Natural Selection - Online Tutor, Practice Problems & Exam Prep

Natural selection is a key mechanism of evolution that alters allele frequencies based on fitness, leading to adaptations. It manifests in three patterns: directional selection shifts traits toward one extreme, stabilizing selection favors average traits, and disruptive selection promotes both extremes. Additionally, balancing selection maintains multiple alleles through frequency-dependent selection and heterozygote advantage, exemplified by sickle cell trait's protection against malaria. Understanding these concepts is crucial for grasping evolutionary dynamics and species adaptation.

Natural Selection

We now want to take a look at natural selection, which I definitely think is the most important mechanism of evolution. That's because it's the only mechanism of evolution that can produce adaptations. Right. So, when you look at nature and wonder what is the major driver of the variation you see? Why are things good at the things they do?

It's natural selection. Alright. Before we go on, we just want to fill in this purple box here and think about what effect natural selection has on allele frequency. We're going to say here that it just changes the frequency of specific alleles, but it's going to change them in very specific and predictable ways, right, based on the fitness of those alleles. And that's going to be different from our other mechanisms.

Now when we think of natural selection and how it affects populations, there's going to be sort of 3 buckets that we can put this into, and we're going to say here 3 basic patterns that depend on which traits have greater fitness. Alright. So let's take a look. Our first type is going to be called directional selection, and directional selection favors one end of the distribution, we're going to say. Alright.

So what do I mean by one end of the distribution? Well, we have a distribution here. We're using Darwin's finches as an example. And on the x-axis, all the way to the left, you see those are birds with small beaks. And as you go across the x-axis, the beaks get larger to the one on the right that has a large beak.

And on the y-axis, we have a number of individuals. So we see that we have this sort of normal distribution there. Most of the individuals in this population have about an average-sized beak. Right? But what would happen if there are only large seeds available in this population?

Can you predict how this population might change? Right. Well, I would predict that those larger-beaked birds would have higher fitness, and that's going to sort of shift the distribution over time. And importantly, the average phenotype in this distribution is going to change in one direction. Right?

Hence the name directional selection. So we're going to say here the average phenotype changes in the population. And when I think of natural selection, I'm usually thinking of directional selection. I'm thinking of a population getting faster, or smaller, or changing color in some way in response to a specific selection pressure. And, you know, that is how populations change, but it's not the only way that natural selection can work.

Another thing that happens a lot out there in nature is called stabilizing selection, and stabilizing selection is selection that favors, we're going to say here, the middle of the distribution. So we have the same distribution here, but now imagine that there is a stable mix of seeds. Right? In a stable mix of seeds, maybe it's best to have that average size beak, and it's bad to have a really small or really big beak. So in that case, you can predict that this is going to narrow the distribution, if it changes it at all, but importantly, the average isn't going to change.

Right? We're really just eliminating those extremes from the population. Right? Now you can imagine how this could work out there in nature a lot. Right?

In any population, it's already pretty well adapted. It's doing well. It's good at the thing it does. Well, those extreme phenotypes might be bad — maybe, you know, they're too big or too small, maybe the color is too bright or too dull. Whatever it is, right, you can imagine how the middle, the average phenotype is good and those extremes are bad, and that average phenotype will be stabilized. Right? The population will be stabilized. It won't change over time.

Alright. Our final type of selection here we're going to say is disruptive selection. Disruptive selection favors, we're going to say here, both ends of the distribution. So now instead of having a mix of seeds, we only have small or big seeds. And in this case, it's going to be best to be a small-beaked bird or a big-beaked bird, but it's going to be bad to have that average size beak.

So you can imagine that over time, this could sort of hollow out this distribution in a way. It could give you what you can think of maybe as a bimodal distribution. Right? We're sort of getting 2 types in this population. So we're going to say disruptive selection, we'll say here, increases variation in the population.

And if it goes on long enough, you could have sort of 2 stable phenotypes that you see in that population. Or if it goes on long enough and other conditions are met, this could cause a population to split into 2 species. Alright. We'll look at these three types of selection more in examples and practice problems, and then we're going to look at natural selection even more after that in future videos. I'll see you there.

Natural Selection Example 1

For this example, it says, in each situation below, identify what type of selection is being discussed, then predict how the average phenotype in the population will change as a result of this selection. If you do not expect the population to change, write no change. Alright. So let's dive right in. A says, in a population of ginkgo trees, trees that are very short do not get adequate sunlight.

Well, trees that are very tall are subject to more wind damage. Medium height trees have the highest fitness. Well, what type of selection does that sound like? To me, it sounds like stabilizing selection. And in stabilizing selection, how would you expect the average phenotype in this population to change over time?

Well, I'm going to say no change. Right? That's the idea of stabilizing selection. The average phenotype is stabilized because it has the highest fitness and it's the extremes of the population that are removed. Alright.

B says then, in a population of oysters, light-colored oysters can camouflage on shallow rocks, while dark oysters can camouflage against dark shadows. Medium-colored oysters are easily caught as prey. Well, what type of selection does that sound like? I'm going to say disruptive. Right here, the average phenotype is bad, and it's the extremes that are better.

So what's the change to the population that we expect to see? I'm going to say this is going to increase that phenotypic variation. Right? So here I expect to see more dark oysters, more light oysters, and fewer of those medium or average oysters. Alright.

Our next one, c, well, it says over the course of many generations, servals, a medium-sized wildcat native to sub-Saharan Africa, have evolved from having normal-sized ears to having large ears optimized for hunting. Alright. So what type of selection does it sound like happens there? I'm going to call that directional. And what's the change that we see to that average phenotype?

Now I'm going to say big ears. Having big ears is something I know about. Alright. So these cats are getting bigger ears over time. That's a directional change in terms of that phenotype.

It's changing in a very clear way, and so we call that directional selection. Alright. We'll practice this more, and practice problems come up. Give them a try.

Which of the following statements regarding disruptive selection is true?

• A. The average phenotype has the highest fitness.
• B. It can lead to an increase in genetic variation.
• C. One particular trait becomes more common.
• D. Genotype frequencies remain constant.

The graph below shows the birthweights and infant mortality rate as recorded by researchers in London between 1935 and 1946. The bars represent the number of births recorded in 0.5-pound increments. The orange dots represent the mortality rate for each birthweight and are shown on a logarithmic scale. What type of selection does this graph suggest directly affects human birth weight?

Graph adapted from: M.N. Karn and L.S. Penrose, Birth Weight and Gestation Time in Relation to Maternal Age, Parity, and Infant Survival. Annals of Eugenics, January 1951, Vol. 16 No. 1., Page 147-164.

• A. Diversifying selection.
• B. Stabilizing selection.
• C. Artificial selection.
• D. Directional selection.

The graph below shows the average tail length of a population of deer mice. The first graph shows the tail length of the mice while living in a prairie ecosystem, and the second shows the tail length many generations after a group of mice moved into a forest ecosystem. Which of the following types of natural selection describes this scenario, and what conclusion could you draw from these data?

• A. Stabilizing selection: shorter-tailed mice had increased fitness in the forest ecosystem.
• B. Stabilizing selection: longer-tailed mice had increased fitness in the forest ecosystem.
• C. Directional selection: shorter-tailed mice had increased fitness in the forest ecosystem.
• D. Directional selection: longer-tailed mice had increased fitness in the forest ecosystem.

Balancing Selection

Natural selection will make alleles with high fitness more common in the population and will remove those alleles with low fitness. So one question people sometimes have is, why do so many genes have multiple alleles? Well, there are multiple answers to that question, but one answer that happens sometimes is called balancing selection. Balancing selection is defined as selection that favors more than one allele in the population. So when we think of the effect on allele frequency, remember, the effect on allele frequency is our measure of evolution.

This process will maintain multiple alleles. It will prevent any one allele from being driven to what we call fixation, becoming the only allele in that population, and it will keep alleles from being removed from the population. We're going to see two common types here: frequency-dependent selection and heterozygote advantage.

Frequency-dependent selection is defined as when selection favors the less common phenotype. For example, consider these snails with different shell patterns. The hypothesis is that birds learn to recognize specific shell patterns of these snails called Cepaea nemoralis. These shell patterns help with camouflage, but if a bird learns a shell pattern, like the common yellowish shells, it will hunt those snails until they become less common. Then the birds will shift their focus to a different, now more common, shell pattern. In this way, the rare phenotype is always favored, preventing any phenotype from becoming most common, thus allowing multiple alleles to be maintained.

The second type, heterozygote advantage, occurs when heterozygotes have higher fitness than either homozygote. Since heterozygotes have two different alleles, this keeps both alleles in the population. A classic example of heterozygote advantage involves malaria and sickle cell. In areas where malaria is prevalent, heterozygote advantage is observed with the sickle cell allele.

Regarding sickle cell, we have three basic phenotypes. Normal hemoglobin refers to AA homozygotes, heterozygotes have the sickle cell trait, and SS homozygotes have sickle cell disease. Sickle cell disease is associated with high childhood mortality rates and shorter lifespans for those untreated. If there is malaria present, individuals with normal hemoglobin (AA homozygotes) have a medium risk of death from malaria but no risk from sickle cell, leading to medium fitness. Individuals with sickle cell disease (SS homozygotes) have high risks from both malaria and sickle cell, resulting in low fitness.

However, individuals with the sickle cell trait (AS heterozygotes) have low risk of death from both malaria and sickle cell. These mild symptoms of sickle cell may protect them from malaria, resulting in high fitness. When heterozygotes mate, they can produce both homozygote forms, keeping both alleles in the population. All three genotypes are expected to be seen in the population, although heterozygotes have the highest fitness. This scenario illustrates why all alleles are retained, despite one genotype having clear fitness advantages. We will explore more about these types of balancing selection soon.

Natural Selection Example 2

This example tells me that a population has the following allele and genotype frequencies for the r gene. We are to assume that this population is not in Hardy-Weinberg equilibrium due to the presence of natural selection. Then we want to know what type of selection may be happening, and how do you know? Alright. So we have these allele frequencies, p equals 0.4 and q equals 0.6.

And we have our genotype frequencies, but we know that they're not in Hardy-Weinberg. So how could we figure this out? Well, we could compare them to what we would expect if it were in Hardy-Weinberg. So I'm going to do that. Right?

So the big RR genotype, well, that if it were in Hardy-Weinberg, that would be equal to p2, which is equal to 0.42, which equals 0.16. Alright. So it's actually pretty close, but there are looks like there's fewer of these bigger homozygotes in the population than I would expect if it were in Hardy-Weinberg. Well, how do I figure out the heterozygote? Well, I can go to pq is what that should equal if it's in Hardy-Weinberg.

So that's 2 times 0.4 times 0.6, which equals well, 2.4 times 0.6 is 0.24 times 2 is 0.48. Alright? So there's more heterozygotes than I would expect. And then what about my little rr? Well, you can probably see where this is going.

I'm going to take q2, and that equals 0.62, and that equals 0.36. Alright. So as I look at this, there are fewer of both homozygotes, despite a little bit, but there are fewer of each homozygotes than I would otherwise expect if it were in Hardy-Weinberg, and there's an excess of heterozygotes. Well, when I see that and it's due to natural selection, what do we call it? That's heterozygote advantage.

Right? So what would be happening in a population like this is it were otherwise in Hardy-Weinberg equilibrium. Each generation would start in Hardy-Weinberg, but then those homozygotes are not surviving at a higher rate. The heterozygotes are surviving more. And so the frequency of them, while they become more frequent in the population as those 2 homozygotes, well, as they die off.

All right. So how do I know this? I'm just going to say there's an excess of heterozygotes compared to homozygotes. When compared, I'm not going to write it all out, but if I'm being complete, I should say when compared to the Hardy-Weinberg expectation. Alright.

With that, more practice after this. Give it a try.

In many species of salmon, there are two mating types in males: large aggressive males and small sneaker males. Large aggressive males fight for territory in which they mate with resident females, while small sneaker males hide and try to quickly enter larger males’ territories to mate with females. “Sneaking” is, on average, a more successful strategy when there are many large males competing for territory, while “Fighting” is, on average, a better strategy when there are few large males present. As described, this scenario describes what two types of selection?

• A. Frequency-dependent selection and stabilizing selection.
• B. Heterozygote advantage and stabilizing selection.
• C. Frequency-dependent selection and disruptive selection.
• D. Heterozygote advantage and disruptive selection.

Which of the following statements about frequency-dependent selection are true?

I. In frequency-dependent selection, the rare phenotype is favored.

II. Frequency-dependent selection leads to a loss of genetic variation over time.

III. Frequency-dependent selection is a form of balancing selection.

• A. I & II only.
• B. I & III only.
• C. II & III only.
• D. I, II, & III.

Sexual Selection

Now, I'm gonna take a closer look at sexual selection and sexual selection, we're gonna say is selection for traits that affect the ability to obtain mates. Alright. When we think about fitness before we've said, you know, it's the ability to survive and reproduce, but an organism may be able to survive it may be able to reproduce, but it cannot find another organism willing to mate with it. That is the heart of sexual selection. Alright.

Now this all plays out the way it does because we're gonna say here during reproduction, there is often a high investment sex and a low investment sex. And when we're talking about investment, we're talking about how much the organism invests in that single mating event. Now females are often, though not always, they're often the high investment sex, and that's because females make large gametes. Right? That's sort of the definition of being female.

They make eggs where males make sperm, and they often give more parental care. Right? So for large gametes, think of birds. Right? It's physiologically expensive to make bird eggs.

And think about parental care. Think about an elephant. Right? A female elephant is going to be pregnant for almost 2 years and then will nurse the calf after birth, and that's from that single mating event. Well, in contrast, males are often though not always they're often the low investment sex, and that's because they have small gametes, and they tend to give less parental care.

Right? Think of the male elephant. It could theoretically mate every single day that that female is pregnant, and it could have new offspring with a new female every single day if it could find enough mates. Alright. So that difference is going to lead to sexual selection, and you can tell that sexual selection is going on because you see sexual dimorphism.

And sexual dimorphism is differences in secondary sexual characteristics. And secondary sexual characteristics are differences between the sexes that aren't directly related to the ability to reproduce. So differences like size between the sexes or differences in color. Right? When you see those differences, it's a hallmark that sexual selection has occurred.

Alright. So we're gonna have 2 ways that this can play out, and this is gonna be intersexual selection and intrasexual selection. Let's look at both of them. Now intersexual selection, that inter means that it's between the two sexes. And we can also sort of think of this as mate choice.

So we have this high investment sex, again often the females. Well if they're investing a lot in this mating event, they're gonna be choosy about it. Right? They are going to be choosy, and if the females are choosy, well, that tends to select for and lead to these showy males that are trying to impress the female and win the mate. Right now, the classic example of this is peacocks, right?

These peacocks here, this giant elaborate tail, that thing is not good for survival. Right? If you're running away from a tiger in the jungle, that thing is slowing you down. But for winning mates, they pop them up, the females evaluate, they find the males that tend to have the largest, most elaborate trains, and that's who they tend to mate with. Now this happens with other organisms too.

Here we see a bowerbird. Bowerbirds, they build these sort of elaborate structures, they decorate them, and females come around and they evaluate them, and they tend to mate with the males that have the most impressive structures. Alright. So that's intersexual selection, that mate choice. We also have intrasexual selection.

This is sexual selection within one sex, and we can think of this as competition. Competition for some competition for some resource that grants access to mates. So in this case, it's typically or it's going to be the low investment sex, typically the males that compete. Right? And they're competing for something like territory, and within that territory are females.

So if you control the territory, then those males can typically control at least most of the matings in that territory. Now the classic example for this are these elephant seals here. These seals fighting for the territory that's the beach, and these seals have some of the biggest sexual dimorphism in any animals. The males are huge compared to the females, because if they win a good space of beach, they control something like 90% of the matings that happen in their territory. And there might be well, often there's like a dozen, but there could be up to like a 100 females in that territory, which they get to have most of those matings.

Again, we see it in other animals too, right? Classic like the rams, bashing heads. They're typically, you know, winning some territory, something like that that grants them access to mates. Now keeping inter and intra separate in my head, it's hard for me the way I remember it. Well, that in intersexual selection, I think that the females are kinda walking around going, do I wanna mate with you?

Right? It's that mate choice. And for the intrasexual selection, I see that raw, and I think that that low investment sex is going raw. I'm gonna win the mate. Raw.

We're competing. Right? Hopefully that helps you. Alright. Before we go on, I just wanna say warning.

Right? Mating systems are complex. Okay? So often when you see intersexual selection, there's also some intrasexual selection going on. Right?

These peacocks are competing for the best display space. When you see intrasexual selection, there's often some mate choice going on. Right? These elephant seals, they control something like 90% of the matings in their territory. That's not all of them.

The females are still choosing who to mate with at least some of the time. I also just want to say, really don't overgeneralize, especially in humans. People love to do this, but just please don't. Right? Humans, well, there is some sexual dimorphism in humans, but it's less than in other organisms.

So we really don't know what's going on, and trying to dissect human mating systems to know what is culturally imposed versus biologically imposed, that is really difficult to do, and I'll just say well beyond the scope of general bio. Alright. With that, we will look at intra- and intersexual selection more coming up. I'll see you there.

Natural Selection Example 3

Our examples are about sexual selection, so let's take a look. It says, 3 bird populations are described below. For each population, determine which sex is the high investment sex, or if both sexes have equal investment. Then circle whether the intersexual selection, intrasexual selection, or neither type of sexual selection is being described. Alright.

A here says that red-necked phalarope males are responsible for paternal care. While males are caring for the clutch, female red-necked phalaropes will often attempt to mate with other males and will compete with other females for access to mates, leading to larger and more aggressive females. Alright. So as described, which sex sounds like the high investment sex? Well, I'm going to say males, and that's because the males are responsible for paternal care.

Right? So in that single mating, they have a high investment because they got to stick around and raise the young. So knowing that, well, do you see intersexual selection, intrasexual selection, or neither as described in this example? Well, the key thing for me is that the females will compete with other females for access to available mates, and that sounds like intrasexual selection. Remember, intrasexual selection is competition within one sex.

And here we have that competition within the females. And the way I remember it, that raw competition—I'm going to compete and fight and win the mates. Right? So we have the females competing for access to available mates, and so we get that sexual dimorphism leading to larger, more aggressive females.

Now quick note, we said that usually females are the high investment sex. Here we have the males. When you see that, sometimes it's called a sex role reversal. But it's just important to remember that it's not strictly females or males that behave certain ways. It really depends on how much they invest in the offspring, and that's just determined by that species' mating system.

Alright. We'll go on to B now. Scroll down so we can see it a little bit better here. B says that both male and female Laysan albatrosses raise their offspring. Birds typically form a pair bond with one other bird of the opposite sex that they maintain for life.

There is little visible difference between male and female albatrosses. So which sounds like the high investment sex? Well, to me, that's a bit of a trick question. I'm not going to say neither. Right?

It just says right off the top. Right? Both male and female albatrosses raise their offspring, right, at least relatively. They're giving about the same investment. So then as you read it, is this intersexual selection, intrasexual selection, or neither?

Well, I'm going to go with neither. And you know something that backs that up for me, there's little visible difference. There's little sexual dimorphism. So sexual dimorphism is a result of sexual selection. It just doesn't look like that's really going on here.

Alright. Finally, we have C red-capped mannequin males perform impressive dances that females evaluate when deciding whether to mate. Males have a black body with a red head and bright yellow legs, while females who are responsible for caring for the young are dull green and brown. So based on that description, which sex is the high investment sex? Well, it says the females are responsible for caring for the young.

That's a high investment in that mating. So I'm going to say the females here. And then, well, what do you think? Intersexual selection, intrasexual selection, or neither? Well, I'm going with inter, and that's because we have these showy males and choosy females.

And remember, intersexual selection is sexual selection that involves both sexes—one choosing to mate while the other is sort of performing or trying to impress. So the other sort of silly memory way I remember this, I say, uh. Right? And I think of them going like, do I want to mate with you?

Right? Making that choice. Alright. So with that, we've answered the question. Just a quick reminder, these are sort of simplified descriptions.

You can have both intra and intersexual selection going on in the same systems because nature is complex. Alright? We got more practice after this. I'll see you.

Mudskippers, fish that leave the water to mate, perform elaborate dances involving jumping a few inches off the ground to attract mates. This is an example of:

• A. Intersexual selection because it is an example of mate choice.
• B. Intrasexual selection because it is an example of mate choice.
• C. Intersexual selection because it is an example of competition within one sex.
• D. Intrasexual selection because it is an example of competition within one sex.

In a 1990 study on caribou by Barrette and Vandal in Behavioral Ecology and Sociobiology, it was observed that in confrontations between male caribou, the males with the larger antlers won 90% of the time. If this is a case of purely intrasexual selection, what else would you expect to be true?

• A. Fighting is a display trait, and females choose males with the larger antlers.
• B. Males with the larger antlers would be expected to provide more paternal care.
• C. Winning a confrontation provides males access to more breeding females.
• D. Winning confrontations would signal to females that the male has higher fitness.