So what are these conditions that lead to changes in allele and genotype frequencies over time? They are five. There they are.
No mutations. So you can have no new alleles being added to the gene pool. There has to be random mating, which means there's no sexual selection, which means that the likelihood of getting a particular allele from either parent is completely left up to chance. There can be no natural selection.
Having a particular allele cannot be adaptive, cannot confer any advantage onto the offspring. You have to have an extremely large population size. Technically, you have to have a large family. To really honor this, you have to have an infinitely large population size.
And then the fifth is that you have no immigration or emigration, which, as we're going to learn, that's what is the heart and soul of gene flow. So no gene flow, no immigration or emigration. An extremely large population size has to do with another principle called genetic drift that we'll get into in a minute.
So as long as these five conditions are met, you're going to have no microevolution, no change in genotypic frequency or allelic frequency from one generation to the next. But we rarely observe that actually happening. So what is the use of the Hardy-Weinberg model? As I said, it is a null model. A null model means let's...
try and come up with a model that predicts what will happen if evolution is not occurring. And by disproving that evolution is not occurring, we demonstrate that evolution actually does occur, that within a population, allelic and genotypic frequency do change from one generation to the next under the influence of these five conditions. So, we know that natural selection, genetic drift, and gene flow can alter allelic frequencies in a population.
And these can lead to changes in allele and genotype frequency in a population. Natural selection is the only one that is a non-random effect. It is non-random.
It is adaptive. Genetic drift and gene flow are the only ones that can alter allelic frequencies in a population. are random, as is mutation. So in the case of natural selection, natural selection can be defined as differential reproductive success due to certain alleles being passed to the next generation in greater proportion.
So what does that mean? It means that, for example, if you're an insect that has an allele, that enables you to resist being sprayed by DDT, the pesticide DDT, you are more likely to leave more offspring than your peers that do not have this allele. What is genetic drift?
Genetic drift A principle that the smaller a sample, the greater the deviation you're going to have from a parent population. So if we, for some reason, go from having a very large gene pool, a very large population, having taken a small sample from a gene pool, due to nothing other than sampling artifact, you're going to get a change. in allele frequency and genotypic frequency. So, for example, if I had a bowl of M&Ms that had all of the colors represented and I take a handful of M&Ms, chances are it's not going to be the same distribution of M&Ms in that sample that I have in my hand, then it's not going to be representative. of what's in the bowl.
And the smaller the handful I take, the less likely that handful is going to be representative of the M&M's distribution in the bowl, the same numbers of relative numbers of reds and blues and browns and light browns and greens and yellows and oranges. So genetic drift, not only do you see changes in allele frequencies and genic frequencies, but often in very small samples from a population, you can lose alleles. So if I take a small enough sample of M&Ms, I'm not going to get all the colors of M&Ms in there. If that represents a population that's moving to say an island, then that allele is gone and there's no way of getting it back.
So what if we had a population of snapdragons or whatever these flowers are with these frequencies of the red allele and the white allele? 0.7% or 70% red allele, 30% white allele. Let's say only five of these plants leave offspring behind.
So in the second generation now the allele frequencies have gone from 0.7% and 0.3 to 0.5 of each. Let's say two plants selected at random leave offspring. Now, all of these plants, since the two plants that left offspring were red, that means that P has gone to one, or 100% of the alleles in this population are the red allele, and there is no representation of the white allele, which means the red allele has become fixed in this second population.
or this third generation here. Now there are a couple of cases where we would expect to see large effects of genetic drift or these sampling effects. One is in what is called the founder effect and this occurs when we only have a few individuals starting a new population from a larger population.
Or we take just a few M&Ms from the bowl and we get a non-representative sample. We don't get all of the colors in the same proportions as what we have in the original bowl of M&Ms. So the smaller the sample, the larger the difference is likely to be from the parent population.
The bottleneck is similar. The bottleneck effect is similar to the founder effect. The result is the same in that you usually get a dramatic shift in genotypic frequency and allelic frequency.
Only the bottleneck effect tends to occur when the population size is suddenly reduced within a population, which means those who survive... may not be necessarily due to who's most adaptive as it is just to who did not get reduced from the population. So if the population remains small, it can be further affected by genetic drift.
So say we have an original population of bees. three different alleles. We've got blue and white in the greatest abundance and yellow in relatively lesser abundance. We take a small sample of the beads in the bottle and we end up with a surviving population that is vastly different in its distribution of alleles from the original population. So one case of...
Genetic bottlenecking has occurred in the Great Plains of the USA. There is a wild chicken, the greater prairie chicken, that used to run free in the Great Plains. But the Great Plains became inhabited.
A lot of their habitat, those prairies, were converted to farmland. Agriculture, corn and soybeans and grazing land and what have you, which has reduced the population of greater prairie chickens in the state of Illinois. And we find that among those that survive within those populations, those greatly reduced populations, the level of heterozygosity has dropped dramatically and has reduced their ability to survive in the wild. So prior to the bottleneck, the range of the greater prairie chicken, you can see covered most of the state of Illinois.
In 1993, there's just two populations of this bird in Illinois. So you can see the number of alleles per locus has gone from 5.2 down to 3.7. The population size has dramatically decreased, and the viability of the eggs has greatly decreased from 93% to less than 50%. In another state, in Kansas, where there's no evidence of bottleneck, the heterozygosity is, again, higher, and the viability of the eggs is higher. And the same in Nebraska.
So, for genetic drift, the effect is significant in small populations. Allel frequencies change at random. It can lead to fixation of alleles. It can also lead to fixation of harmful alleles, and you lose genetic variability because of genetic drift.
Gene flow is the other factor that can dramatically affect frequencies of alleles, and this is more due to immigration and emigration, so movement of alleles among populations from one population to another. This can be transferred through fertile individuals or just through moving the gametes. So, for example, if you're a plant and you release your pollen into the wind, if that pollen can move, you don't have to move an entire tree or individual plant if you just have the pollen that can move, or seeds for that matter.
So gene flow tends to reduce genetic variation. among populations over time because it has sort of a homogenizing effect. So typically we think of genetic, of gene flow as increasing genetic diversity, or at least mixing it up and homogenizing it. However, there is a case where gene flow has had a negative effect on a population of birds. So here's a bird found in the Netherlands, in Europe, called Paris Major.
And on this island, in the North Sea, so the north of the Netherlands, there are a couple of different populations of this bird. One of these populations is the Seems to be getting a number of individuals from the mainland of Europe that can get into these populations of birds and mate with them. The eastern population, being a little bit further away, has less of an impact from birds from the mainland. And that difference has led to increased survival.
in the eastern populations because the mainland populations aren't as well adapted to life on the island. So when you get an influx of mainland birds into the central population, they mate with the females in that population, and they are less well adapted because you're getting these alleles that are mainland alleles into this island population. which means they're less well adapted for island living. Gene flow can also increase the fitness of a population. So the spread of alleles for resistance to insecticides, such as malarial mosquitoes, alleles that have evolved to resist insecticide spraying.
have spread from pockets of populations where those alleles were abundant into other populations, which has spread those alleles, increased the frequency of those alleles across the species. Oh, I'll go to the next video for this one.