Hi. It's Mr. Andersen and in this podcast I'm going to talk about the cladogram.
Cladogram is essentially taking characteristics that are shared by organisms and then putting
together a hypothesis of how you think they may be related. Or a phylogenetic tree. It's
not a phylogenetic tree, it's just a guess at how that might occur. And so luckily I
was just working in some pond water yesterday and I discovered these seven creatures. And
I've named them after my self of course. They're called Andersenii aquaticus. And they're all
subspecies of that. But basically they have a lot of different characteristics. And so
if we were to put those together in a cladogram, well, when you were growing up you probably
watched Sesame Street and they used to play a game called one of these things is not like
the other. And so that's a good way to think about a cladogram. And so basically which
of these doesn't look like the others? Which of these is an out group? Well you should
be able to figure this out. First one would be C. And then try to guess before I actually
click on it. The next one would branch of according to this would be B. Because it doesn't
have an eye. But we're going to have that in G. So the next would be G. Try playing
along with me. What's going to be the next one? That's right. That's going to be D. And
then after that? A. And then after that? F. And then finally after that we have E. So
you could probably do that. Just put them in order looking at what characteristics they
have. So the out group is going to be the one that doesn't have any of these characteristics
or maybe one. And so basically a cladogram is going to look like this. It's going to
be a hypothesis of how these things came to be. And so I could look down here and I could
say well right here there's one characteristic. And that would be that there is a cell membrane.
In other words all of them have a cell membrane. And so to show you how this is read, basically
everything that branches out from this point is going to be a cell membrane. What is this
branching point? You can think of that as a common ancestor between all of the organisms
that are on here. Or if I were to go right here this would be a circle. Or if I were
to move up a little bit what's going to be the next thing that comes, the next thing
is going to be the eye. And so I could keep going all the way up. These are characteristics
that are added. And we use those to make a cladogram. Now there's some interesting things
you should know. So if we look at this cladogram and say that it's true. Who's more related?
Is F more related to E? Or is it more related to A? That's a really good question. And so
basically how do we know which it's related to? And I would say the right answer is E.
You have to understand that a cladogram or a phylogenetic tree for that matter, time
is going to go in this direction. So we're going to put time on this side. In other words
this is going to be the most ancient common ancestor. And then we're going to work progressively
up. And so that means that F and E would share a common ancestor right here. But F and A
are going to share a common ancestor all the way back down here. And since this is way
farther back in time, we would say that F is more related to E. But this is a pretty
simplistic one. And again it's just a guess as to how it might occur. A tougher question
might be, well let's add a new one. Let's say I discover something like this. This is
species H. Or subspecies H. Where would you put this on this cladogram? And when I talk
to students in class there's three areas or four areas where they might want to add that.
So some people think we should add it kind of like branching off here. Some think we
should do it right here, branching off D. Some think it should be something like this.
And some think it should look something like that. And so these are all plausible. In other
words these are all guesses. And we could put way more in there than this. But what's
the quote unquote right answer? Well if I had to put this somewhere, what I would do
is I would put it right here. In other words I would have H branching off here. And the
reason why is that if I were to put it way back here before the eye evolved because we
didn't have that eye, well it could have occurred like that, but it would be odd that it would
also evolve the same exact teeth and the same defensive spikes that we find farther up.
So a more plausible guess would be that it branched off of this A and then it lost its
eye. Now how could it lose its eye? Well if we were to put one of these in an area where
there's no light, then the eye wouldn't be an advantage. And so lots of times when we're
adding a cladogram we just add characteristics that are added. But know this, that it's just
as likely to lose something that we've gained as to gain something new. And so this would
be kind of a pretty good guess as to where H goes. And what I'm talking about now is
ockham's razor. In other words I'm talking about the theory of parsimony. What the heck
does that even mean? Well it basically means this. If all things are equal, the simplest
explanation for how that occurred is probably the most right. In other words, if I come
out in the morning and there's a glass of milk that's spilled in the kitchen, I could
come up with a bunch of different hypotheses. It could be aliens. It could be big foot.
It could be a massive earthquake. Or it could be my cat. And if I were to look at all of
these, well it's most likely going to be my cat. It doesn't mean that it's right. It doesn't
mean that aliens didn't come down and spill my milk. It just means that it's more than
likely the cat that did it. And so when you're building a cladogram you kind of try to do
that. And you try to use this idea of parsimony. The idea that the simplest explanation is
the best. So let me show you an example of one of these. This would be a cladogram that's
created for primates. And if we look at this one again to go back, in general in biology
we used to us all of these characteristics to put organisms in a cladogram. Physical
characteristics. But you can imagine that's not a great way to classify life. Or to even
show its evolutionary relationships. Today we now have DNA. We have genetics. And so
you can create a cladogram like this by just sequencing the DNA in all these different
organisms. And then throwing them into computer. And it could create a plausible kind of a
cladogram. And so where are you? You're right here on the human side. And so if we were
to go across, from the left. So if we look at these two, the lemur and the loris, they
share a common ancestor right here. But they also show a distant common ancestor with all
of the primates on here all the way back here. If we were to work across to the tarsier or
the new world and old world monkeys, is kind of a weird term. New world is a South American
monkey. So an example could be the squirrel monkey. An old world monkey would be like
a baboon. And so all of these things have tails. And now we move up to the apes. The
first one would be the gibbon. That would be this first branch point. Then we have the
orangoutang, the gorilla, the chimp. And we could put bonobo branching off here. And they
we finally have the humans. And so again this is just a cladogram. It's a hypothesis of
how these things are related. And you're looking at characteristics that are shared by all
of these. But the big characteristics we're looking at now are homologous structures of
DNA. In other words where the genetics match up, that means they had a common ancestor.
Does it mean that we turned from gorillas to humans? No. It just means that we had an
ancestor way back at this point. And so those are cladograms and I hope that's helpful.