Hi. It's Mr. Andersen and welcome to Biology Essentials video number 3. This
is on genetic drift or random evolutionary change. What I am going to talk about in here
is how we can get changes in a gene pool. And so this right here represents a gene pool
and then these represent the alleles inside that gene pool. And so let me start it going
for just a second. And so inside here we've got the original population. If you weren't
paying attention, I could write some of this stuff out. So we had a p value and a q value.
P value is the allele frequency of the dominant. In this case let's say the dominant is red.
And the q value is going to be the recessive. And so in the first time we've got 10 and
10, and so our p value is going to be 0.5 and our q value is going to be 0.5. So that
would be in the first generation. Now we randomly choose, or we have sex, from that original
population to create the second generation or we get this remixing of the genes and you
can see in the second one that our allele frequency has dropped quite a bit. It's now,
of the dominant, we have 0.3 and the recessive we have 0.7. On the next one, it even drops
more. Now what's making it drop is simply chance. So now it's 0.15 and 0.85. And now
it goes to 0.2 and .08. And then eventually it goes to 0 and 1.0. And so what we've seen
is a drift or just due to random size of the
population we get the elimination of that red allele or that red color. And that's genetic
drift. If we were to graph that out and we were to put the p value here at 0.5 and 0.5.
And I were to graph that, let's get it in the right color, the
\i I \i0 value went from 0.5 down to 0.3 down to
0.15 and then up to 0.2 and then it eventually dropped down to 0. So this would be zero here.
And then if I do the blue color, the blue color went all the way to 1.0 and kind of
mirrored that. So it went like this, like that, like that and then it actually went
like that. And so when we graph those allele frequencies over time, what we would find
is that the values would tend to drift. In other words just due to random chance, those
allele frequencies are going to change like that, or move in different directions. And
so, let me give you some actual data based on that. Let's look here on the internet.
And so this is a simulator. It's a population simulator. And so what I can do is I can actually
have it do the coin flips inside it. And so we're going to start with a population of
50 right now. And if I just let it run, you'll see that there's going to be random drifting.
And so we could think of this as the p value and this as the q value. And you can see that
it eventually eliminated over time. Now let's try to increase the number though. So let's
say we go from 50 to 500 and we reset the population this time you'll see that there's
actually less drift. And the reason why, we still get a little bit of drifting here is,
that you have the law of large numbers taking over and so it's less likely to get that change
just due to chance. Well let's really make the number large. Let's make it like like
5000. And if I click on go, you can see it's going to take a lot longer for my computer
to actually do this, because it takes awhile to do all of those coin flips, but you can
see there's very little drift. And so one of the things that's going to keep an allele
or a gene pool the same is going to be the law of large numbers. So let me talk about
what I am going to talk about. So in this we knowthat the gene pool should remain at
equilibrium and if we ever change our gene pool we call that evolution. And so in the
first two podcasts I talk about natural selection and how natural selection can cause changes
in the gene pool. In this one I am dealing with more of these one up here. These things
that are based more on random chance. Because random chance has a lot to do with natural
selection as well. And the big thing I am talking about is the size. And so if we decrease
the size then genetic drift starts to take over. So for example let's say we have a population
right here and then we have a smaller population that breaks off of that. Maybe they're birds
that are blown to an island. Now chance is going to take over and so these two populations
are going to start to vary. Two real world examples of genetic drift are the bottleneck
effect. Bottleneck effect is when you have a large population and that large population
kind of gets squeezed through a bottle neck. In other words the population is going to
get smaller. Even though the population may recover on the other side of that bottleneck
there are going to be huge implications as a result of losing all of that genetic diversity.
An example I'll talk about is the the almost extinction of the Northern Elephant Seal.
And that is a human cause. The next thing I want to talk about is the founder effect.
The founder effect is similar but essentially it is one founding population making up the
forming population. So I am going to talk about an island in the Pacific called Pingelap
where a founding population made some huge changes in the phenotypes of the offspring
that came from that. So this would be genetic drift for a population of 20. As we increase
the size, you'll see that less of that is going to be the change. And so when we talk
about an isolated population, we could say that this here represents a gene pool and
then all of these colors inside it are going to be the alleles. And maybe it starts with
a 50-50 frequency to start with. But once you break off and have smaller isolated populations
from that, chance can just take over. And so you can have the elimination just due to
chance. Not new adaptation at all. And so an example of where this might play out, let's
say this is a population of tortoises that float to an island. They're exactly the same
as the tortoises on the mainland, but due to just random chance you're going to get
changes to the point where this is actually becomes a new species. Now there could be
adaptation here as well, but just chance in itself has huge implications when it comes
to populations. Let me give you a couple of real world examples of the bottleneck effect
and the founder effect. Bottleneck effect remember occurs when the population gets squeezed
through a small bottleneck. So these are elephant seals. Elephant seals were hunted almost to
extinction. The Northern Elephant Seal live along the West Coast of the US down through
into Mexico. And their population had dropped down to, some scientists think maybe dropped
down to the point of there were just 50 left on one island in Mexico. So this was in the
1800s. And so their population was squeezed through a huge bottleneck. If you look at
the Southern Elephant Seal, the southern elephant seal lives way down here by Antarctica and
didn't see that pressure. They look very similar, they just live in different areas. So they
are very closely related. But some scientists and you can see the citation right here, some
scientists took a look at the bottleneck effect and what effect it had on the Northern Elephant
Seal. It's population, remember, had gone from a few hundred thousand down to fifty
and then had gone up to, I think now it's over 150,000 left. So it had been squeezed
through this tiny bottleneck. And so what they did is they actually looked at elephant
seals before the bottleneck and after the bottleneck to see how they were effected.
Now you might think to yourself how could we do that? How could we look at them before
they actually went extinct, which was in the 1800s? Well what they did was that they grabbed
a number of skulls from the Smithsonian. So they've got about 11 skulls pre-bottleneck
and then they were able to extract DNA. And so in this case what they did is they looked
at the DNA. They looked at mitochondrial DNA. That's the first thing that they did in their
study. And what they found is that they had lost a huge amount of genetic diversity. In
other words, when they looked at these pre-bottleneck Northern Elephant Seal DNA there was a huge
amount of diversity, but after the bottleneck they're very very similar. The second thing
they looked at was, they wanted to look at symmetry. They wanted to see if that actually
can effect the skull itself. Can it effect what they're like? Can it make them less fit
phenotypically? And so what they did here to kind of put out the data is that they had,
here's Southern Elephant Seal, here's pre- bottleneck Northern Elephant Seal and then
here's post bottleneck Northern Elephant Seal. And what they were doing was looking at symmetry,
symmetry of the skull. Does the right side look like the left side? And so they measured
this value right here on the lower mandible. And they graphed the left mandible distance
versus the right mandible distance and you can see that the Southern Elephant Seal, there's
a really nice linear relationship. They then looked at the same thing with these old skulls
from the Smithsonian and they found that there's a linear relationship as well and you can
see the r values are close to one which means it is really close to a linear relationship.
But then they looked at the symmetry of the post bottleneck Northern Elephant Seal and
they find that the data is all over the place. In other words their skull is very asymmetrical.
What does that mean? Well when you decrease the DNA you're actually having it manifesting
itself on the morphology or the outward appearance of the skull. And so that could make them
less fit to changes in the environment or more susceptible to another extinction or
another bottlenecking effect. Another example related to humans would be the founder effect.
So the founder effect can happen in any organism. But the founder effect is when you have a
small population that finds a new area or is the founding population of that new area.
And so I didn't know about this, but maybe you did. In the, in the South Pacific we have
what's called the Federated States of Micronesia and in there is a tiny little island called
Pingelap. And it was doing well back in the day but In the 1700s they had a massive typhoon.
And that typhoon swept through Pingelap. And as it did that it killed almost everybody
on the island. So their population dropped down to around 20. So those 20 people were
the new founding population of Pingelap. And all people who live there today are descendants
of that first 20 people. Now what's interesting is that the leader of Pingelap, I'm not going
to try to pronounce is his name, Nahnmwaki Mwanenised, but the leader of Pingelap one
of these 20 survivors, actually had an odd form of color blindness, where he had complete
colorblindness. In other words, when he looked at a Macaw, like this he sees it in gray scale.
He couldn't see any color at all. This is really rare, something like 1 in every 33,000
people in the United States have this. But since this was the founding population, very
quickly 5 percent of the people and now I think the numbers up to around 10 percent of the people
on Pingelap have complete colorblindness. And that's due to just the random chance in
that founding population. Now the number is going higher than it was originally and so
that suggests that there's inbreeding as well. And if this is the leader of the population
there could have been a quite a bit of inbreeding as well. And so that's an example of a founding
effect. It's just the chance of who happens to survive or who happens to land on that
island or in that one area that can create huge repercussions for the rest of the time
of that population. And so that's genetic drift. Again, it is randomness but it gets
bigger and bigger and bigger the smaller the population is. I hope that's helpful.