Monday, February 25, 2013

Hey! That worked! (A useful exercise on the four forces of evolution)

On the first exam in my Physical Anthropology class, I always ask either "Define genetic drift", or "Define gene flow". Every year, about half the class gets the question wrong, because they can't remember which is which. I explain the concepts in class, we do an exercise on them, they're in the book, they're listed on the study guide. Yet, every year, students confuse the two concepts.

But if students can't remember the difference, then later readings/lectures/activities on human evolution will not make much sense. (Seriously, try explaining multiregional evolution when half the class hears the words "gene flow" and thinks "Oh, like Pitcairn Island?".) 

This year, I tried something new. I created an activity that made students apply the concepts, not just see them acted out, as was the case with my old activity. And - joy of joys! - only about 15% of the students got the question wrong on the exam! Success!

Here's the activity:

Students were divided into groups of four, and given the following scenario:

You are a member of the Lovegood-Scamander Expeditionary Force, charged with tracking down the elusive Crumple Horned Snorkack (Snorkackus crumpcornius). After an arduous trek to a nearly inaccessible mountain valley in Sweden, you found the only known populations of this elusive animal, living on a series of isolated islands in the middle of a cold, deep lake. You have taken extensive notes and measurements, and your results are shown on the following slide.

Note: all of the snorkacks are the same species, but there is variation within the species by population.

I gave the students a simple map, made very quickly with Google docs. Since this activity worked well, I'll have to make a nicer one for next year.

[Drat, I can't figure out how to paste the map here.]

The students were asked to look at the variation in snorkacks as described on the map and to do the following:

1- As a group, take five minutes to describe the variation (differences) that you see in the different populations of snorkacks. You should describe only, without adding your interpretations or explanations. What was actually seen or measured about these populations?

2 - As a group, spend fifteen minutes creating at least three hypotheses to account for the variation you described. You must use the concepts of natural selection, gene flow, and genetic drift in at least one hypothesis each. (For example, you could have one hypothesis that explains the variation entirely through natural selection, one hypothesis that explains it through genetic drift, and a third that explains it through gene flow. Or, these could be combined in various ways. Just make sure you use each concept at least once.)

3 - As a group, take ten minutes to discuss what data you would need to test your hypotheses. Think widely and creatively. Do you need to take genetic samples from the populations? Do you need to measure the average temperature on each island? Make sure you know how the data would help support or refute your hypothesis.

Afterward, the students shared their hypotheses. It was a useful exercise. Not only did students apply the evolutionary concepts, but we had a useful conversation about the nature of scientific inquiry.

Best of all, students remembered the difference between gene flow and genetic drift on their exams!

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