questions and followup activities that could be useful, depending on the instructor’s priorities.

For my part, I am happy to see more ways to connect selection, phenotypes, and genotypes. This simulation appears to be well-thought out, and the materials are freely available on the evolution-of-life website. By the way, I encourage you to explore the entire site. The “Teach” page has links to materials for the shrinking cod activity described here, as well as the origin of life, Darwin, evolution in fast motion, and coevolution. If I find other gems, I will certainly let you know; if you do, please leave a comment and let me know!


JördensJanina, Roman Asshoff, Harald Kullmann, and Marcus Hammann. 2018. The American Biology Teacher 80(2):132-138.
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3 Responses to Another Way to Connect Selection, Phenotype, and Genotype

  1. Doug Gaffin says:

    Dr. Hoefnagels: thanks for this great post. You described the exercise well and I really appreciate the direct links to the needed resources. I think I will use this early in my Beyond Darwin class this fall!

  2. Pingback: Natural Selection in Tortoises: A (Homemade) Video | Teaching nonmajors biology

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