Hi there, I'm Jayme Dyer. This video is about evolutionary trees, which are diagrams that depict
the relationships between groups of organisms. Evolutionary trees can be used to answer some
really interesting questions, like:
What did the first form of life look like?
Where did the COVID-19 coronavirus come from?
Did a doctor actually get in a fight with his ex-girlfriend
and spitefully infect her with HIV?
It was 1994
and the doctor had been having an affair with his ex-girlfriend,
the victim,
for 10 years.
One night after a big fight where they broke up, he went over to her house
and gave her a "vitamin B" shot.
Six months later, she was diagnosed with HIV.
According to the medical records at the doctor's office
the night he administered the "vitamin B" shot
he had also drawn blood from a patient who was HIV positive.
The victim suspected that the "vitamin B" shot was actually blood from the HIV-positive patient
but the doctor argued that she could have gotten HIV from someone else.
I mean, is it even possible to prove where somebody got their HIV from?
Of course it is, I wouldn't be telling you this if it wasn't possible.
The police in the case hired some scientists.
They published this paper and in order to prove whether the doctor was guilty or not
they used an evolutionary tree.
Because HIV, like all viruses,
and all living things,
uses nucleic acids
like RNA or DNA
to store its genetic information.
Any time one organism makes a copy of
its genetic code, mutations might happen.
So as one virus copies itself into two
and two into four and so on,
sometimes the mutation will happen that
will be passed on to all of that virus's offspring
The scientists can take the genetic sequence of different HIV viruses
and they can reconstruct the family tree of those viruses by comparing the differences in the sequences.
The more differences between two sequences, the more distantly related those viruses are.
By the way, sometimes scientists want to construct an evolutionary tree for organisms that we don't have DNA sequences for.
So in that case the scientists use differences in body shape instead of DNA sequences in order to determine relatedness.
So if the doctor infected the victim with HIV from the patient, then her
HIV viruses would be more closely related to the patient's viruses than
to the viruses of anyone else in the community.
To test that, the scientists determined the sequence of viruses from the victim, the patient,
and several HIV-positive people from the community.
And they determined the evolutionary relationship between all those viruses.
Here are the actual results.
But how do we look at an evolutionary tree like this and tell whether the victim's viruses are more closely related to the patients?
To answer that question we need to know how to read evolutionary trees.
An evolutionary tree is a lot like a family tree.
This is my family tree.
The current generation is at the tips of the tree, down here. Ancestors are up here, farther away from the tips of the tree.
One major difference between a family tree and an evolutionary tree is that we don't know who the
ancestors are, since they usually aren't alive anymore. But because they gave rise to their
descendants, we know they existed. The ancestors are represented by each branching point in the
tree. We call those nodes. Each node represents the most recent common ancestor of all the branches
coming off of it. Nodes that are farther away from the tips of the tree represent older ancestors.
But how do you look at an evolutionary tree and tell which organisms are more closely related to which?
You could count the steps?
In this tree, elephants and lions are one step away from each other. But elephants and eagles are two steps away from each other.
Does that mean that elephants and lions are more closely related than elephants and eagles?
The problem is, the real world has more than five animals in it.
There are lions and tigers and bears (oh my!)
and many other species.
On this tree, if I count the steps, now it seems like elephants are really distantly related to lions, but,
of course the relationship between elephants and lions hasn't actually changed, I've just included more animals on the tree.
So, don't count the steps.
Whether there are just a few organisms on the tree, or a lot, it doesn't change the relationships between the organisms.
Another possibility is to see which organisms are closest to each other on the tips of the tree?
On this tree, eagles are farther away from elephants than they are to squid.
Are eagles more closely related to squid than they are to elephants?
To help us think about this, let's go back to my family tree.
I've constructed my family tree as a mobile. There's me.
Now, you know intuitively that I'm more closely related to my cousin, Michael, than I am to my second cousin, Megan.
Now, if I rotate here where my grandpa George is,
now, I'm close to Megan in the tree and I'm far away from my cousin Michael. But I haven't actually
gotten more closely related to my second cousin Megan, right? I'm still more closely related to my
cousin Michael. I've just changed the way the tree looks.
So this is true for any evolutionary tree.
In this tree, eagles are closer to squid than they are to elephants. But if I rotate around this node here,
now, eagles are closer to elephants and they're really far away from squid. But I haven't actually
changed the relationships between the animals, I've just changed the way the tree looks. In fact,
I can rotate around any node and it will change the way the tree looks, but it doesn't change the
relationships between the organisms on the tree.
So, don't see which organisms are closest to each other on the tips of the tree, because
the tree can rotate around any node without changing the relationships between the organisms.
Another option is to compare the most recent common ancestors?
You know that I'm more closely related to my cousin Michael than I am to my second cousin Megan.
That's because my cousin and I share the same grandfather. He's our most recent
common ancestor. But he is not an ancestor of our second cousin, Megan. The most recent common
ancestor between me and my second cousin Megan is our great-grandmother Agnes. She's a generation
older than my grandfather, which is why she's farther away from the tips of the tree. Now, in an
evolutionary tree, we don't know what the ancestors looked like, but we still know they existed.
Each node in the tree represents an ancestor. So the most recent common ancestor between eagles and
elephants is here. It gave rise to elephants, lions, and eagles, but not to squid or octopus. There is
a most recent common ancestor between eagles and squid but it's older, farther away from the tips of
the tree. Since the most recent common ancestor between eagles and elephants is more recent
eagles are more closely related to elephants than they are to squid. So, yes, when you want to
know which organism is more closely related to which, compare the most recent common ancestors.
It turns out, this logic works no matter what the tree looks like. I can rearrange the tree this way
or this way or this way and always the most recent common ancestor between eagles and elephants is
more recent than the most recent common ancestor between eagles and squid. Remember the doctor and
the victim? So the scientists predicted that if the doctor infected the victim with HIV from the
patient, then the victim's HIV viruses should be most closely related to the patient's HIV viruses.
Here are the results. These are the HIV viruses from the victim. It turns out that HIV mutates
so rapidly that there are genetically different viruses even in one person.
These are the viruses from the patient. And the most recent common ancestor between them is here.
And the most recent common ancestor between the victim and anyone else in the community is back
here. And since this most recent common ancestor is closer to the tips of the tree, it's more recent.
And that's why the scientists concluded that the victim and the patient's viruses were
very closely related, suggesting that the doctor did in fact infect her with HIV from his patient.
So, this was the first time that an evolutionary tree was ever used in a criminal court in America.
As for the doctor, well,
he was convicted of attempted murder and sent to prison,
based in part on what we learned from an evolutionary tree!