We now want to take a closer look at the 5 assumptions of Hardy Weinberg equilibrium. And these five assumptions are something that you are very likely to be responsible for, so we'll go through them one by one. Now when we start talking about Hardy Weinberg, we said that there must be random mating in the population and no evolution. Now that's only 2 assumptions. So, how do we get to 5?
Well, if we look at evolution a little bit in a more specific way, we can break it up into four ways that a population can evolve. So you take those four ways plus that non-random mating. That means when we look at things more specifically, Hardy Weinberg is going to make 5 assumptions. Alright. So let's look at them.
Our first assumption is going to be that there needs to be random mating in that population, and that's because non-random mating will affect genotype frequencies. Remember when we introduced Hardy Weinberg, we said, you know, imagine you're taking all the alleles in the population, putting them in one bag, shaking it up, then to get the individuals for the next generation, we sort of reach in that bag randomly and pick out pairs of alleles. But if you have non-random mating, right, if certain genotypes are more likely to mate with each other, that is not like putting all those alleles in one bag. And in that next generation, you're going to have more or less of certain genotypes than you would otherwise expect from Hardy Weinberg. Now importantly note, the same individuals are still mating, they're still passing on the same alleles.
They're just getting paired in a non-random way. That means that this does not affect allele frequency, and that's important because remember, a change in allele frequency is our measure of evolution. So nonrandom mating will push things out of Hardy Weinberg because it affects genotype frequency, but on its own, it is not a cause of evolution. Alright. We'll look more specifically at what happens to populations when there is nonrandom mating in a future video.
Alright, Benoit. Our next assumption is that this population needs to have no mutation, and that's because mutation introduces new alleles into a population. And so you imagine your population in Hardy Weinberg, there's your mutation, that new allele. Well, that's going to change the genotype frequency and push it out. But now remember early on in this section, we said that sometimes mutation is considered a mechanism of evolution.
Sometimes people don't consider it a mechanism for kind of technical reasons, and you need to know what is expected in your class. So let's talk about that now. Remember, when we talked about mutation, we said that it's rare. So in a big population, if you have a mutation, it's going to introduce a new allele, but it's not going to change that genotype or allele frequency very much. Right?
The change of changing a single allele is going to be kind of insignificant in a really large population. So, yes, technically, mutation will change genotype frequency, and it will change allele frequency, but it's not going to change it much at all. It's going to be other mechanisms of evolution that sort of, you know, grab on to that new mutation, that new allele, and possibly make it more common. Alright. Now we've already talked about mutation, so we are not going to look at mutation in any more detail going forward.
Alright. Our next assumption that we want to look at here though is going to be that we need to have no natural selection in our population. That's because natural selection removes specific alleles from the population. So remember we had our white and brown rabbits. Right?
Imagine that population was in Hardy Weinberg equilibrium, but the fox comes along and it eats a whole bunch of white rabbits. That's going to change that genotype frequency, and it's not going to be in equilibrium anymore. Alright. We will take a closer look at natural selection and look at very specific ways that natural selection can affect population more in future videos. But for now, our next assumption is going to be there needs to be an infinite or at least very large population size, and that's because allele frequency changes by chance in small populations.
Again, think of that bag where we're pulling alleles out. Right? That gave us our predictions for what should happen in the next population. But if we're not pulling a lot of alleles, if we're not making a very big population, what's the chance that what happens is perfectly matching to our expectations? Right?
With probability, there's some chance involved. Now the bigger the population, the closer you expect your expectations to match that probability. And in an infinite population, it should match perfectly. But in small populations, they won't. Right now, when populations change due to just random chance, we call that genetic drift, and we will look at genetic drift in more detail in future videos.
Alright. Our final assumption here though is going to be that this population needs to have no gene flow, and that's because gene flow moves alleles either into or out of the population. Right? So imagine you have your population in Hardy Weinberg equilibrium.
There's another population nearby with a very different allele frequency, and a bunch of those individuals move in. Well, that's going to change the genotype frequency in that original population and push it out of Hardy Weinberg. Again, we'll look at gene flow more in an upcoming video. Alright. That means well, we said that random mating does not affect allele frequency, so it's not a mechanism of evolution.
Well, that means that these four assumptions, if broken, do affect allele frequency. And so these 4 can be considered mechanisms of evolution, though I'm just going to put my little asterisk there on mutation for the reasons we've already said. Right now, I also said you're very likely going to need to remember these five assumptions, so we have a memory tool to help you here. Right? We have these 2 funny looking plants, and they look like they're in love.
We're going to say here, mating mutants, it's natural in flowers. Alright. Mating, we need to assume random mating. Mutants, no mutation.
Natural, no natural selection. In, infinite or at least very large population size. And flowers, the first part of that word, gene flow. No gene flow. Alright.
So with that, we'll practice this more and practice problems to come. I'll see you there.
