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clades rotate = equivalent phylogenies


Stephanie Keep, Misconception Monday: Do You Talk Tree? Part 3 June 29, 2015

Also see the pipe-cleaner manipulative:

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!

Using Pipe Cleaners to Bring the Tree of Life to Life. Kristy L. Halverson. The American Biology Teacher, Vol. 72, No. 4 (April 2010), pp. 223-224

Using Pipe Cleaners to Bring the Tree of Life to Life. Kristy L. Halverson. The American Biology Teacher, Vol. 72, No. 4 (April 2010), pp. 223-224

Take every opportunity to remind them that any living organism—be it bacterium, fungus, or ape—is equally evolved….

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:

Pop Quiz!

  • Troodontids and oviraptors are both positioned equidistant from the dromaeosaurs on this tree. Is one of them more closely related to dromaeosaurs than the other? How do you know?
  • Which group on this tree is most closely related to birds? (Hint: This is a trick question.)
  • True or false: The producers of Jurassic World missed a golden opportunity to inspire a new generation of paleontologists by showing off all the cool things we have discovered in the last twenty years.

There is a chance that one of those questions has nothing to do with this misconception…

Misconception: The fewer nodes separating two taxa, the more closely related they are.

Correction: The more recently two organisms share a common ancestor, the more closely related they are.

Number of nodes might seem at first like a reasonable basis to judge relatedness. The more nodes you pass through along a lineage, the further back in time you’re going, right? But it doesn’t take long to realize that it just doesn’t work.

Remember that trees are models. The person constructing the tree can collapse and expand the tree as needed, and even add and remove groups. And that affects the number of nodes between taxa. For example, here are two trees—one includes sharks and their relatives and the other doesn’t. So in the one tree, amphibians and starfish are separated by three nodes; but in the other, they are separated by four nodes. But, of course, how closely related these two groups are does not shift depending on the tree you’re looking at.

What if we compare number of nodes within the same tree? Does counting nodes to ascertain closeness of relation work then? Nope. Look at the tree here: How many nodes separate the oval and the star? Two (purple circles). How many nodes separate the triangle and the star? Two (green circles).

But applying what we learned in the last misconception, we know that the star is more closely related to the triangle than the oval. In sum, the only way to judge relatedness is by determining how recently the taxa shared a common ancestor—there aren’t any shortcuts.

All images are courtesy University of California Museum of Paleontology’s Understanding Evolution (http://evolution.berkeley.edu).

Related articles

What’s in a Name? Taxonomy Problems Vex Biologists: Researchers struggle to incorporate ongoing evolutionary discoveries into an animal classification scheme older than Darwin. – Christie Wilcox, June 24, 2019, Quanta Magazine

Carl Linnaeus was probably not the first scientist to realize the inherent connectedness of life on this planet. But he articulated and codified it. In the 10th edition of his Systema Naturae, published in 1758, he established a system of naming and organizing life that endures to this day — what we still call Linnaean taxonomy, although today’s system is somewhat different from the five-rank hierarchy he proposed. The principle is the same, though: Life is organized into nested ranks, with each higher tier representing a larger group of related organisms to which the species at the bottom belong.

This ranked taxonomy — domain, kingdom, phylum, class, order, family, genus, species — is foundational to biology pedagogy. Every student learns it, often through a mnemonic like “Didn’t Know Popeyes Chicken Offered Free Gizzard Strips” or “Dear King Phillip Came Over For Great Spaghetti.”

But a growing number of researchers think it’s time for taxonomy to move away from these ranks, or even abandon them altogether. “When a student has to learn it, it also suggests to the student that there’s something special about these groups,” said Andreas Hejnol, a comparative developmental biologist at the University of Bergen in Norway. Yet there isn’t.

The problem that Hejnol sees with the whole system is that the ranks don’t mean anything specific or uniform across all groups of life. Even though species is arguably the most important rank across multiple fields of biology, there are dozens of species concepts in use — and biologists working with different groups of organisms can’t seem to agree on just one. You might think that the other end of the hierarchy would be more settled, but it wasn’t so long ago that domains simply didn’t exist — the three domains we use today (Archaea, Bacteria and Eucarya) were only proposed in 1990. At that time, the top rank was kingdom, and there were five of those; now there are at least six, though some say there should be as many as 32. Similar ambiguities plague all the taxonomic ranks in between — even those often considered to be major, distinct and unambiguous, like phyla.

Perhaps this could all be resolved if the scientific community simply agreed upon a definition for each rank, but there’s no consensus for that. “The [taxonomic ranks] could be used in a more meaningful way, and people have even come up with proposals about how to do that. But those have never caught on,” said Kevin de Queiroz, a research zoologist with the Smithsonian Institution’s National Museum of Natural History in Washington, D.C. The obstacle, he believes, is that biologists are accustomed to thinking about the ranks as they are. Small changes might be acceptable, “but in order to standardize them across all of life, some of them would have to change radically.”

These changes may occur over time. But in the interim, it’s all too easy to forget that the ranks aren’t consistently or broadly meaningful. And when people lean on these ranks to form evolutionary hypotheses or examine biodiversity, their science can be fundamentally flawed.

“I don’t think we need to get rid of [taxonomic ranks],” said Ronald Jenner, a phylogeneticist at the Natural History Museum in London. “What we need to get rid of is the unwarranted connotations that higher-level taxa are comparable. Because they aren’t. They are abstractions. They are information storage boxes. That’s all they are.” We can see this clearly by looking at phyla.

What’s in a Name? Taxonomy Problems Vex Biologists. Quanta Magazine


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