|Figure 1. An 1837 sketch from Charles Darwin's notebooks.|
Modern biologists are just as concerned with appropriately grouping related organisms as was Linnaeus. Since the time of Darwin we have used tree diagrams to represented the relationships among related organisms (Figure 1). Humans and chimpanzees for example can be placed on the tips of branches that can be followed back to a larger branch that represents the ancestor we share in common. To see how we are related to monkeys, bats, dogs, or sharks, we'd have to track back closer and closer to tree trunk and then trace paths back out along more distant branches. The length of that branch-tracking journey represents the distance of the relationship between any two organisms on the tree.
When I started teaching evolution at Saint Michael's College I wanted an authentic activity that students could complete to make an actual evolutionary tree. It's quite frankly boring to simply study trees completed by others and memorizing the branching patterns seems utterly pointless. My quest was for a prepared procedure that would walk students through the actual process used by evolutionary biologists to make the trees that we call phylogenies.
|Figure 2. Data from 2015 Saint Michael's College students.|
|Figure 3. The arbitrary starting tree; click to enlarge.|
We refer to the gain or loss of traits as "transitions" and we used software to count the 251 transitions needed to explain the arbitrary tree. My students then move branches around on the tree and the software automatically recalculates the number of transitions.The goal is to generate the most parsimonious tree, or the tree with the smallest number of transitions. This becomes competitive as student groups report out on their shorter and shorter trees during the lab session. As a homework assignment, the students compare their trees to trees published by evolutionary biologists. This places their work in the larger context and I have found that the comparisons generally fare very well. As a result of writing this blog I think I'll ask my students to use their trees, together with published trees to write several hypotheses about the traits of skulls they have not yet seen. Sounds like a whole new lab!
|Figure 4. A tree made based upon the observed skull traits.|
What I don't tell my students is that the software has a feature that does the work automatically. The software is unbiased; it won't place the polar bears near the brown bears just because that might make most sense. Instead, the software makes groups that minimize the number of transitions and generates a tree based solely on the data (Figure 4). This data-based tree requires 121 transitions and places organisms together in ways that very closely match the tree of life, a phylogeny generated by professional biologists.
Importantly, 15 students dreamed up 46 traits without consulting published work and without reference to what some other biologist might think of as a 'good trait'. They worked in groups and the only criteria for choosing traits were: that they could communicate the trait to their peers; the trait should occur in at least 2 skulls; the trait could not occur in every skull.
This is our third year running this experiment. We use different skulls each year to keep it interesting. The list of traits that students come up with changes each year also. If other teachers would like to run this exercise on skulls or on other organisms you can find all of the needed information in this short paper.
Figure 1 from Wikimedia Commons. Other figures generated at Saint Michael's College.