another in-class activity: four forces of evolution

We had a surprisingly fervent and productive discussion on belching in my intro to cultural anthropology class on Tuesday, but that's not what I want to discuss in this post. Here, I'm going to explain the very simple exercise that I used to help students in Intro to Bio Anth understand the four forces of evolution. I devised this because my students frequently confused genetic drift with gene flow (the names are too similar, I think), and they often have trouble understanding that genetic drift and founder's effect aren't the same thing, rather one is an example of the other.

My class has 60 students, and we meet in a small auditorium with an aisle down the center. First, I had everyone stand up, and I shifted them a bit so there were equal numbers of people on each side of the aisle. I told them they were two squirrel populations, and they lived on opposite sides of a significant, but not completely impenetrable barrier, like a river. Then, I told them that their shirt colors represented phenotypic variation in the populations. I wrote each shirt color on the board, along with the numbers of individuals wearing each. We discussed the differences in variation in the two populations (only one population had a pink variant, for example, and black was much more common in one population than the other, although it was common in both).

Next, I had them "act out" gene flow (in a completely PG manner). I had everyone from Population 1 who was standing closest to the aisle walk across to the other population. These represented squirrels who rafted across on logs, or whatever. Then, we re-calculated the frequencies of the variants, and talked about how this caused evolution to occur by changing frequencies of alleles. (Yes, we talked about the difference between allele variation and phenotypic variation.)

Then, I had them "act out" genetic drift. I told them that the lowland areas of Population 2's territory had flooded, and every squirrel who happened to be in the first two rows died. The first two rows sat down, and we again re-calculated the variation in the population. We just happened to lose a unique variant in this exercise, which was a useful illustration.

Then, I had them "act out" founder's effect, by bringing the first row of Population 1 out onto the stage and forming their own, new population. It turned out there were no women sitting in the first row on that side of the auditorium, which put a damper on their reproductive potential as a population, so we joked about picking up female squirrels from floating logs. We discussed the genetic diversity of the new population, and how that genetic diversity would change (or not) through time as the population grew.

Finally, we "acted out" selection. I picked the most common shirt color in Population 1 (black), and told them that black-shirted squirrels were more susceptible to squirrel plague than other squirrels. I had the black-shirted students count off 1,2,1,2, and then told all #2s to sit down. We then re-calculated the frequencies of the variants, and discussed the fact that different environmental conditions, with different selective pressures, would lead the different populations down different evolutionary paths. (I asked the students what would happen if the squirrel plague continued to be a problem in Population 1, but didn't effect Population 2. One of the students replied that, in the future, when Population 1 sailed over the river to conquer Population 2, they would likely wipe them out from disease. Nice image of squirrel conquistadors rowing their logs across the river, huh? Kids today read too much Jared Diamond.)

It was a very simple exercise (although time-consuming), and it conveyed only a very basic understanding of evolutionary forces. But, it more graphically demonstrated the concepts than any lecture or textbook example. I'm waiting with baited breath to see if it actually helps them on the exam.