Natural Selection - Online Tutor, Practice Problems & Exam Prep

Natural selection, a key mechanism of evolution, is based on two observations: variation is inherited and species tend to overproduce offspring. This leads to the inference that certain traits enhance survival and reproduction, resulting in evolutionary changes in populations over time. For example, in snowshoe hares, coat color variation affects camouflage and survival rates. Natural selection acts on existing genetic variation, increasing adaptations to current environments, while fitness is relative and context-dependent. Ultimately, populations evolve, not individuals, as traits become more prevalent through generations.

Natural Selection: Observations and Inferences

We now want to spend some time really diving into natural selection and how natural selection leads to evolution. But we're just going to start by saying that natural selection is based on 2 observations and 2 inferences. Now depending on your book or your professor, you may break these ideas up slightly differently, or sometimes these are called Darwin's 4 postulates. But these ideas are going to be central to any description of natural selection. Now by observations, we mean that there are going to be 2 things that if you go out look around in the world, they are just unquestionably true.

And our 2 inferences are going to be 2 very logical conclusions that we can draw from those observations. Alright. So let's take a look. Our first observation is going to be that variation is inherited, and the first part of that is just that there is variation in populations. Right?

In a population, not all organisms are identical, and the second part of that is that a lot of that variation is inherited from organisms' parents. Variation is passed from parent to offspring. Alright. Now, we're going to use rabbits as an example. We have here a population of white and brown rabbits, and well, that white and brown coat, that's our variation, and you can imagine that this type of variation would be passed from parent to offspring.

Now today we understand that when things are passed from parent to offspring in that way, we're talking about genetic variation. But of course, when Darwin was coming up with this idea, that was before genetics was a science. Alright. Well, observation 1 is that variation is inherited. Observation 2 is the idea of overproduction.

Species produce more offspring than the environment can ever possibly support. Right? If you know anything about rabbits, you know they have the potential to make a lot more rabbits very quickly. Right? Now depending on the species, it can vary.

But usually, a female rabbit can have more than 10 offspring every year, and those offspring can start reproducing when they're just a few months old. So a small population of rabbits has the potential to get really big, really fast, and yet, right, the world's not covered in rabbits. So what's happening? Well, not all the rabbits live, not all the rabbits get to reproduce. The sad fact is most baby rabbits die.

Well, we can take that observation, and that's going to lead us to our first inference. If not all rabbits get to reproduce, well, then we have this idea of selection. Certain traits will make survival and reproduction more likely, and the inverse of that is certain traits will make survival and reproduction less likely. Alright. In this story, you could imagine maybe being a white rabbit makes it harder to be camouflaged.

And so, well, the fox is going to catch those white rabbits at a higher rate, and inversely, it's not going to be catching and eating the brown rabbits quite as much. So being a white rabbit, in this case, means you're less likely to survive and reproduce. Being a brown rabbit means you're more likely to. Well, if that's the case, we can really clearly lead to our next inference here, and that's going to be around the idea of evolution. Those traits that help organisms survive and reproduce will accumulate in the population.

If brown rabbits are surviving and reproducing at a higher rate, well, they're going to pass that trait on to the next generation, and our population will end up having more brown rabbits and fewer white rabbits than at the start. Alright. That was our definition of evolution. A change in a population over time, and we just saw it there. Now this may seem kind of straightforward, maybe kind of simple, and in a lot of ways, it is.

But this is an idea that has a lot of depth to it and some real nuance as well. So we're going to spend some time really looking at it closely. Before we do, we have some examples and practice on these ideas, and I'll see you in the next video.

Natural Selection Example 1

In this example, it asks, how do giraffes get such long necks? This is a classic question in evolution. We want to use the observations and inferences of natural selection to describe in simple terms how long necks could evolve in giraffes. Alright. So we have this little illustration here showing we have maybe some shorter-neck giraffes, and over time these giraffes get longer necks, the long-neck giraffes that we see today.

Alright. So remember, we start with variation. Natural selection needs variation to work. So what would be the variation that you see here? Alright.

I'm just going to go with neck length. It's perfectly reasonable that in the population of some ancestors of the giraffe, some had a slightly longer neck, some had a slightly shorter neck. Alright. Well, what about overproduction?

What are we going to say there? Well, we'll just say more giraffes are born than can reproduce. Alright. We said that is true for really all organisms. Alright.

Well, what's the selection going to be in this case? Well, we'll say that longer necks get more food. Right? That's the classic story. You have this very tall tree here and the giraffes with the longer necks will be able to get those leaves up in that high tree easier.

That means they're more likely to survive, because they're less likely to starve to death. If you survive, you reproduce. Alright. Well, that means evolution. What's going to happen to this population?

Well, average neck length increases. Right? If the individuals with shorter necks are not reproducing, they're not passing on that variation. Those individuals with longer necks are reproducing. They're passing on that variation, and the population changes.

The average neck length gets longer. Alright. Practice this more in problems after this. I'll see you there.

Which statement below best restates one of the key observations central to the concept of Natural Selection?

• A. New environments will introduce new variation to a population.
• B. Some variation has a developmental basis or is influenced by the environment.
• C. Some individuals will have more offspring survive and reproduce than others.
• D. Traits become more common in a population when they are used more often.

In a population of crickets, all crickets make a “chirp” to attract mates at the same rate. Chirping also attracts predators, however. In one season, more than 50% of the crickets are eaten as prey as a result of chirping. In this simplified example, using only the information given, do you think this population will evolve by natural selection and why?

• A. Yes, because chirping increases reproduction, you can expect chirping to be more frequent in the future.
• B. Yes, because chirping attracts predators, there is a different rate of survivorship among crickets.
• C. No, because chirping attracts both predators and mates, the two effects will cancel each other out.
• D. No, because there is no initial variation in chirping, the population cannot change through natural selection.

Important Insights for Understanding Natural Selection

When we introduced natural selection, we said that it's an idea that can feel kind of simple and straightforward, but it has a lot of depth and nuance to it. Here, we want to explore some of that depth. Alright. In our previous example, we used white and brown rabbits. Well, it turns out this is a real example.

We're going to be talking about snowshoe hares. Now most snowshoe hares, you probably know, they live in areas like in North America and Canada, where there's snow during the winter, and so the snowshoe hare turns white during the winter to be more camouflaged. But importantly, not all snowshoe hares do. Some snowshoe hares remain brown, and especially in areas sort of near the edge of their range where it's a little warmer, where there's not always as much snow, you have more hares that remain brown. So that's going to be our variation.

Well, we talked about overproduction. It turns out that snowshoe hares, the females can birth 12 offspring or even more than 12. We'll say 12 plus offspring per year, and those rabbits are sexually mature at just a few months. So you have a real chance for a lot of rabbits to be born very quickly. Well, not all of them can survive, but in a year with little snow, you can expect what's going to happen.

The white hares are going to be killed at a higher rate. Right? So we have our new population here from reproduction, but the animals that are catching these, they're going to be picking off those white ones a lot more frequently than they're picking off the brown ones because they're not as camouflaged. Well, then we have evolution. If we come back, right, the population will have fewer white hares in the future.

We expect that the population is going to look different. It's going to have more brown hares because they pass that variation on. Alright. So let's take a closer look at these and some insights from what we just talked about. The first thing that I want to know is that natural selection requires existing variation.

The population changed because there are already brown and white rabbits. If there is not variation in the population, natural selection can't do anything. If all the rabbits turn white every year, well, we got no story here. There's going to be no change. Natural selection does not introduce variation.

Natural selection selects from variation that's already in the population. Our next insight is that members of the same species are competing. Right? We may have been thinking about, well, can these rabbits get away from the fox? Right?

And you think of that as the competition. But what's really important is not, Does this rabbit get away from the fox? But rather, Does this type of the rabbit get away from the fox more often than other types of rabbits? What gets passed on depends on which rabbits in this population are surviving. That leads us to this idea of fitness.

Remember fitness is the likelihood that an organism is able to reproduce. Well, fitness, we're going to say is relative. Now there are ideas of you know, there is something called absolute fitness, but in very general terms, the way you want to think about it is in relative terms. Right? If you're able to survive and reproduce and have one offspring, that might be great, but it's not very good if the next rabbit over has 82 offspring.

Right? That is not a very good fitness. You're not putting your traits in the next generation nearly as much as the next one over. It's also, though, going to depend on environment. Our fitness here, which rabbit was more likely to survive and reproduce, completely depended on this idea that there was not much snow this year.

If it snows a lot, well, these fitness values are going to be completely different. Different rabbits are going to be surviving and reproducing. Our next idea though is that survival and reproduction is based on probability. In our example here, rabbits of both colors were caught by the fox. And, right, if you have a population, rabbits have something like 80% mortality.

80% of the offspring die. Well, if your coat color means that you only have 70% mortality, that's pretty good. However, it still means that that one rabbit is probably going to die. It just means that as this gets played out thousands of times over, hundreds of thousands of times over across big populations, the results are going to be very predictable. Alright.

Our next idea that we want to talk about is that the populations evolve, individuals don't. Right. In this example, the individuals either survived or they didn't. They reproduced or they didn't. And at the beginning, at the end, we have white and brown rabbits.

Right? We have that same variation. What changed in the population was the frequency of the trait. Right? That idea, the frequency of the trait, that's a population level idea.

The population changes, individuals survive and reproduce, or they don't. Our next idea is that natural selection increases the number of adaptations to the current environment. Right? Over time, natural selection is going to make populations more adapted to the environments that they live in. As more and more traits become more and more adapted, natural selection sort of homes these organisms to survive in these environments.

But let me go to the current environment. Right? If this environment changes, which environments often do, then the population, well, it's going to evolve as different organisms, and different variations now have higher fitness. Alright. We're going to play with these sort of, like, nuances around natural selection some more.

Give it a try, and I'll see you there.

Natural Selection Example 2

Our example here says the graph below shows beak depth of medium ground finches on Daphne Major in the Galapagos before and after a major drought. The graph displays both beak depth for the population before the drought in white and for the survivors that reproduced after the drought in black. Use the graph to answer the following questions. Alright. Let's analyze this graph.

We see the white bars, and those black bars overlaid and on the y-axis, which represents a count, and on the x-axis, which represents beak depth — the height of the beak where it attaches to the face of the bird. Note that this data comes from Rosemary and Peter Grant, two scientists who spent years on Daphne Major cataloging every single bird and measuring all kinds of traits to track evolution in this population over time. In the legend, it shows the average beak depth for both groups.

Our first question here is, by how much did the average beak size change after the 1977 drought? The average for the breeding population before the drought in 1976 was 9.2 millimeters, and for those who bred after the drought, the average beak depth was 9.9 millimeters. So the calculation would be: 9.9−9.2=0.7 mm. Thus, the beak size change is 0.7 millimeters, showing a small yet documented change in this population.

Next, which birds had higher fitness during the drought? Observing the graph, those birds with deeper beaks appear to have had a better chance of surviving and reproducing. Even though each size class had survivors, the percentage was higher in classes with deeper beaks. Thus, birds with deeper beaks had a higher fitness.

Do you think your answer to the previous question is always true on Daphne Major? From the given information, it's hard to generalize, but the answer is likely no. If deeper beaks were always advantageous, it would be reasonable to expect all birds to have developed deeper beaks over time, yet the beak size before the drought was smaller. Historical populations had smaller beaks.

Finally, what do you think the beak depth of the offspring produced in 1977 will most closely resemble? Considering those that survived and reproduced had deeper beaks and assuming this trait is heritable, their offspring likely had larger beaks. The average beak depth for the offspring was approximately 9.7 millimeters, indicative of a slight evolutionary change towards deeper beaks, driven by natural selection. Even though the change was about half a millimeter, it still represents an evolutionary adaptation.

Herbicides, chemicals administered by farmers to kill weeds, have been hugely beneficial in increasing the efficiency of modern agriculture. Many of the herbicides used today, however, are less effective than when they were originally introduced because weed populations have evolved resistance. Knowing this, which of the following statements is likely true?

• A. Individual weeds that were able to adapt to the presence of herbicides survived, allowing the species to evolve.
• B. The herbicides must negatively affect both the survival and reproductive ability of the weed species.
• C. Herbicide resistance evolved because some species were able to outcompete other species in the presence of the herbicide.
• D. Species that developed herbicide resistance must have had pre-existing heritable variation as to how likely they were to survive in the presence of an herbicide.

Which of the following statements correctly explains a way in which evolution by natural selection and Lamarck's ideas about evolution are different?

• A. Lamarck focused on an organism's ability to reproduce, whereas natural selection focuses on survival.
• B. Natural selection requires variation to already exist in a population, whereas Lamarck said that variation could be created through use and disuse.
• C. Natural selection explains how individuals will adapt to new environments, whereas Lamarck explained how populations change in their current environments.
• D. Lamarckian explanations did not explain how populations become better adapted overtime, whereas natural selection did explain adaptation.

Here’s what students ask on this topic:

What are the main observations and inferences that form the basis of natural selection?

Natural selection is based on two main observations and two inferences. The first observation is that variation is inherited; not all organisms in a population are identical, and much of this variation is passed from parent to offspring. The second observation is overproduction; species produce more offspring than the environment can support, leading to competition for resources. From these observations, we infer that certain traits enhance survival and reproduction, making those traits more common in the population over time. This leads to the second inference: evolution, defined as a change in the population over time as advantageous traits accumulate.

How does natural selection lead to evolution?

Natural selection leads to evolution by favoring traits that enhance an organism's survival and reproduction. When a population has genetic variation, certain traits may provide a survival advantage in a given environment. These advantageous traits increase an organism's fitness, or likelihood to reproduce. Over time, these traits become more common in the population as they are passed on to successive generations. This gradual change in the frequency of traits within a population is what we call evolution. For example, in snowshoe hares, coat color affects camouflage and survival rates, leading to more brown hares in areas with less snow.

Why is genetic variation important for natural selection?

Genetic variation is crucial for natural selection because it provides the raw material for evolution. Without variation, all individuals in a population would be identical, and natural selection would have no traits to favor or disfavor. Variation allows some individuals to possess traits that give them a survival or reproductive advantage in a specific environment. These advantageous traits become more common over generations, leading to evolutionary changes. For instance, in snowshoe hares, the presence of both white and brown coat colors allows natural selection to favor the color that provides better camouflage in varying snow conditions.

What is the role of fitness in natural selection?

Fitness in natural selection refers to an organism's ability to survive and reproduce in its environment. It is a relative measure, meaning an organism's fitness is compared to others in the population. Traits that enhance fitness increase an organism's chances of surviving and reproducing, thereby passing those traits to the next generation. For example, in a population of snowshoe hares, brown hares may have higher fitness in areas with less snow because they are better camouflaged from predators. Over time, the frequency of the brown coat trait increases in the population, demonstrating evolution driven by natural selection.

How does the environment influence natural selection?

The environment plays a critical role in natural selection by determining which traits are advantageous. Environmental conditions, such as climate, availability of resources, and presence of predators, influence which traits enhance an organism's survival and reproduction. For example, in snowshoe hares, the amount of snow affects which coat color provides better camouflage. In snowy environments, white hares are less visible to predators, while in areas with less snow, brown hares have a survival advantage. As environmental conditions change, the traits that confer higher fitness may also change, leading to shifts in the population's genetic makeup over time.