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Phylogenetic Tree

Teacher/Instructor Patrick Roisen
Patrick Roisen

M.Ed., Stanford University
Winner of multiple teaching awards

Patrick has been teaching AP Biology for 14 years and is the winner of multiple teaching awards.

A phylogenetic tree is a branching diagram that shows evolutionary relationships. These trees, often compared to family trees, are constructed using a variety of evidence generally using DNA. In phylogenetic trees, sometimes the lengths of the branches represent time since a group split from each other.

People learn best, a lot of times, by looking at pictures and diagrams. It helps us really understand what's going on. So scientists when they're trying to figure out who's related to who, and how best to explain it, they often turn to diagrams called Phylogenetic Trees.

Now you look at that name and you think wow that's a scary idea. But it's actually not a hard thing to understand, because Phylogenetic Trees are essentially the same thing as a family tree. What does a family tree tell you? Well, a family tree shows you who's related to who, and that's what the purpose of a Phylogenetic Tree is for. It's a branching diagram that shows evolutionary relationships. Which organisms gave rise and are most closely related to which other organisms. Like I said here, it's very similar to a family tree.

Now how do you construct a family tree? Well, you might start looking at records that give evidence of who gave birth to who. But a lot of times you may not have those written documents, so we can turn to DNA testing, and see whose DNA is found in who. That's how they've constructed some of the family relationships of royal families like Thomas Jefferson's family or his descendants, have been tracked that way.

Well, similarly, scientists will study the DNA of various different species. Then compare their DNA, compare their RNA, compare their proteins, all these different molecules to kind of figure out which organisms, which species are mostly closely related to who.

Now when you look at these diagrams, sometimes they're constructed very, I want to say diagrammatically, but other times they'll show different lengths of branches. If you're seeing these very specifically laid out lengths of branches, then they're trying to give you a sense of roughly, how much times is between those organisms. As in how long ago did they branch off from each other.

If we take a look at this. This is an example of a phylogenetic tree. You can see down here at the bottom, just like in a real tree, there is a central trunk. Well, this is implying that there is some kind of common ancestor. From that common ancestor, we have these three major branches. From those, they branched out again to give rise to all sorts of different organisms.

You can see way down here at the bottom, there is the common ancestor, you're up here in the animal kingdom. You can see that there's been major periods of time that have separated us from all these other creatures. So that's a phylogenetic tree.