Allelic Frequency Changes: Study with Video Lessons, Practice Problems & Examples

Natural selection, genetic drift, mutation, migration, and non-random mating are key mechanisms influencing allele frequencies in populations. Natural selection favors advantageous traits, while genetic drift causes random changes, especially in small populations. Mutations introduce new alleles, and migration enhances genetic diversity through gene flow. Non-random mating, including assortative mating and inbreeding, affects genetic variation and can lead to reproductive isolation and speciation. Understanding these concepts is crucial for grasping population genetics and evolutionary processes.

Natural Selection

So Hardy Weinberg exists in this ideal world of these five specific assumptions. But reality isn't like that, right? And these assumptions are way too restrictive to exist in real life. So we're going to go through each of these five and talk about what happens in real life populations. So the first one is selection, right? No selection for Hardy Weinberg, but in real life, there is selection. And that selection is called natural selection. And natural selection is when organisms have genes or alleles that allow them to survive better and allow them to reproduce. And the reason that natural selection exists is simply because there's a struggle for survival. There are many organisms that are born on this Earth. There are so many organisms born on this Earth, not all of them can survive. Because not all of them can survive, there have to be a few that are more apt to survive and reproduce than others. And those few usually have some kind of gene that allows them to survive and reproduce better. So because not everyone survives, the ones that do have particular phenotypes that allow them to, and they can pass that on to their offspring. That's how selection pretty much works. Now there are many different types of selection. One is called directional selection, and that is when an allele moves in one direction towards what's called fixation or loss. Loss makes sense, right? That allele will be completely lost in the population. But fixation, what does that mean? The exact opposite of loss. So fixation, fixed alleles, are alleles found in every organism. So when an allele is fixed, that means every organism in that population has only that one allele. There are no other allele options for them. And so Directional Selection moves what could be two or three or four alleles toward one direction, whether they are fixed in a population or they're lost from the population. Then, there's positive and purifying, which are separate selections. So what happens in both of these is that there's some kind of mutation. Right? So when there's a mutation, that can either be beneficial, or it can be harmful, or it could potentially be neutral. But in selection, it either has to be beneficial or harmful. So when a mutation is beneficial, positive selection says, okay, that's great. We need this. It's beneficial. It helps the organism survive and reproduce, so let's make it a higher frequency. Purifying selection says that the mutation is harmful. It makes the organism less likely to reproduce, so we're going to get rid of it. We're going to remove it from the population. And then, finally, there is balancing selection. And this is when there are two or more alleles, but essentially they're both useful. They're both beneficial to the population. And so they are maintained at some kind of equilibrium, where they both exist and selection sort of weeds out many other alleles for these, to balance these. The majority of the organisms have these alleles. So an example of this would be directional selection, where the red represents before. So this is before over here, and the blue represents after. Before Directional Selection, there's a really high amount of this one phenotype, less of this intermediate, and a very low amount of the phenotype. After selection, this one, the allele for phenotype 1, has been lost, and this one has been gained, moving towards fixed. Right? And, after, this means that there's a low amount of phenotype 1, intermediate of intermediate, but a very high amount of Phenotype 2, and that's due to Directional Selection, where this allele becomes lost and this one is gained at a higher frequency. Then there's stabilizing selection. This is also kind of like balancing selection, where before there was sort of this broad allelic frequency distribution over three different phenotypes. Whereas, after the selection, the alleles for this intermediate phenotype here in the middle have been selected for, and that intermediate phenotype is usually a mixture of multiple alleles, and so those multiple alleles have been selected for. Now, let me disappear so we can talk about this. Another measurement of natural selection includes fitness. And what fitness is, it's a measure of how well an individual's genetic makeup contributed to the next generations. And so that says, how well are those genes passed on? There are two kinds of fitnesses. There's absolute fitness, and this just says the number of offspring an individual has. Right? The more offspring, the more genes you passed on. The second type is relative fitness, and that's the fitness, or the number of offspring an individual has, compared to another individual. So if we say that we have a brown fish and a blue fish, and the brown fish produce two offspring, and the blue fish produce ten. Well, the relative fitness is two to ten. Right? And so that compares how fit the blue fish was to the brown fish. So it says brown. Sorry for my handwriting. And so fitness is another way to measure selection. Now selection exists, right? Natural selection exists all the time. We can actually see it in our lifetimes. And it is a reason why, because this exists, it is also why Hardy Weinberg equilibrium is not a good representation of current populations living at the time, and instead, it's just an estimation. So that's selection. Now, let's turn the page.

New Alleles and Migration

Okay. So now let's talk about the new alleles and the mutation, which are the a and the m of Sameer. New alleles are created all the time, and those change the allelic frequencies. One of the main ways that new alleles are created is through mutation, which is happening consistently. Mutation can create new alleles in a population, but it also can convert one allele into another. To measure that, the creation of new alleles, we actually look at the mutation rate, which can just be calculated, without knowing anything about evolution. Right? It's just calculating the mutation rate of that organism. And this is the rate at which mutations occur in the population. There are various formulas that you can use to calculate the mutation rate, and how frequently that will introduce new alleles. Also, this formula here can calculate how the mutation rate affects, the mutation rate on allele p causes the change in the frequency of q, because that mutation is changing the allele p into the allele q. Now, some of your books are very heavy on these types of calculations, but most of you will get lucky and won't have to calculate these yourself. Just know that alleles are created all the time, and this is often through mutation.

The third way and or the third letter in Sameer is m, it's migration. It's also called gene flow. These are the exact same thing, so whichever your book uses, make sure you use that one. And that's the movement of individuals between different populations. You can also use subpopulations. Right? Because it's different populations. And so, when one subpopulation, sort of, has an individual that migrates into the next one, that can create this concept called genetic admixture, which is a mix of genes and individuals that arose from multiple populations. Right? So, for instance, if we have this black circle population and this red circle population, when the black circle population has an individual move over here and mates, then that's going to create this genetic admixture in the offspring because that genetic information is coming from more than one population. This allows adding genetic variation in the population that does not meet the assumptions necessary for Hardy-Weinberg equilibrium and the Hardy-Weinberg formula.

The introduction of new alleles, either through mutation or through migration or gene flow, are super important things that are happening all the time. Mutations are happening all the time, and individuals in different populations are constantly migrating throughout different subpopulations, and that's creating a lot of genetic variation in these populations. So with that, let's now turn the page.

Genetic Drift

Okay. So now let's talk about genetic drift. This is important particularly for the case of Samir, who assumed an infinite population size, because real populations aren't infinitely large. Right? Populations can't be infinite. There isn't an infinite number of individuals in any population. And because there is a limitation on population size, it can't be infinite. Since populations are actually finite, only a limited number of offspring will be produced, and those offspring only represent a subsection of the total alleles in a population. What this means is that even though the population is finite, there's an unlimited number of gametes, say, 3,000 gametes per person, which is actually much lower than the actual number. However, not every gamete that's created will produce an individual. And because not every gamete produces an individual, each generation only contains a sample of alleles. A small subsection of alleles from the previous generation are passed on to the next. These are the ones that stay in the gene pool. But because there is a loss of alleles, essentially, not every allele is passed on, there's this phenomenon called genetic drift.

How does genetic drift happen? Take yourself as an example. You have many alleles that represent your genetic material. When you create gametes, whether sperm or eggs, there's a random selection of alleles. Remember, there's random assortment, so you only get half of your total alleles in every single egg or sperm. Millions of eggs and sperm are produced, but we don't have millions of children. Thus, only a small subsection of all the alleles will actually be passed on to the next generation, and that's completely by chance, just randomly chosen. Moreover, the more offspring that you have, the more alleles that are passed on, and this is again, all by chance. If the number of gametes produced is small, which is true for humans, then a smaller number of offspring is produced. This increases the chance that the gametes will differ from the entire gene pool.

There's this entire parental gene pool that contains all the genes from both of our parents, but we only inherit half of those. And there's a great chance that this half will differ significantly from the frequencies found in the parents. This is referred to as sampling error. It is a deviation from the expected ratio due to a limited sample size. For example, if I have 5 pennies and 5 nickels and I choose 2 of them, there's only a small chance that I'll choose 1 penny and 1 nickel, representing the frequency found initially. There's a higher chance I would choose 2 pennies or 2 nickels. This is an example of sampling error, and the same thing happens with alleles. We only inherit half of the alleles from our parents, and only some of these produce an individual. Because we're producing only a small number of individuals, there's a very high occurrence of what's called genetic drift. Genetic drift is this change in allelic frequencies. As illustrated earlier, the frequency of half pennies to half nickels changed to either 2 pennies or 2 nickels when I picked that sample. Similarly, genetic drift is this change in allelic frequency due to random choices, the random disappearance of genes in a small population.

Genetic drift occurs at a much higher rate in small populations, and when allelic frequencies are equal. Now, in Hardy-Weinberg assumptions of infinitely large populations, genetic drift doesn't occur. However, in real situations where populations are finite, genetic drift happens due to random occurrences. Sometimes we might randomly choose more blue alleles over red, and sometimes vice versa. This affects different alleles and genes in finite populations, especially in smaller populations producing a small number of offspring, happening at a very high rate.

Genetic drift can lead to the fixation or loss of an allele. Fixation occurs when all individuals in a population are homozygous for one allele, for instance, if all individuals have two copies of the blue allele, it's fixed. Loss occurs when no individual in a population carries an allele, like when the red allele is completely lost. These effects of genetic drift can restrict genetic variation, which is crucial for the adaptability and survival of species.

Selective pressures like the founder effect and the bottleneck effect can also cause genetic drift. The founder effect occurs when a new, smaller population is formed from a few individuals who do not carry all the alleles from the original, larger population. This reduces variation in the new population. The bottleneck effect occurs when there's a significant reduction in population size due to environmental events or other catastrophes, which results in a loss of genetic variance.

Overall, genetic drift is a crucial concept in understanding population genetics, particularly in non-infinite populations, a core aspect of the Hardy-Weinberg principle.
With that explained, let's now move on.

Non-Random Mating

Okay. So now we're going to talk about non-random mating, which is the last part of this seminar, labeled as 'M'. In Hardy-Weinberg, there has to be random mating, but in real life, that doesn't actually happen, and non-random mating due to phenotypes, which are caused by different alleles, occurs in every organism on Earth, including humans. It's called assortative mating when individuals choose mates based on phenotypes. There are two types, positive and negative. It's positive mating when it's chosen based on similar phenotypes, so how similar the two mates are phenotypically, and it's negative when it's based on if females and males have any kind of assortative mating based on smell. What they did is they got males to wear white T-shirts, and they lived in these shirts essentially for a few days. They exercised, they got them real sweaty, real smelly, it was nasty. Then they put them in plastic bags and gave them to women to rate, based on attractiveness. It turns out that women prefer the odor of males who have a certain genotype, and that genotype is very different from them. The alleles that they were looking at were these alleles called MHC. You may or may not know what they are. It doesn't really matter what they are. Just know that they're involved in the immune system. Females always chose men with different MHC molecules, and I could tell this simply by the odor that those MHC molecules were different in the men. There's obviously something in the odor of probably both females and males that allow humans to differentiate different potential mates. This is an example of non-random mating. There are examples of this all throughout the animal kingdom: different mating dances, feathers or colors, locations, geographical locations, all of this is an example of nonrandom mating.

So, there is another type of non-random mating, which is isolation by distance. Two populations that live in different areas that are either separated by continents or countries or even something as simple as a mountain or a stream, aren't going to mate with each other because they can't get there. They're not like humans with planes that can fly all over the world. Generally, organisms are restricted to a very small geographical location. When there are two populations or more populations that are separated from each other for an extended period of time due to selection or genetic drift or any of these things that we've talked about previously, genetic variations begin to develop between these new populations. When those variations get to a certain point, we call this speciation, which is the creation of a new species. Speciation occurs through reproductive isolation, so isolating two populations that now can't reproduce with each other. There are two types of reproductive isolation, there can be prezygotic, so this is before the formation of the zygote, something that prohibits them from mating, and that reduces breeding. Try to think of some things that could be prezygotic, preventing the organisms from mating. Right? It could be isolation by distance, which is what we're talking about. It could also be that the organisms don't have the genitalia to be able to mate, or that they would never think of each other as mating populations to begin with. For instance, hummingbirds and bears are never going to look at each other and say, that's probably a good mate. And so, those are all prezygotic mechanisms. Then there are postzygotic mechanisms, and these are things that after the zygote is formed, prevent further reproduction. Usually, what happens is there's some kind of offspring created from these mates, but they're either inviable, meaning that they die, or they are born but they're infertile because they're sterile and they can't reproduce. So if you can't make more offspring, then it's considered reproductive isolation. An example of this is a mule, which is a mating between a horse and a donkey. Mules are an example of postzygotic isolation because they are sterile, and they cannot reproduce, and so you can't get more mules by mating mules, and therefore, it's an example of reproductive isolation.

Another form of random mating or non-random mating is inbreeding, which I know is kind of a taboo subject among humans, but essentially it's the mating between relatives. Inbred individuals are much more likely to be homozygous for harmful recessive alleles. A lot of genetic diseases are recessive genetic diseases, and inbred individuals are much more likely to be homozygous for those and are much more likely to have recessive genetic diseases. It's called inbreeding depression when inbreeding leads to a reduction in vigor or reproductive success, which would happen in the case of a recessive disease. If someone has a recessive disease, they're much less likely to reproduce, and be healthy enough to reproduce. We usually think of this as super taboo in humans, but actually, inbreeding in plants, especially through self-fertilization, can actually be a positive process. But generally, inbreeding is non-random mating, right? Because you're mating with your family. You can measure inbreeding through \( f \), and that's the probability that two alleles in an individual trace back to the same ancestor. And, of course, inbreeding is much more common amongst small populations because there's not that diversity of choice of mates. It's very limited, and so it's much more common there. So the inbreeding coefficients for something like a father-daughter is 25%. So there's a 25% chance that if you choose two alleles from this offspring, it will go back to the same ancestor, and that's very high, especially when you look at recessive genetic diseases. That's a really high inbreeding coefficient. You can look through the rest of these and see that, you know, as you get further away, with second cousins for instance, it's 1.56%. All of these are examples of inbreeding, an example that happens especially in the animal kingdom more often. This is how all these assumptions that Hardy-Weinberg makes in its formulas aren't necessarily that relatable to real life situations. So with that, let's now move on.

Which of the following terms describes a change in allelic frequency due to random disappearance of genes in a small population?

• A. Natural Selection
• B. Allele creation through mutation
• C. Gene Flow
• D. Genetic Drift
• E. Non-random mating

A group of finches live on a small, isolate island. One day, a few finches travel to a distant island and start a new population of finches. This type of change in a population is called what?

• A. Speciation
• B. Founder effect
• C. Bottleneck effect
• D. Genetic drift

Which of the following is an example of natural selection?

• A. A neutral mutation is carried from generation to generation
• B. A rabbit migrated to a new location and brought new alleles to the endogenous rabbit population
• C. A mutation causes a finch to develop a stronger beak, which makes it more likely to grow, survive, and reproduce.
• D. One allele becomes fixed in a population due to random genetic drift over time.

Here’s what students ask on this topic:

What is the Hardy-Weinberg principle and its assumptions?

The Hardy-Weinberg principle states that allele and genotype frequencies in a population will remain constant from generation to generation in the absence of evolutionary influences. The five key assumptions for a population to be in Hardy-Weinberg equilibrium are:

• No selection: No differential survival or reproduction of individuals.
• No mutation: No new alleles are introduced into the population.
• Large population size: The population is infinitely large to prevent genetic drift.
• No migration: No gene flow between populations.
• Random mating: Individuals pair by chance, not according to their genotypes or phenotypes.

These conditions are rarely met in real populations, making the principle a useful null model for studying evolutionary processes.

How does natural selection affect allele frequencies in a population?

Natural selection affects allele frequencies by favoring alleles that enhance survival and reproduction. This process increases the frequency of advantageous alleles and decreases the frequency of harmful ones. There are different types of selection:

• Directional selection: Favors one extreme phenotype, leading to allele fixation or loss.
• Stabilizing selection: Favors intermediate phenotypes, reducing variation.
• Disruptive selection: Favors extreme phenotypes at both ends, increasing variation.

Overall, natural selection drives populations toward greater adaptation to their environments.

What is genetic drift and how does it influence allele frequencies?

Genetic drift is the random fluctuation of allele frequencies in a population, especially significant in small populations. It occurs due to random sampling of alleles during gamete formation and reproduction. Over time, genetic drift can lead to the fixation or loss of alleles, reducing genetic variation. Two key phenomena associated with genetic drift are:

• Founder effect: A new population is established by a small number of individuals, carrying only a fraction of the original population's genetic diversity.
• Bottleneck effect: A significant reduction in population size due to environmental events, leading to a loss of genetic diversity.

Genetic drift can have profound effects on the genetic structure of populations, independent of natural selection.

How do mutations contribute to changes in allele frequencies?

Mutations are changes in the DNA sequence that create new alleles, contributing to genetic diversity. They can occur spontaneously or due to environmental factors. Mutations can be:

• Beneficial: Increase an organism's fitness, potentially becoming more common through positive selection.
• Neutral: Have no effect on fitness and may persist in the population by chance.
• Harmful: Decrease fitness and are often removed by purifying selection.

Although mutation rates are generally low, their cumulative effect over generations can significantly alter allele frequencies and drive evolutionary change.

What is non-random mating and how does it affect genetic variation?

Non-random mating occurs when individuals select mates based on specific traits, rather than randomly. This can affect genetic variation in several ways:

• Assortative mating: Individuals mate with others that have similar phenotypes, increasing homozygosity and potentially leading to inbreeding.
• Disassortative mating: Individuals mate with those that have different phenotypes, increasing heterozygosity and genetic diversity.
• Inbreeding: Mating between relatives, increasing the likelihood of homozygosity for harmful recessive alleles, leading to inbreeding depression.

Non-random mating can lead to changes in allele frequencies and influence the genetic structure of populations, potentially leading to reproductive isolation and speciation.