Evidence of Natural Selection - Online Tutor, Practice Problems & Exam Prep

Natural and Artificial Selection

One of the best ways to wrap your head around natural selection is to compare it to artificial selection. Now, those are two terms that were both coined by Charles Darwin in his book on the origin of species. So we're going to start here just by saying in the origin of species, Darwin explained natural selection by comparing it to artificial selection. Alright. So let's figure out what artificial selection is.

Artificial selection, we're going to say here, is selective breeding of individuals with traits favored by people. Right? Whether it's for crops or animals, for agriculture, or for pets, people know how to do this. Right? People know that if you want certain traits in organisms, you pick parents with those traits and breed them together, and you're more likely to get offspring with those traits.

Alright. People have been doing this for a very, very long time. Now in the Origin of the Species, some people are kind of surprised to know that Darwin spends a decent part of the first part of the book talking about pigeon breeding. That's because in the 1800s, breeding what were called fancy pigeons was a popular pastime, and people who bred pigeons were called pigeon fanciers. So here we have, an image of a pigeon, and people know well, you know what a pigeon looks like, but you probably haven't seen these fancy pigeons that people have bred.

So, just by choosing traits that people are interested in, people get all these kinds of crazy-looking birds. Right? You get ones like this with a big puffed-out chest, or people have bred them with these feathers covering their feet. So it almost looks like it has big boots on. That third one there, I don't know, kind of looks like a chicken or something.

Right? Those don't look like normal pigeons, and they come just from people selecting the traits that they want in the offspring. Now you may be less familiar with pigeons, but I'm sure you're familiar with dogs. Right? So dogs are descended from the gray wolf.

So we have a wolf here, but we know that people, just by selecting traits they were interested in, have created all sorts of different dogs. Right? A bulldog, a wiener dog there, a poodle. Right? All of that comes from people just saying, hey.

That's a trait I would like to see in these puppies. Let's breed these organisms together and, well, that trait is more likely to come out. Alright. That's really no different from natural selection. But in natural selection, we're gonna say that reproduction of individuals with traits favored in the environment.

Right? The ideas are the same. Some organisms are more likely to reproduce. In artificial selection, it's because of people choosing the traits. In natural selection, it's in the environment.

Now in the Origin of Species, Darwin talks about, for example, the finches on the Galapagos. So this is what people think the ancestor of those Galapagos finches looked like. But depending on what island those birds landed on, depending on the food source of those on that island, that selection favored different traits. We have some species with big heavy beaks to crack open big hard seeds. There are smaller-beaked finches that feed on different things.

Well, we can apply this to other animals as well. Right? We've been talking about the big cats, the genus Panthera. So this is sort of a proposed ancestor of those big cats. And you can imagine that in different environments, slightly different traits were favored, leading to different organisms reproducing more and passing on different traits.

And we can see how we could get, you know, lions and jaguars and tigers depending on the environment they live in. Alright. We'll practice this more coming up, and I'll see you there.

Evidence of Natural Selection Example 1

For this example, we want to place the statements below in the Venn diagram, depending on whether they describe natural selection, artificial selection, or both. Alright. So here's our Venn diagram. Natural selection on the left, both in the middle, and artificial selection on the right. Alright.

So let's go through these statements one by one. Statement A says, "Over time, the frequency of a trait in the population will change." So what do you think? Natural selection, artificial selection, or both? Both natural selection and artificial selection will cause a change in the trait over time in the population.

So that's going to get A in the middle there. Alright. Next, we have B: "Organisms with certain traits reproduce at a higher rate." Natural selection, artificial selection, or both?

That's going to be true for both again, and it's just we're talking about the reason that they're reproducing at a higher rate. With artificial selection, it's due to people selecting, whereas with natural selection, it's due to the environment selecting, but both have differences in reproduction. Alright. Next, we have C: "Breeding is controlled by humans."

Where does that go? Well, just said it. Right? Breeding controlled by humans. That's the key definition of artificial selection.

We'll put the C there. Next, we have D: "More offspring are produced than will survive or reproduce." Where does that go? Alright.

Again, that's going to be true for both. More offspring are produced than could possibly survive or reproduce in both cases. Again, the difference is just why they're surviving or reproducing. So D goes in the middle. Then we have E, "Competition for environmental resources or mates determines survival and reproduction." Where does that go?

Right. Well, that's keying in on that reason, and that reason is the environment. So, that's natural selection. We'll put the E out here, and our last one, F: "Requires variation in the population." Where does that go?

For change to happen, we must have variation in the population. So F goes in the middle for both. Alright. So remember, natural selection and artificial selection—some people really argue they're just the same thing. The key difference is what is choosing which organisms survive and reproduce.

If it's the environment, it's natural selection. If it's humans, it's artificial selection. Practice problems after this, and I'll see you there.

Kale, brussels sprouts, cabbage, broccoli, and kohlrabi are all vegetables that come from selecting for different traits in the wild mustard (Brassica oleracea). For example, to produce kale, humans selected plants with the large leaves. Over repeated generations of selection this led to the modern plant kale. This is an example of:

• A. Natural Selection.
• B. Artificial Selection.
• C. Homology.
• D. Use and Disuse.

Which statement identifies the main difference between natural selection and artificial selection?

• A. Natural selection is controlled by the environment, while artificial selection is controlled by humans.
• B. Natural selection only influences phenotype, while artificial selection influences genotype.
• C. Natural selection works on existing variation in the community, while artificial selection introduces variation into the community.
• D. Natural selection has the goal of cultivating specific characteristics, while artificial selection does not.

Evidence of Natural Selection

Natural selection is an explanation for how populations change, but it's also something that we can just see in real time. So here we're going to look at some evidence of natural selection. We're just going to start by saying natural selection can be observed in relatively short time intervals, and we're going to look at 2 examples of this. First is methicillin resistant Staphylococcus aureus or really just antibiotic resistance in bacteria, and then we're going to be looking at coat color in the rock pocket mouse. Now we're using these two examples because these are really common examples that you're likely to see either in lecture, your textbook, or maybe even on a test.

Alright. So let's talk about this methicillin resistant Staphylococcus aureus or MRSA. Now again, this is really just the same story for any antibiotic resistant bacteria though. Now in any population for natural selection to occur, we said we need to start with variation. So we're going to have this population of bacteria here that could cause an infection.

And we have sort of 2 types here that we're calling out with different colors. They are either the not resistant or the resistant bacteria based on whether they are resistant to antibiotics. Now, when people get an infection, you take antibiotics to kill the bacteria. So we're going to add a selection pressure to this population. Alright.

Our selection is going to be the presence of antibiotics. All right. Now antibiotics are supposed to kill the bacteria, but we have variation in this population as to whether or not those bacteria are resistant. So when we add the selection pressure, that antibiotics, well, a lot of the bacteria die, but not all of them. We have a difference in fitness.

The resistant bacteria have higher fitness because we see here the resistant bacteria were more likely to survive the presence of that antibiotics. And so when that happens and the population then reproduces, and maybe people use more antibiotics as this infection spreads to someone else, and then they use antibiotics, so on and so on. Well, then eventually that population evolves resistance. So resistant bacteria become more and more common. And this is a story that is played out in all sorts of different bacteria.

And I'll just note that antibiotics were one of the great public health advances of the 20th century, and most public health experts agree that antibiotic resistance is going to be one of the great public health challenges of the 21st century. Alright. Next example here is the rock pocket mouse. Now this is a story that's really developed in the past 20 or 30 years or so that people have been doing research on this. And this is the idea well, there are these mice that live in the desert southwest, and they have different coat colors, brown or dark coats, and most of these mice live well, in the desert southwest, the ground is largely brown, stone-colored, but there's going to be areas where, well, these mice live on this black volcanic rock.

So we're going to talk about these mice on this black volcanic rock. So the population starts out mostly with tan fur, some black fur though. And predators though, as this population moves on this black volcanic rock, they are hunting mice on this black rock. And so that's our selection pressure. You can imagine what's going to happen.

Who's more likely to survive? Well, our fitness difference. Black mice have higher fitness because they're better camouflaged, and then as this population now reproduces, well, what's going to happen? The population evolves. Right?

That black fur becomes more common in that environment with that black volcanic rock. Right? Now this story is so powerful and has become so widespread because people know the genetics of mice really well. So the researchers have been able to look at not just the coat color, but also they've been able to track the underlying genes as the population changes. And they've seen a very similar story play out in all sorts of different populations of mice as they adapt to different substrate colors, different ground colors.

Alright. So again, these two examples, things you're very likely to see. I'd recommend being at least a little bit familiar with them. We have more examples in practice file. I'll see you there.

Evidence of Natural Selection Example 2

This example is about the peppered moths. Let's take a look. It says industrial melanism in the peppered moths is one of the classic examples of evolution by natural selection. I just want to pause right here. This is one of the classic examples.

This is something you've likely heard about before, and if not, I definitely recommend you'd be familiar with this story. Alright. Let's keep going. It says at the start of the 1800s, almost all moths were light colored with small dark spots, which allowed them to camouflage on the lichen of trees. During the industrial revolution in the mid to late 1800s, much of London and surrounding areas were covered with a layer of black soot.

By the 1900s, most moths were fully black colored. Today, the population again is composed of mostly light colored moths. Based on this evidence, answer the questions below. Alright. Before we answer the questions, let's just look at the image here.

So here we see a dark background, sort of that soot colored background, and here you see that light colored moth really being very visible on it. And this dark colored moth right there, very hard to see. Now on this light colored lichen background here, the light colored moth, very hard to see. The dark colored moth, very easy to see. Alright.

So now that we have that background, let's see these questions. The first question here, what variation did natural selection act upon? Remember natural selection can only act if there is variation in the population. So what was the variation we're talking about? Alright, should be pretty straightforward here.

It's just talking about the light versus dark coloring, I'll say. Alright. Next, it says in the early 1800s, what variant had higher fitness? How do you know? Alright.

Well, what do you think? Which variant in the early 1800s had higher fitness? Well, it says here at the start of the 1800s, almost all moths were light colored. Alright. Well, that seems that the light colored moths probably had higher fitness at that time.

Right? If they're very common, it's likely because they are surviving and reproducing better and that's our definition of fitness. So light, I'm just going to say they're more common and we can assume why that would be, right? In the start of the 1800s, there wasn't black soot everywhere. There was this lichen on the trees, and so they were better able to survive because they're better camouflaged.

Alright, in the late 1800s, which moths had higher fitness? Well, in the late 1800s, it says here by the 1900s most moths were almost fully black colored. So that means in the late 1800s, those black colored moths were reproducing more. They have higher fitness. So I'm going to say the dark colored moths have higher fitness then, and again it's because this population is changing to become dark colored and sort of that's our definition of fitness, they're surviving and reproducing more.

So finally, why do you think the light colored moth is again more common today? Well, I don't know if you've been to London, but it's not covered in black soot. Okay? So the environment, is no longer dark colored. Right?

So remember fitness depends on the environment and when the environment changes fitness changes. So when there's a dark layer of soot everywhere, everything's black and grimy. It's advantageous. You have higher fitness to be a dark colored moth. When that soot goes away and lichen comes back, it's light again, well, fitness changes.

Now the light colored moths are more common again, and that's natural selection. Alright. More practice after this. Give it a try.

Bedbugs are a pest that has been a nuisance throughout human history. In the 1940s, the widespread introduction of the pesticide DDT allowed people to effectively kill bedbugs at home. However, a mutation in the gene that codes for sodium channels in these insects provided resistance to DDT in some bedbugs. DDT was banned in 1974, but most bedbugs today are still resistant to DDT. What could be an explanation for why most bedbugs still have resistance to DDT?

• A. The mutation that provides resistance to DDT also provides resistance to other commonly used pesticides.
• B. Bedbugs have retained resistance in case DDT is used in the future.
• C. The large amount of DDT used before 1974 bred for exceptionally high levels of resistance, meaning resistance is unlikely to wear off for a very long time.
• D. Once a trait has been selected for, a trait will remain in the population going forward.

A proposed strategy for combatting the spread of antibiotic resistance is to allow doctors to prescribe only a certain subset of antibiotic drugs and regularly rotate which antibiotics are allowed to be prescribed over several years. Why could this approach limit the spread of antibiotic resistance?

• A. Using different antibiotics at different times will prevent your body from developing too strong a resistance to any one drug.
• B. During times when a specific antibiotic is NOT being used, those bacteria with resistance will not have higher fitness and are more likely to be removed from the population.
• C. Removing the use of certain antibiotics lessens the chance that bacterial genomes will mutate in ways that provide resistance to that specific antibiotic.
• D. This approach would allow bacteria to evolve resistance to less powerful and, therefore, less important drugs while keeping resistance low to more powerful and important antibiotics.