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22. Evolutionary Genetics

Phylogenetic Trees


Phylogenetic Trees

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Hi in this video, we're gonna be talking about philo genetic trees. So we've mentioned philo genetic trees and a few different topics throughout this course. But I want to give a little bit more vocab into how to actually read them and what all the lines and dots mean. So philo genetic trees are evolution. Revolutionary representations of what's called Phila ginny. So what is file a ginny? Villa ginny is the evolutionary relationship of a group of organisms. So fill a genetic trees are showing the evolutionary relationships of a group of organisms. And so this tree consists of multiple structures. First we have a node and each node represents a different organism being compared to types of notes. There's a terminal node and this is where the line ends and we can see the organisms usually present day organisms for which we have data. So we know what they are, we know what they eat, we know what they do because they're living today. For the most part, the internal notes are representing common ancestors that existed before divergence. We may or may not have information about them, but we know that they exist because they were their common ancestor that then created multiple organisms that now exist today. So those are the notes. Then we have the branches and the branches represent evolutionary connections between organisms. So um and these branches length usually represents the amount of time between divergence. So I'm going to show you that on this tree down here and just once this tree down here in just one second. But first I'm gonna explain a rooted tree. So a rooted tree is a tree with an internal node. So this common ancestor node um that is a common ancestor to all other nodes on the tree. So this is the one common ancestor that created all the organisms that are present on that tree. So when we look at this, this philo genetic tree of life, it's consists of every organism on earth. So here are some of the organisms that these terminal nodes that exist today, Right. And these are kind of big ones. Right, This is plants. So that is huge. If you were to go this out further, it would be very detailed. Um But we're not gonna do that. So we have these terminal nodes here. What you can see is that there are these internal nodes where the branches which are these lines meet. So internal node, here's another internal node. And each one of these nodes represents a different organism that used to exist that eventually evolved and became these two different organisms. And so um so here are the terminal nodes, all these things out here again. Remember that this is a very big philo genetic tree that the terminal nodes go out for each one of these classifications much farther. Then you have the internal nodes, which are these internal ones. The branches represent this relationship and the branches length determines the length of time between divergent. So this one is very long. Whereas these three here are very short compared to this one. So that's going to represent how long it's been since this diverged from this and this these three diverged from that internal node. And then finally we would say that this tree is rooted because all of these organisms can be referred back to this um internal note here that is representing this single common ancestor that all of these um different organisms eventually evolved from. So that is all the terminology you need to know about a philo genetic tree. Now how are follow genetic trees constructed. So they're often constructed using hm Ology. And so we've probably heard this a lot in reference to DNA sequences but it can also be for more. So hm Ology refers to similarities and the similarities are shared between various species and there due to some kind of common shared ancestor that eventually developed into both of them. And so hm Ology can refer to D. N. A. Sequence but it also can result to fanatic traits. I'm gonna show you an example of a fanatic trait that would represent apology in a second. So um a mono fill ethnic group or clade is a group of species, all descended from the group's most common ancestors. So they're all um descended from this one single common ancestor. And then we say that the logistics approach to constructing a fila genetic tree sort of makes every tree. So if you have six organisms and you want to know how they evolved. Well, there's all these potentially different ways that they could have evolved. If you label them 123456, the first one could have diverged the second one. It could have evolved this way. It also could have done it this way, so on and so forth. So it's actually if you are interested in the math of it, it's actually six um bacterial different ways that this biology genetic tree could have formed. If you don't necessarily remember that from math, it's fine. It's not important. Just know it's a big number. Um and the there's obviously a ton of pathways, right? It's going to be a huge number of potential ways that all six of these organisms could have evolved. And so the clinic sticks approach actually looks at all of them, they write out all of them and then they use this process called the principal of parsimony, which says that the tree that is correct, is the simplest one. So you look at all the trees that you've written out six factorial for our example and you say which one of these is the most simplest and that one you say is the correct one. And so um these uh it can be very difficult. I'm not saying that it's an easy way or that it's a foolproof way. It's definitely not. But it is a fairly good way of getting the file a genetic tree to be as close to accurate as possible. So here are examples of hm Ology in a fanatic trait. So each one of these colors representing these bones. So this is here a fin I think. And this is obviously an arm or a claw of some kind. And you can see that um the colors of these bones are the bone structures are shared between these two very different structures. And therefore it's very likely that even though this organism has kind of a fin or a wing and this one has an armored claw that eventually there is some common ancestor here that had the blue bone, the yellow bone and the red bone that it then passed on to these two different organisms which then evolved them to become different things. Whereas this became claws. These kind of shrunk these bones became very the yellow bones became sort of tall and long where these ones became short and thick and so on and so forth. But these are still homologous structures because usually they're um they they resemble each other the way that they're formed and therefore they likely came from some common ancestors. And by examining these. Hm. Ology is both in terms of the epic traits here but also in terms of the D. N. A. Sequences um scientists are able to piece apart, you know which organisms came from a common ancestor and how far apart or how long ago did they diverge. Um So that is sort of how they scientists go about designing these bio genetic trees. So with that let's now move on.

Which of the following structures on a phylogenetic tree represents the evolutionary connections between organisms?


Which principle is NOT used to construct a phylogenetic tree?