Introduction to Evolution and Natural Selection - Online Tutor, Practice Problems & Exam Prep

Evolution is the gradual change of populations over time, defined by "descent with modification," where species share common ancestors. Natural selection, a key mechanism of evolution, favors traits that enhance survival and reproduction, leading to adaptations that suit organisms to their environments. Understanding species relationships through phylogenetic trees emphasizes common descent rather than a hierarchy of complexity, highlighting the importance of variation in defining species. This framework is essential for grasping evolutionary processes and the diversity of life.

Evolution and Natural Selection

Hello folks, and welcome to Evolution and Natural Selection. Alright. Evolution is one of the big ideas in biology. It's an idea that explains so much that's going on in the world of biology. And here we're going to start talking about it.

Here, we're really just going to sort of set the stage and then these ideas we're going to go into a lot more detail coming up. You'll notice though here that we said evolution and natural selection. A lot of times you hear evolution by natural selection. What we want to do first is just figure out what these words mean separately, so then we can weave them together and understand how all this ties together really nicely. Alright.

So let's start with this term evolution. Evolution, we are going to define as the gradual change of a population through time. Alright? So if there's a population, you come back a year later, a million years later, whatever it is. If that population is different, we're going to say that it has evolved.

Now we can sum this up in the next phrase here, actually coined by Charles Darwin, descent with modification. Descent means, you know, there you have offspring, those offspring have offspring. That's the idea of descent. And just as that happens over time, things are modified. Things change. Now this brings up the idea of common descent, and common descent is the idea that species alive today share common ancestors. So to illustrate that we have this evolutionary tree here and this evolutionary tree shows the relationships between the organisms in the genus Panthera or sometimes just called the big cats. And what you see here is that all these cats, well, you can trace their ancestry back. You can trace their descent back to a common ancestor. So why do these big cats share so many traits in common?

Well, they have common descent. They share a common ancestor. But you can also ask, well, why is a jaguar different than a leopard or a tiger? Well, descent with modification. Yes.

They share a common ancestor. But through these lineages of descent, things have changed over time. They're now different organisms. That's the idea of evolution. Alright.

But what about natural selection? Natural selection, we're going to say, is a mechanism, the major mechanism, of how and why populations evolve. This is what's going to cause that change over time. And we can state natural selection in the simplest terms I possibly can think of is just to say that traits that help individuals survive and reproduce become more common. Right?

Reproduction passing your traits on to your offspring. That's the idea of descent. Well, if there are traits that make it more likely that you reproduce, well, those traits are going to be more common in the next generation and the population will have changed because of it. That causes evolution. Now you've probably seen this phrase before, survival of the fittest.

That's a phrase that sums up natural selection. It's actually a phrase that dates back to the time of Darwin, though he didn't coin it. And we just want to look at this for a second because we want to think what this really means. Well, survival of the fittest, the things that are fittest are going to survive and then pass on those traits. We just want to note, we also said that really reproducing is what's so important because you want to pass those traits on.

Then we also want to look at this term, the fittest, because this causes some people some confusion. And the term fitness, that's actually going to be a vocabulary word that you need to know. So when we're talking about evolution and natural selection, when we're talking about fitness, we're not talking about physical fitness. The word in the scientific context has a different meaning. What it means here is the likelihood that an individual contributes offspring to the next generation.

So an organism that is very fit is likely to have offspring. Its traits will be passed on. Now, survival of the fittest. It's really important to survive because you need to survive to reproduce. Alright, so natural selection is so interesting because natural selection leads to adaptation.

An adaptation is just any trait that makes an organism well suited to its environment. So when you look at organisms out there in the world, we have the question just why are organisms so good at what they do? Why do they seem to fit so well in their environment? Well, they develop adaptations because of natural selection. Alright.

We're going to look at all this in a lot more detail coming up. It's going to be a heck of a good time, and I'll see you there.

Introduction to Evolution and Natual Selection Example 1

Our example here tells us that evolution and natural selection are related concepts and can be difficult to separate completely. We want to read the following statements and decide whether the statement is more directly related to the concept of evolution or the concept of natural selection. For concepts relating to natural selection, write ns. For those relating to evolution, write e. Alright.

So let's try and remember how natural selection and evolution are different from each other, and let's look at these statements. Our first statement says a caterpillar that is camouflaged is more likely to reproduce because it is less likely to be eaten by a bird. So what do you think? Natural selection or evolution? Well, we're talking here about the likelihood it is to reproduce because of how likely it is to survive, and that likelihood is based on a trait.

It's camouflage. And so I can surmise that it's more likely to reproduce. It's going to pass that camouflage on to the next generation. All that sounds like I'm describing natural selection. Alright.

Next up, we have the closest living relatives of birds are alligators and crocodiles. Therefore, we expect birds to share many traits with reptiles. What do you think? Evolution or natural selection? Well, to me, this sounds like we are talking about common descent.

This idea that things share common ancestors. Birds and alligators are related to each other because they have a common ancestor, so we expect them to share some traits. That's all tied up in that concept of evolution. Alright. Our next one here says that fossil data indicates that early ancestors of modern horses were less than a half a meter tall.

And a trend in horse evolution is that horse ancestors got larger over time. Alright. First off, I just think it's fun to think of little half meter tall horses running around, but let's think natural selection or evolution. Alright. To me, this sounds very much like descent with modification.

Right? We're talking about the ancestors of horses. That's that idea of descent. But they've changed over time. They've been modified.

Descent with modification. That is evolution. Alright. Our next one here, it says, When thinking of fitness, attracting mates can be just as important as surviving. What do you think?

Evolution or natural selection? Well, when we're thinking about fitness, we said fitness was the likelihood to reproduce. Here, we're talking about that likelihood. Well, attracting mates is just as important or maybe more important than surviving sometimes. But that is all tied into that idea of natural selection.

The things that reproduce the most will pass their traits on to the next generation. So I'm putting the ns on this one. Our final one here says dandelion seeds are able to travel great distances in the wind. The dandelions whose seeds travel the farthest have the best chance of spreading their offspring widely. What do you think?

Evolution or natural selection. So, again, we're talking about how likely it is for these things to reproduce and for the offspring of that reproduction to be successful. That's all tied into that idea of natural selection. All right. Again, we're going to be looking at evolution and natural selection, both of them very closely going forward.

But we can practice this some more and practice problems coming up. Check them out.

Explain the relationship between natural selection and evolution.

• A. Evolution is an explanation for how natural selection happens.
• B. Evolution and natural selection explain the same phenomena but use different timescales.
• C. Evolution explains how life begins, while natural selection explains how life changes.
• D. Natural selection is an explanation for how evolution happens.

Which organism will have the highest fitness?

• A. Organism A lives for 1 year, is relatively large, and produces 1 offspring.
• B. Organism B lives for 5 years, is relatively small, and produces 1 offspring.
• C. Organism C lives for 1 year, is relatively small, and produces 3 offspring.
• D. Organism D lives for 3 years, is medium-sized, and produces 2 offspring.

Which statement would be supported by the concept of common descent?

• A. Adaptations allow species to survive well in the environments in which they live.
• B. Many similarities between species exist because they were inherited from the same ancestor.
• C. Organisms that reproduce the most are considered to be the most fit in an evolutionary context.
• D. Species change over time in response to environmental pressures.

Typological vs. Population Thinking

Before we really dive into evolution and natural selection, we just want to think about how we talk and think about organisms. And so this is going to lead us to contrast two types of thinking: typological and population thinking. So let's start out by talking about typological thinking. Typological thinking is when you have sort of one ideal or one sort of idealized form to which we compare other things in the world. And this is an idea that sort of came from Plato, the ancient Greek philosopher.

And when we think of that one ideal, when we go out into nature and we see variation, well, we're going to think of variation as a deviation from that ideal. All right. To give an example of this, we're going to look at some lions. So if I sort of picture a lion in my head, this is kind of my idealized form for a male lion. But of course, if you go out into nature and you look at lions, you're going to see that there's actually a lot of variation out there.

These are all sort of variants of manes of lions. You have some that have dark manes, some big old dark manes that go way under their belly. You have some males that actually don't grow manes at all. So typological thinking says, well, all this variation, that's not really what a lion is. A lion is this idealized form, and those variations, that's just sort of a deviation from that ideal.

Now, if we take this approach, that means that a species description is going to be unchanging over time. And importantly, it does not include its diversity. All right. So now let's contrast that to population thinking. Well, in population thinking we're going to start out by saying that variation from all individuals is important.

So now if I wanted to find what a lion is, I don't think of, you know, in my mind's eye, what is a lion. I need to go out and I need to find all the lions or at least the representative subset of lions. So I start with the group of lions, and I look at all this variation. And that's how I define my species.

Now from all this variation, I can take an average, and I might get an average and say okay this is my average lion. But we want to be really clear here. What is average in population thinking? Was average or typical? It depends on the variation.

Right? There is no lion that is average in every single way, that is typical in every single way. This average lion in population thinking, it's really more of a statistic. So when we use this way of thinking, well, yeah, we can have an average, but we acknowledge that the variation is real. This means that a species description will change over time and very importantly includes its diversity.

All right. Now you may look at this and say, like, okay. Yeah. Well, that makes sense. Why would we ever do typological thinking?

But we do forms of typological thinking a lot. It's a common way to think, and it's a common way to think in biology. Now when you hear in genetics when we say something like the gene for hemoglobin or in physiology when we say the human heart, we're usually thinking of sort of a single standard form of that and not really worrying about the variation at that time. Now that's a typological way of thinking that helps us out because we really just want to understand how these things work at one level, and we can worry about the variation later. But for evolution, it is really important.

To understand evolution and natural selection, we must use population thinking. We must focus on that variation. All right. Examples and practice to follow. I'll see you there.

Introduction to Evolution and Natual Selection Example 2

Alright. Let's take a look at this example. It says modern biologists agree that species are not fixed entities. But sometimes, typological ways of speaking and describing organisms are used. Of the following statements, mark those that you think are more aligned with typological thinking with a t.

Mark those that you think are more aligned with population thinking with a p. Alright. So we're gonna read these statements. Just wanna know, all these statements are true. There's nothing, sort of, inherently wrong with these statements.

But some of them just, sort of, lean more towards describing the world in a population viewpoint, and some lean more to describing the world from a typological viewpoint. So let's take a look. Our first one here says that the normal height for a human female is 5 foot 5 inches tall. What do you think? Typological or population viewpoint?

Well, that word "normal" keys me into that typological viewpoint. We have sort of one thing that is normal and variation outside of that we consider abnormal. Now the average height for a female, at least in the United States, is about 5 foot 5 inches tall. But it's perfectly normal to be 5 feet tall or 5 foot 10. Right?

That variation is expected in the population. And this statement doesn't sort of include that viewpoint that includes that variation. So this is definitely a typological way of describing at describing average height. Alright. Our next statement here is mature white oak trees often live for more than 200 years with rare examples living more than 400 years.

Alright. What do you think? Is that more of a typological or population thinking type statement? Well, does it include the variation? Yeah.

it really does. Right? They often live this long, so they don't always. Some of them even live over that over 400 years. But it's not giving me sort of a type and saying different from that is wrong.

It's just saying this is what we see out there. This is really more of a population statement because it's taking that variation into account. Alright. Our next one. The eastern gray squirrel is a rodent with uniform gray fur weighing between 406-600 grams.

What do you think? Population or typological thinking? Well, I'll ask you this. If you found a gray squirrel that didn't have uniform gray fur, would it still be a gray squirrel? I mean, I think so.

It's part of that species. If you found one that weighed 350 grams or 650 grams, would it still be a gray squirrel? Yeah. It would. Right?

So this is really a typological way of thinking. It's giving us sort of a narrow species definition. Now identifying species, we often talk this way because it gives us sort of one thing to look for, but it is more of a typological way of talking. Our next one says that a jaguar or leopard with abnormally dark coloration is called a black panther. What do you think?

Typological or population? Well, jaguars or leopards that have black fur and don't have the normal spots, that's what we do call a black panther. But that variation is normal. Right? That variation is out there in nature, and it's just a type of cat that is part of that species.

In this statement, it says that that's sort of an abnormal dark coloration. Whenever we see something abnormal, that's really a typological way of thinking. We have sort of a normal jaguar and the abnormal one. Well, black panthers, those are just part of the variation that you see in leopards and jaguars. Alright.

Our final statement here says that the mean length of a birch tree leaf is 7 plus or minus 2 and a half centimeters. What do you think? Population or typological type thinking? Well, when I look at this, the mean length, that's an average. But in this average too, it's giving me the variation, plus or minus 2 and a half centimeters.

So this statement is just sort of acknowledging that this average is a statistic. This average came from measuring a lot of birch leaves out there. And so this is a population way of thinking. Right? It is describing the variation.

Even though it's just giving us an average, it's acknowledging that that average is, is made up of lots of different variation. Alright. So again, none of these are necessarily wrong statements, but we do want to practice thinking in a population viewpoint because that really, really helps us in understanding how evolution works. We're gonna practice a little bit more. I'll see you there.

What is one reason population thinking is so important in biology?

• A. Population thinking requires scientists to consider the full variation that is present in nature.
• B. Population thinking allows for fast and accurate species identification.
• C. Population thinking makes identifying abnormalities and diseases more straightforward.
• D. Population thinking simplifies making comparisons between different species.

Dog breeds are defined by denoting breed standards, a list of agreed-upon traits that individuals of that breed should possess (such as size, coat, coloring, temperament, etc.). Dog shows rank dogs based on their adherence to these breed standards. Based on this information, which statement about dog breeds below is correct?

• A. Breed standards describe many different traits; because many traits are considered, this reflects population thinking.
• B. Because there are many breeds of dogs, dog breeds reflect population thinking.
• C. Dog breeds reflect typological thinking because through breeding, humans control what traits a dog inherits.
• D. Breed standards use an idealized form to which dogs are compared; this reflects typological thinking.

Ladder vs. Tree Thinking

Now again, before we really dive into evolution and natural selection, we need to spend a little bit of time just sort of thinking about how we think about things. And in this video, we're going to look at ladder versus tree thinking. This is just sort of this idea that historically, when we thought about organisms, people would create this hierarchy of complexity. Where today, we really try to focus on evolutionary relationships.

So let's look at these two ways of thinking. First, we'll talk about ladder thinking, and ladder thinking focuses on this hierarchy of species. Now this really comes from Aristotle, the Greek philosopher, and he said that species were unchanging. They had different levels of complexity, and that's really what divided the species, their complexity levels. He created what became known as the Scala Naturae or the great chain of being.

This ordered species from low complexity. At the lowest complexity, Aristotle put plants. And it went up to high complexity, which I bet you can imagine what he put on top, put humans. This way of thinking really dominated Western thought up through and past the Middle Ages. So if we look at this sort of ladder here, here's our hierarchy.

You see plants at the bottom and as you go up, you get invertebrates, fish, mammals, humans, and often people would even just put gods at the top there. Well, that's well and good. But if you really think about it, are humans more complex? Certainly, if you think about the central nervous system, yeah. We probably are more complex than other organisms.

But if you think about other traits, like camouflage. An octopus, well, it's an invertebrate, but it can change its shape, its skin texture, and its skin color to hide. If we think about plants, they can do photosynthesis. They can process sunlight to produce usable energy in the cell.

We can't do that at all. So today, we really want to think about tree thinking. In tree thinking, we're going to say that species are related through common descent. Thinking about relatedness gives us this evolutionary tree view of the world, and that allows us to put our focus on species relationships. Right?

So when I look at this tree, an important thing is there is no hierarchy. There's no thing on top. These things are all at the top of the tree because they're all alive today. And that gives us sort of this little bit of insight that I really want you to know that all populations have been evolving for the same amount of time. So what differentiates species isn't necessarily complexity.

Lots of species are complex in specific ways and not complex in other ways. What differentiates them is how well they fit in their environment. Species have been changing through time to fit in their environment where they reproduce and survive the best. Now when we think of variation, this means that these shared evolutionary histories can explain a lot of species variation. So now when we look at a tree like this, we can ask a question like, well, why do these organisms have bones?

The organisms on this tree that have bones, we can follow their descent with modification. We can follow their ancestry back, and we see they share a common ancestor. So answering questions like that, understanding why things have specific adaptations, those relationships provide us a lot of insights.

Now, again, you may think, well, that seems obvious. I wouldn’t use ladder thinking, but we do need to be careful with our thought. Because a lot of times, words saying things like, bacteria are a lower organism, slip into our language. And while you may know what you mean, it can undermine our thinking in subtle but important ways.

So we're going to practice with tree thinking some more coming up. You should check it out.

Introduction to Evolution and Natual Selection Example 3

In this example, we want to match the statements on the left with the type of thinking that they best represent on the right, and it says here some statements may match more than one type of thinking. Alright. So our types of thinking that we've talked about are ladder-based thinking, population thinking, tree-based thinking, and typological thinking. Alright. So let's go through these statements one by one.

A) Species are unchanging. Which way of thinking or ways of thinking do you think that applies to? Well, we said that ladder-based thinking often treats these unchanging species. So I'm going to put an A there. And also, we said typological thinking. Typological thinking, we said that species match an unchanging type, and that idea came from Plato. Alright. So that's A. Now we have B) Humans are the most complex organism.

Which type of thinking does that align with? Well, that's really that ladder-based thinking. Right? The ladder, or that great chain of being, the scala naturae, that puts organisms on a hierarchy from most simple to most complex, with humans being at the top. Alright.

Next, we have C) Species can change over time. Well, species can change over time. That aligns with both population thinking and tree-based thinking. Alright. Next, we have D) Common ancestry can explain similarities between organisms.

Which way of thinking does that represent? Alright. Really, that's at the heart of that tree-based thinking. We want to think of the relationships between organisms as their evolutionary relationships, finding their common ancestors. Alright.

That brings us to E) Species descriptions should include their variation. Well, that we said was at the heart of that population thinking. Thinking that when you're describing a species, you need to know all the variation that's out there, not describing that species by a single type. Alright. And then finally, we have variation as is seen as imperfection.

Alright. Variation is seen as imperfection. We say that that is most closely aligned with this typological thinking. Right? You have a perfect type, this ideal, and the variation that you see is some deviation from that type.

Alright. There are more videos to come. I'll see you there.

True or False: if false, choose the answer that best corrects the statement.

In tree thinking, species are organized from low complexity to high complexity.

• A. True.
• B. False; in tree thinking, species are thought of as unchanging.
• C. False; in tree thinking, species are organized from high complexity to low complexity.
• D. False; in tree thinking, species are organized based on evolutionary relationships.

The Latin term Scala Naturae reflects which argument?

• A. Darwin’s argument for descent with modification.
• B. Aristotle’s argument that organisms can be arranged hierarchically by complexity.
• C. Plato’s argument that all individuals deviate from a divine ideal form.
• D. The modern biological argument that organisms should be organized by evolutionary relationships.

Reading Trees

We now want to spend a little bit of time reading trees or learning how to read phylogenetic or evolutionary trees. Now in this chapter, it's very unlikely that you're going to have a lot of test questions about interpreting phylogenetic or evolutionary trees, but you are going to see them. So we just want to make sure that when we see them, we know what we're looking at and we can read them in a basic way. So we'll start off here by saying phylogenetic trees, which is just another word for evolutionary trees. This is a way to understand species relationships.

And those relationships are going to be based on common descent, this idea that organisms share a common ancestor. Now when you look at a tree, there's really just two things you're looking at. You're looking at branches and nodes. The branches of the trees, those are the lines of the tree. They represent a population over time as it changes and evolves.

The nodes, well, the nodes are where the branches meet. Those represent a common ancestor of the different populations on the tree. Now there's all different ways that trees are drawn. Sometimes, like the one we have here, organisms are sort of lined up on the side and the tree sort of goes back off to one side. Sometimes the organisms are on the top, the tree comes down.

Sometimes they're even drawn in a circle. Sometimes like this one, the lines meet at points. Sometimes they make right angles. All of that doesn't matter. What we want to focus on when we look at a tree, we're going to say here to read a tree, focus on how branches connect and they connect at the nodes.

All right. So to see what we mean here, we're going to use this phylogenetic tree, this evolutionary tree of the genus Panthera here, the big cats. And we see a common ancestor and we can trace this population through time. So we find this branch. This is that branch, the population of that common ancestor evolving through time.

Well, then we see it splits. Now there are 2 populations evolving through time. We have 2 branches on tree. Well, these branches split again as these populations separate. Here, we see one population that leads to the evolves into the modern jaguar.

We see another population that leads this way and it splits again. One part of that population evolves into the modern lion, and the other group that split off, well, it evolves into the modern leopard. Down at the bottom, we see this other population. Well, it splits. One of those populations that splits off becomes the modern snow leopard, while the other population evolves into the modern tiger.

So those are all our different branches and to find our nodes, we just find where they meet. So here's a node. That's a common ancestor of different populations of this tree. It's a common ancestor of all the cats on this tree. They can trace their branches back to that node.

Alright. We can find different nodes. All of these are different splitting points that are the last common ancestor of different populations on this tree. Now on this tree, we have this arrow here and this arrow represents time. In all these trees, we have this sort of inherent axis moving through time where the organisms that we're looking at are at least alive today or at least alive more recently in time.

And the common ancestor, as you go back through the tree, you're going deeper and deeper into time. So a way to think of this is that relatedness, how closely related organisms are equals how deep in the tree is that common node or in other words, that common ancestor. So to see what we mean, let's look at an example here. Well, take the leopard and the snow leopard. If you want to know how closely related the leopard and the snow leopard are, well, they both have the name leopard in their names.

They both well, they kind of look a little similar. And in space, they're right next to each other on this tree, so you might assume they're closely related. But if we look at how deep in the tree we find the common node, well, I gotta go back all the way down these branches, all the way to find way back here. This first node where that first population split was, that's the last common node that these cats share. They're not very closely related at all.

That node is the ancestor of all the cats on this tree. So we can ask, well, who is the leopard most closely related to? Well, we follow its branch back. We follow it to this node and we see, well, what other population came out of that node? The lion.

So the lion and leopard, they're most closely related to each other. Alright. We're going to practice this type of thinking. More coming up. We'll see you there.

Introduction to Evolution and Natual Selection Example 4

Our example here says, answer the following questions using the tree below, and well, here's the tree. So it's, our tree. Let's introduce you to the organisms. We have the ostrich, the rhea, the tinamou, the cassowary, the emu, and the kiwi.

The ostrich and the rhea and the cassowary and the emu, those are all large flightless birds. The kiwi is a sort of small flightless bird that lives in New Zealand, and the tinamou, that's actually a group of bird species that live in South America. All right. That's not really relevant to any questions we want to answer though. So let's go through our questions.

First, it says, a) which organism is most closely related to the emu? All right. Well, here's the emu. Take a look at the tree. What's most closely related to the emu?

Well, remember to answer these questions. What's most closely related? You follow the branches back to find the nodes. And the deeper in the tree, the farther back in time or the closer to the top of the tree, the more recent. So as I follow this branch back from the emu, this is its sort of most recent node, the most recent splitting event in that population, and I can follow that node up and it shows me that it's most closely related to the cassowary.

So I'm going to write the cassowary here. Alright. Next, we have is a Tinamou more closely related to a kiwi or a rhea? So here's the Tinamou. Here's the rhea and here's the kiwi.

Take a look at the tree and tell me what you think. All right. Well, when I look at this, I'm kind of instantly tempted to say the rhea because, well, you look here, they're sort of close to each other on the screen or on the paper. But remember, we have to find where these nodes meet, and I follow this branch back to a common node. That's the common node here with the kiwi, and I have to go deeper in the tree to find that common node with the rhea.

So more recently, I'm trying to not go as deep in the tree. So both the kiwi and the tinamou come out of this node here. So the Tinamou is going to be more closely related to the kiwi. Alright, next it says which bird or birds is the ostrich most closely related to? Alright, well here's the ostrich.

Take a quick look. Which birds is it most closely related to? Well, as I look, when I follow this back and I find this is the most recent node that the ostrich sort of branches out of and I look, well, everything else comes out of that node. So that's really a bit of a trick question. The ostrich is going to be equally related to all other birds on this tree.

So I'll write that equally related to all other birds. Birds with an r. Birds on the tree. Alright. Next, we have which bird or birds would you expect to share the most traits with the Tinamou?

Alright. Again, here are those birds. The Tinamou. Which things would you think have the most in common? Well, I would think that things that share a more common ancestor, a more recent common ancestor with the Tinamou would share traits because those traits would have been present in that common ancestor.

So I'm going to say what's most closely related to the Tinamou. I'm going to follow this back. Here's its, the last node that the Tinamou branches off from. What else comes out of that node? Well, the Emu and the Cassowary.

So the Emu and the Cassowary, those are both equally related to the Tinamou. I would think that they share a lot of traits with that, and with the Tinamou. Alright, so I'm going to say the Cassowary and the emu. All right. Next, we have e.

Circle all the descendants of the bird that lived at node a. Right. Here's the, here is node a. So what birds are descendants of the organisms that lived at that time? I'm just going to follow the branches that come out of node A.

Right? So coming out here. Well, that leads to all those and coming out this way that leads to the rhea. So it's really just going to be all of these birds here. I'll circle in there.

Those are all the descendants of the birds that lived at node a. And our final one here. Put a star on the node of the tree that represents the most recent common ancestor of the cassowary and the kiwi. Alright. Take a look.

Where do you think you should put that star? Well, I trace my nodes back. The kiwi back to this node right here. The cassowary also can trace its lineage back to that node. That's the last, that's the most recent place where those two branches meet.

So I'm going to put a star there, and with that, we've answered all the questions and I'll see you in the next video.

Based on the following simplified tree, which of the following is more closely related to the pear?

• A. Cucumber.
• B. Cotton.
• C. Orange.
• D. California Poppy.

Based on the simplified tree below, identify which node represents the most recent common ancestor between pears and cucumbers.

• A. Node A.
• B. Node B.
• C. Node C.
• D. Node D.

Which of the following statements is FALSE based on the simplified phylogenetic tree shown?

• A. The cucumber is equally related to the apple and the pear.
• B. The cucumber and the California poppy share a common ancestor at node E.
• C. The most recent common ancestor between the apple and the orange is also the most recent common ancestor of the apple and the California poppy.
• D. The pear and the cucumber are more closely related than the pear and the orange.