If you're seeing this message, it means we're having trouble loading external resources on our website.

If you're behind a web filter, please make sure that the domains *.kastatic.org and *.kasandbox.org are unblocked.

Main content
Current time:0:00Total duration:10:57

Understanding and building phylogenetic trees

EVO‑3 (EU)
EVO‑3.B (LO)
EVO‑3.B.1 (EK)
EVO‑3.C (LO)
EVO‑3.C.1 (EK)
EVO‑3.C.2 (EK)
EVO‑3.C.3 (EK)

Video transcript

when we look at all of the living diversity around us then a natural question is well how related are the different species to each other and if you put that into an evolutionary context relatedness should be tied to how recent did two species share a common ancestor and what we're going to try to do in this video is construct a tree for showing how different species evolved from common ancestors and we're going to do it based on some of these observable traits that we see but this is going to be a huge oversimplification I'm only doing it with five species in five very simple traits as we'll see or as we'll talk about in future videos this can be done in a much more complex way and that's what biologists would do they would look at much more than five traits and they would look at molecular evidence molecular evidence in terms of protein differences in terms of DNA differences to really start to build out what we call a phylogenetic tree so let me write this down that's what we're going to create file o genetic genetic tree Philo comes from the Greek for group or kind or tribe and then genetic comes related to the word Genesis how do these things come about how do the different groups or tribes or in this case how do the different species come about well when you're trying to make one of these trees it's important to realize that this is a hypothesis but you're like always trying to come up with the simplest hypothesis that can explain the observations that you actually see and when we look at these at least the species that we have listed here it looks like there's one that is more different than all the other ones the lamprey here does not have any of these five traits that we are observing so this we would call the out-group the lamprey is the out-group and a lot of times when you need to construct a phylogenetic tree they might provide you something with something that is clearly an out-group here it doesn't have any of these observable traits and sometimes if we're looking at genetic differences it might have the largest number of genetic differences relative to everything else and so it makes sense the the simplest hypothesis is it's common ancestor is most distant into the past with everything else and so let me start to draw this tree so I am going to put deep into the past so deep into the past there is a branching out point where you have the common ancestor of the lamprey and everything else we see here so eventually you have that common ancestor and there's many many species along the way and eventually we get we get a lamprey in present time in present time and so the next thing to think about is all right well how did everything else end up branching well what's what's common about everything else that maybe wasn't common about the lamprey well one common thing is we see that everything else at least that we have listed here have have jaws and so it's reasonable to say all right we have this common ancestor this between the lamprey and everything else at this branching point right over here and then it branched off into multiple species and one of those species must have evolved jaws so let me put jaws right over here so jaws right over there and jaws that's called the jaws are considered a derived trait this in ancestral species that this root did not have jaws we're assuming but at some point they evolved and they stuck around because they proved to be favorable in certain environments or it could have even been things like genetic drift who knows but I'm guessing that it was favorable in certain environments so let's see let's see if we can classify everyone else so now out of the four so we've let's actually cross out the lamprey just for simplification since we've already classified that character now of everyone else we've already thought about everyone's got jaws so now let's go to the next most common trait so and actually let me cross out the jaws to just for keep things simple so I can do it a little bit cleaner so I'm going to cross out the jaws and now let's see the most the next most common trait is are the lungs but not every species that we have left has lungs the sea bass does not have lungs it does not breathe air the way that the animals that live outside of the water breathe air and so they the the next the next point of divergence must be between the sea bass and everything that we have left over so let me draw that so once again I said must be this is a hypothesis I think it's a reasonable hypothesis so let me draw that so this is the sea sea bass and there's a common ancestor between the sea bass and everything else and the antelope the bald eagle and the alligator and at some point that common ancestor diverged into multiple species and one of those one of those child species must have evolved lungs so lungs must have evolved at some point but we're assuming that that wasn't that wasn't on this lineage for the sea bass and once again I'm just trying to find the simplest explanation there might have been some situation where maybe lungs evolve and then Dior then went away at some point you revert you reverted to an ancestral form but we like to go with the simplest explanation this is a property that the bile just will also often called parsimony and actually let me write this down parsimony which in everyday language means cheap if someone tells you you're parsimonious it's a nice sounding word but it means that you are cheap but parsimony in this context say hey we're trying to we're trying to be cheap with come with complexity we're trying to be as simple as possible in our explanation of what's going on but anyway let's go back to what we were doing so we've already put into consideration we have already talked about the sea bass here and we have already talked about lungs alright so what do we have left so we have to talk about the Antelope the bald eagle the alligator and gizzard and fur all right and looks like the bald eagle and alligator have gizzard the antelope has fur and actually we haven't talked about the bald eagle and feathers as yet either all right so it is possible so let's make the next thing between well we could do it we could do it this way and once again I'm trying to do this in real time something that seems so let's let's make a branch here and let's say that that is the branch for let's say that's the branch for the bald eagle I'll say be eagle that's the branch for the bald eagle let's see if I can construct one that will explain the differences between the bald eagle the antelope and the alligator well the evolved eagle and the alligator have something in common they have a gizzard in common so let me make a branching point make them a little bit closer then the bald eagle is to the antelope so let me do that so let me put the alligator there and then I'm going to talk about when we get these derived traits so that is the alligator and obviously I could have written the alligator on this side and the bald eagle on that side or I could have rotated I could rotate at any one of these branching points and then what we would have left is the antelope now let's see if I can account for all of these derived traits and to loap alright so we have the common ancestor of the sea bass the bald eagle the alligator the antelope right over here we have a branching point at some point the lungs where we are hypothesizing evolve in this branch and then this branch well let's say that this branch this is the common ancestor between the antelope alligator and bald eagle and a common ancestor of the bald eagle and alligator they have to they have to get the gizzard so let's put the gizzard down right over here this is where the gizzard this is our hypothesis doing that same colors so that's the gizzard gizzard right over there and so everything after everything that descended from that ancestor that had the gizzard well they're going to have gizzards that's what we're assuming but once again that can be lost this is this is a hypothesis and so we have accounted for the gizzard let me let me cross that out so we have accounted for the Ezard and so let's see we have to account for the feathers and the bald eagle is the only one that has feathers so let me put that here so at some point you have a common ancestor of an alligator and a bald eagle it branches off into multiple species one of which one of which gets a feather or gets feathers and once again I you know that could have branched off into many many things because we know that the bald eagle isn't the only species with feathers but the bald eagle for sure is a species that has feathers and let's see so we've accounted for the feathers now feathers and now we just have to account for the fur the fur of the antelope and so we don't know where this could have happened we might want to look for more evidence to come up with a good hypothesis but someplace along this right branch we could put we could put the fur and so there you have it this is actually a reasonable phylogenetic tree I I practice the part of the biologics of parsimony to come up with the simplest explanation but there are more complicated explanations and we don't know some of those more complicated explanations could very well be true but from this we have a very quick and easy graphical representation of how related different species could be and where they share common ancestors so bald eagle and an alligator based on this phylogenetic tree we would say are more related than a bald eagle is to an antelope they have a they have a more the bald eagle and the alligator have a more common common ancestor or more recent common ancestor right there than both of their common ancestors with the antelope and that would make them more related and if we were doing this for real we would want to look at genetic evidence and and look at the various proteins and say okay do we does does that back this up are bald eagle and alligators DNA do they have more in common with each other than they do with the DNA say of an antelope and many for especially once you get complex there could be many different explanations that we just want to get more and more and more evidence to keep refining our phylogenetic trees
Biology is brought to you with support from the Amgen Foundation