Natural Selection in Tortoises: A (Homemade) Video

[Doug Gaffin and Marielle Hoefnagels worked together to develop the materials used in this post.]

A while back, I wrote a post on an activity that connects genotype, phenotype, and natural selection. In a nutshell, the activity uses colored chips to represent alleles in cod. Students selectively “harvest” the largest fish over multiple generations and observe what happens to allele frequencies in the population.

Tortoise on Galapagos with open mouth

What’s so funny about natural selection? (Galapagos tortoise photo by M. Hoefnagels)

This activity leaped to our minds when my husband and I were invited to give a lecture about natural selection to a group of non-biologists as we were all visiting the Galapagos Islands. We decided to adapt the cod activity to illustrate how natural selection could select for long necks in tortoises—we were particularly interested in driving home the point that tortoises didn’t “decide” or “try” to evolve in response to their environment. Our original intention was to have the group actually do the activity, but in practicing it before the lecture, we quickly realized that the activity involves quite a bit of counting and other tedious tasks that would use up our entire lecture time.

So we decided to do the activity in our hotel room, film it, and turn it into a video that we could show to our companions; you can see it here. The video, which is 2 minutes and 45 seconds long, requires a bit of explanation. It begins with a founding population of 10 tortoises, each of which inherits a total of six alleles for neck length. The alleles are represented by puzzle pieces of different colors: pink = 4 “length units,” yellow = 3 units, blue = 2 units, and purple = 1 unit. To assign genotypes to each tortoise in the founding population, equal numbers of alleles of each color are placed in a bag and shaken. Each tortoise receives a random assortment of six alleles. In the first 25 seconds of the video, we randomly select alleles from the bag to assemble the tortoise genotypes.

Illustrates genes and alleles controlling neck length

A tortoise from the “game board,” annotated to show the three genes and four possible alleles controlling neck length.

Next, the number of alleles of each color is tallied for each tortoise. The number of pink alleles is multiplied by 4, the number of yellow alleles is multiplied by 3, the number of blue alleles is multiplied by 2, and the number of purple alleles is multiplied by 1. These numbers are added to calculate the neck length of each tortoise. The five tortoises with the shortest necks do not survive (their totals are circled in the video), and the average neck length for the founding population is calculated. We are now through the first minute of the video.

For the next 27 seconds, the alleles for the shortest-necked tortoises are wiped off the board, the five survivors have their alleles doubled, and all of the alleles are swept into the bag and scrambled. At 1:27, we are ready to begin Generation 2. Each of 10 tortoises again gets a random assortment of six alleles, the numbers are tallied, the unfortunate ones with the shortest necks are identified for elimination, and the generation’s average neck length is calculated. We’re now about 2 minutes into the video. Then the survivors of Generation 2 have their alleles doubled and swept into the bag, and the start of Generation 3 is signaled at the 2:22 mark. By 2:45, the Generation 3 numbers are tallied.

The selection pressure against short necks in this simulation is very strong, so it’s not surprising that three generations is enough to produce a prominent trend toward long necks:

Graph of neck length

Whether using cod or tortoises, this activity is really good, and we are indebted to the inventors of the cod activity. If you want the tortoise version of the worksheet and student instructions, please leave a comment and we’ll send them your way. Or, if you simply want to show the video and explain it to students as you go along, feel free to do that as well.

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3 Responses to Natural Selection in Tortoises: A (Homemade) Video

  1. Gaffin, Douglas D. says:

    Great job Dr. Hoefnagels!!!

  2. Mary E Gard says:

    I would love a copy of the lab for using in class! Thanks!!

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