Node-Counting Is Alive and Well

Evolutionary tree

Image credit: Gregory, 2008

In a previous post I offered help for students struggling with evolutionary trees. In that post, I talked about a particularly difficult final exam question requiring students to interpret an evolutionary tree (a labeled version of the image to the right). I surmised that the incorrect strategy of “node-counting” was responsible for the failure of many students to get the question right.

This semester, I used the same question as a “pop quiz” in lecture. I asked the same true/false question (“The evolutionary tree at right shows that a human is more closely related to a sea lily than it is to a sea urchin”) but further asked students to explain how they arrived at their answer.

Of the 50 students present that day, 34 incorrectly responded that the statement is true. Two typical explanations were, “The sea lily has less nodes between the human than the sea urchin does” and “The human goes through two common ancestors to get to the sea lily and four for the urchin.” The 16 who got it right typically explained their tree-reading strategy correctly. For example, one wrote, “A human shares the same last common ancestor with both the sea lily and the sea urchin, regardless of how close the human and the sea lily are horizontally.”

I followed up the quiz by emailing each of the 16 students who got the question right and asked how they knew the answer. Eleven of those students wrote me back. Of those students, four mentioned the “DNA and evolutionary trees” lab we completed in the previous week. Three remembered it from a brief lecture at the start of the semester. One learned it in 6th grade, and one figured it out by logic (“Since all species to the right of humans were descendants of a common ancestor that’s a sibling of humans, I assumed they were all equally similar to humans.”)

If students had not been allowed to work together, I suspect the number of correct answers would have been much lower. Of the 11 students who emailed me back, five mentioned help from fellow students either as the sole source of information (three students) or as an important supplement to what they learned lab (two students). The four students who never emailed me back all sit near one of the students who remembered the material from an early lecture, so I suspect help was an important factor for them as well.

After seeing the quiz results, I took time in the next class to explain once again how to read evolutionary trees. But I also decided to write a blog post, because I thought it was interesting to learn that many students find conversations with their peers to be useful in figuring out challenging biology concepts. I also enjoyed seeing the diversity of experiences that were memorable to those students who correctly solved the quiz question.

Of course, the bigger problem remains: Node counting really is alive and well. So, this morning I shared the resources I highlighted in the “struggling” blog post with my students, and I will keep reinforcing tree-reading techniques in each lecture that features evolutionary trees. If you have more ideas, please share!

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