3:41

So, it's random indirection over one generation,

whether you're going to increase in frequency, or decrease the frequency.

So assuming that there is more than one allele, so assuming you have

at least two alleles, any allele is about equally likely to increase or

decrease in frequency after one generation of sampling error or genetic drift.

So for example, if the allele frequency of big A, assuming there's two alleles,

big A and little a.

If the allele frequency of big A is 0.6, you're about equally likely to have

a frequency above 0.6 or below 0.6 in the next generation.

However, it's very unlikely you're going to have exactly 0.6 again,

because there is a decent chance you're going to have some non-representation.

Even if you have a very large sample,

it might end up being 0.603 or something like that.

So allele frequencies tend to drift due to this sampling error, and

this is where the term genetic drift came from.

I'm going to show you what would happen in the context of this.

Imagine tossing a coin ten times this is similar to having P equals 0.5.

You may get five heads from these tosses.

Right?

So that would be exactly what's expected in the sense of the average, right?

Since you're equally likely to get heads or tails.

Now getting more than five heads or

getting fewer than five heads is equally likely.

This shows you the distribution that you expect the probabilities.

I'm probably getting exactly five heads is actually less than a quarter.

Your probability of having ten heads is extremely low, probability of having zero

heads is extremely low, but you're about equally likely to have slightly more or

slightly less than five heads when you toss the coin.

Okay, in fact, actually the probability of getting ten heads is about one

in a thousand, so very unlikely unless it's a weighted coin.

Well, the same concept applies to populations.

So if the original population has allele frequency of big A 0.6,

we see below what happened in one generation of genetic drift.

The probability of having it greater than 0.6 versus

less than 0.6 is about the same.

And this is the case if you have 10 diploid offspring.

5:39

Okay, now let's see what happens as we look at this over

multiple generations briefly, but just looking at the individual steps.

So this magnitude change compounds as it relates to the population size.

Now, regular changes are going to occur if the population size is smaller.

You'll have greater individual deviations in allele frequency per generation

as the population is smaller.

So let's look at three different population sizes.

Let's look at population size 400.

You're starting in this case, this is looking at generations on the x-axis and

allele frequency on the y-axis.

You're starting at 0.5.

And what's happened here is we're starting one, two,

three, four, five, six, seven, about eight different populations at 0.5.

And this shows random changes over all eight population.

After about a hundred generations we see there's a few

that are kind of close to 0.5.

Some of them are higher, and some of them are lower.

So we have this genetic drift that's compounded over time.

This shows this little green figure shows the approximate average size of

a change in one generation in each population.

Notice what happens if we reduce this.

Instead of 400, what if we looked at a population of size 40?

What we see in this case, much bigger changes, we have several alleles that have

gone all the way to 100% or 0% in some of these populations, and

some are still segregating, but again, you're equally likely to go up or

down on an individual generation, and the individual step size now is much bigger.

You get bigger random changes in allele frequency with smaller population sizes.

All right?

Let's do the same thing.

This was with population size of 40.

Let's look at it in the extreme with population size four.

7:20

Wow, all variation is lost.

We'll come back to this, actually, very shortly.

But you see that the individual step sizes here is very large.

Again, in each of these case, these are eight different simulations,

each with population size four and starting the allele frequency at 0.5.

So with those examples in mind, can we solve, mathematically, how

big the individual steps are that result from genetic drift in a single generation.

On average, the answer is yes.

So we can use the variance.

Now recall we used the variance before when we were

studying heritability a couple of lectures back.

We look at the variance in allele frequency due to one

generation of genetic drift.

So looking at how much of a spread is there.

The answer to that is pq divided by 2N.

Where p and q are the two allele frequencies and N is the population size.

We use 2N because we tend to work with diploid organisms.

Now, how do we use this to actually look at average changes?

Well, we can look at the standard deviation.

The standard deviation of this would be an estimate

of the average allele frequency change in one generation.

In fact, mathematically, it would actually be a slight overestimate, but

it still gives us an idea for illustration purposes.

So, how do we get the standard deviation from the variance?

Well, the standard deviation is always the square root of the variance.

So we have this formula here for

the variance, we take the square root of that, as illustrated here, and that gives

us the average of the frequency change from one generation of genetic drift.

So let's apply this to examples that I just showed you,

the figures from a couple of slides back.

When we had a population size of four,

we had starting allele frequencies of 0.5 and 0.5.

The average change based on this formula should be about 0.18.

What these means by the average change is that if you start with an allele frequency

of 0.5, it's likely that you will go up to about 0.68 or

likely that you might go down to 0.32.

That's sort of an average change.

The change could be more than that you could go to 0.70, you could go to 0.62,

so maybe is larger, maybe smaller of a change, it also could be up or

it could be down.

We don't know the direction of genetic drift from one generation.

This gives you an idea of the average step size, 'kay?

Now notice, for this one is 0.18.

If a population size is 40, the average changes is quite a bit smaller,

as we witnessed.

In this case,

the average change should be only an allele frequency change of about 0.06.

If the population size is 400,

the average change in one generation of genetic drift is 0.02.

Now, you notice with those very small ones, like the population size of 400,

that's why in the example when we're looking at a population size of 400,

no allele was ever lost or fixed.

We always still had variation in the population.

Because individual steps are very small, and

even over a hundred generations, we still retained variation in that population.

In contrast, when we looked at the very small population, population size of four.

We lost all variation very quickly,

because the step sizes are very big, there's very likely to get to 100% or 0%.

Okay, so what does it take home messages from this lecture as a whole?

So the take home messages from this video, drift is strongest in small populations,

drift is neither predictable in direction, nor

exactly replicable in degree in one generation.

That you saw with all those different populations even when we started it over

and did it again, they didn't all follow the exact same track.

They all have the same on average change allele frequency but

some went up, some went down, some had a little bit more than average,

some had a little bit less than average at times.

It's not exactly replicable in degree, and it's not predictable in one generation.

Very important to end on there.

And finally, drift can change big changes in allele frequency over time.

We'll pick up on this in the next video.

Thank you.