This week I am cutting to the chase! No long jargon-laden intros, no musings about jargon—now it’s all about the trees, baby! In fact, it’s all misconceptions about trees, and we’re going to tackle three of them.
Misconception number one, come on down!
Misconception: Evolutionary trees have “main lines” of evolution and “side tracks” of evolution.
Correction: There are no “main lines” or “side tracks” in evolution or (consequently) in evolutionary trees.
It might seem intuitive that the lineage from the root to the furthest branch tip represents the main artery of evolution, and all other paths are secondary vessels. But that just isn’t how evolution works. Evolution does not have a trajectory—humans may love to regard themselves as occupying the pinnacle of life, but I’d wager that just about any other creature on the planet (except, maybe, teacup yorkies or hamsters) would beg to differ. The best way to counter this misconception with your students is to show them diagrams like the one here from UCMP.
Remind them that nodes can rotate! If you have 15 minutes, you can have students make their own manipulable evolutionary tree with pipe cleaners so they can actually rotate nodes themselves! And take every opportunity to remind them that any living organism—be it bacterium, fungus, or ape—is 100% equally “evolved.” Remind them that sea cucumbers can spew out their guts and run away! (And to answer your next question, no, I will never ever stop talking about gut-spewing sea cucumbers. Holothuroidea FTW!)
Misconception: The closer two organisms are on an evolutionary tree, the more closely related they are.
Correction: The more recently two organisms share a common ancestor, the more closely related they are.
You can’t draw any conclusions from the left-right positions of organisms in an evolutionary tree. (And why is that? Say it loud; say it proud: nodes can rotate!) The only way to figure out how closely two taxa (organisms or groups of organisms) are related is to trace their ancestry back until you hit their common ancestor. So in the tree to the right, the circle and triangle are close together, but they are more distantly related than the triangle and the oval. Why? Because the common ancestor of triangle and oval lived more recently than the common ancestor of circle and triangle. Remember that time runs from root to tips on evolutionary trees, so the further back toward the root you have to go to get to the common ancestor of two taxa, the further back in time that ancestor lived. The further back in time the common ancestor, the longer the two taxa have been evolving independently, and therefore the more distantly related the taxa are.
How do you reinforce this idea with your students? Like this: