Evidence of Evolution - Online Tutor, Practice Problems & Exam Prep

Homology indicates shared traits among species due to common ancestry, evident in structures like upper limb bones across tetrapods. Key types include vestigial structures, embryologic homology, and molecular homology, which reveal evolutionary relationships. Fossils provide crucial evidence, showing changes over time and specific transitions, such as the evolution of whales from land-dwelling ancestors. Understanding these concepts is vital for grasping evolutionary processes and the interconnectedness of life forms.

Homology and Analogy

We now want to spend some time looking at the evidence for evolution. So remember, we're talking about this idea of common descent, and that idea is that species share common ancestors. Well, how do scientists tell when species share a common ancestor? Well, we're going to really rely on this idea of homology. Homology is the idea that traits with similar underlying structures suggest common ancestry.

Alright. Now we're going to use the upper limb bones of tetrapods as an example, but the idea of homology spans all levels of biological organization. We can see homology in genetic sequences. We can see it in protein structure. We can see it in bones.

So let's take a look at this to really understand what we're talking about. Alright. Tetrapods. These are organisms that live on land and have a backbone. So these are all the reptiles, the amphibians, the birds, and the mammals. Well, if we look at the upper limb bones of any one of these organisms, you're going to see that they have very similar bone structure. Alright. Now here's an example. We show the limb structure of a whale, a frog, a horse, a lion, a human, and a bird.

Alright. Very different organisms here that use their limbs very, very differently. We have a whale where it's almost like a flipper. We have a horse that's very specialized for running, a human that can pick things up, and a bird that can fly. When you look at the bones though, they're really similar.

Right? We see this blue bone. They all start with this sort of big, heavy bone at the top, one bone that's the humerus. Then, we see these two bones running in parallel in a lot of them. That's the radius and the ulna.

And even the horse and the frog here, they only have one bone, but that bone starts as two bones that grow together. So really, all of these organisms start with a radius and an ulna, two parallel bones in that sort of forearm part of their arm. Well, then we get this little group of bones. Those in us are the wrist bones, the carpals, and then we get this series of sort of short, long bones, what in us are our hand and our finger bones. Why would these organisms with such different use for their limbs have such similar underlying structure?

Right? If you were to build these structures from scratch, you probably wouldn't use the same pieces to do it. But if these organisms share a common ancestor and that ancestor had that bone structure, well, remember we had this idea of descent with modification. That bone structure is sort of passed on, and that just sort of small changes to that are what happened. Right?

Evolution doesn't just sort of throw out a body plan and start over. It takes variation that is there, and it makes certain parts of that variation more common or less common. And so you end up with these really common traits that you can see through really large groups of organisms. All right. Well, that's the idea of homology.

We also need to then talk about analogy because analogy well, these are functionally similar traits with different evolutionary origins. And analogy can really sort of confuse the situation when you're trying to figure out how things are related. So for example, wings. Well, bees have wings. Birds have wings.

Bats have wings. But we don't think the common ancestor of birds, bees, and bats had wings. And when you look at these, well, it makes sense that the common ancestor didn't have that because these types of wings are structured really, really differently. They're built very differently. So analogy can confuse us when we're trying to figure out how things are related.

We need to put all our chips on the homology. Alright. Now if you need to remember these two words, remember homo? Well, that's Greek for meaning the same. And here, we're talking about the same structures in different organisms because those organisms have the same common ancestor.

Where ana in analogy well, remember Greek, the Greek ana means again, and these are traits that have evolved again and again from different evolutionary origins. All right. We're going to look at different types of homology. More coming up, and I suggest you check it out.

Evidence of Evolution Example 1

In this example, we're going to be looking at homology and analogy. So let's take a look. It says, given the evolutionary tree below, do you expect the ability to glide using skin flaps in the North American flying squirrel and the Australian sugar glider to be an example of homology or analogy, and we need to explain our reasoning. So we see a flying squirrel here and we see a sugar glider, and these things, well, they seem very similar in some ways. Right?

They both jump from trees and hold out their arms and legs, and they have these flaps of skin that allow them to glide very well. So is this a homology or an analogy? Well, let's first just remember what these words mean. Right? Remember homo?

Well, that means the same. So a homology means that they inherited this trait from the same ancestor. Whereas an analogy, remember Anna? We said that means again. It means it's evolved again. It's evolved more than one time. So let's look at this tree. Take a look at this tree, and does it look like these things inherited that trait from the same common ancestor, or does it look like it evolved more than once? Well, when I look at this tree, I want to find the node that is the ancestor of both these organisms. So I follow these branches back, and they meet at this node way down here at the bottom of the tree.

Now this is the root of our tree here, and the thing that lived there is the ancestor of all placental and all marsupial mammals. So all placental, these are the mammals that you're more familiar with, right? Rats, tigers, lions, but most mammals. And marsupials, well those are sort of like all the kangaroos, etc. So, did the ancestor of all those organisms have flaps of skin that it could jump out of the tree and glide with?

I don't think so. Alright? So that means that this would be an analogy. Alright. Now what's my reasoning?

Well, I would say that the common ancestor did not have the trait. The trait in this case, the ability to glide using those flaps of skin between the arms and the legs. Alright. We'll practice this some more and more problems will come. I encourage you to check them out.

A similar arrangement of internal organs is observed in birds, rodents, and frogs. This is likely an example of:

• A. Analogy.
• B. Use and Disuse.
• C. Homology.
• D. Uniformitarianism.

The potato (Solanum tuberosum) and the sweet potato (Ipomoea batatas) are both plants that form starchy tubers. A tuber is an underground part of plant used for nutrient storage and is the part of the potato and sweet potato that we eat. Most other plants of both the genus Solanum, which includes the tomato and the eggplant, and the genus Ipomoea, which includes the morning glory, do not form tubers. As described do you think the tubers found in potatoes and sweet potatoes are more likely an example of analogy or homology and why?

• A. Homology, because the tuber phenotype is extremely similar in both plants.
• B. Homology, because potatoes and sweet potatoes likely share a common ancestor.
• C. Analogy, because the tuber is a structure that likely first evolved in the common ancestor of these plants.
• D. Analogy, because the presence of a tuber in these plants likely arose separately in each population.

Types of Homologies

The concept of homology, that organisms share traits because those traits were present in their common ancestor, is going to be an incredibly important idea for understanding evolutionary relationships. But there's also going to be some specific types of homology that you're probably going to need to know about. So here we're going to go through some of those. We're going to talk about vestigial structures, embryologic homology, and molecular homology. Alright.

Well, let's start with vestigial structures. This is something you've probably heard about before. The idea of a vestigial structure is a trait that has lost most or all of their ancestral function. Alright. There are some traits when you see them.

It's just very clear it's a reduced form or not as functional as it once was in the ancestor. Now importantly, that doesn't mean that it has no use. It's just clearly a reduced function. So some great examples of this, right?

The pelvic girdle and femur in some snakes. Right? Snakes don't have legs. Why do they have a pelvic girdle? Wings on an ostrich.

Right? An ostrich can't fly. Now it does flap those wings around sometimes, but it's very clear that it's descended from birds that had flying wings, and it can't fly. And my favorite example is going to be human goosebumps. Right?

We see here an image of goosebumps. Right? If you get cold or scared or excited, the hairs on your body stand up on end. But why do they do that? Well, if you had fur, right, standing the hair up on end, that's going to increase the warmth of that fur coat, or it could be used as a threat display.

Think of, like, a dog growling. The hair is standing up on the back of their neck. Right? But it doesn't do much for us because, well, I don't have very much hair. I don't have enough hair to keep me warm when it stands up on end, and I don't have enough hair to be threatening when it stands up on end.

Alright. Our next example here is going to be embryologic homology. Now, embryologic homology is one that people realize fairly early in the study of evolution. And this is just this idea that organisms share traits as embryos that are not shared in adults. Right.

So we can see here we have an embryo of a fish, a bird, a reptile, and a mammal. And for how differently these organisms look as adults, they look pretty similar as embryos. And we can even call out specific traits, like the pharyngeal arches and the post-anal tail. Right? We see here these little bumps here, these are all the pharyngeal arches.

Sometimes they're called gill slits, but technically that's correct because all these organisms don't have gills. But early in development, all these organisms have these sorts of same bumps, and then they go on to develop into sort of different but related structures in the adult organisms. They also all have tails, even though not all these organisms have tails as adults. So why would that be? Well, we said that evolution, right, it works on existing variation.

It doesn't just sort of blow up body plans and start all over. And changes in an early developing embryo, that's probably going to cause a lot of changes down through development as that organism turns into an adult. Where small changes sort of at the end of development, they're less likely to sort of blow things up, and those are the type of traits that evolution is more likely to work on. So these body plans, these early developments are well, it's remarkably stable through evolutionary time, and so you can look at things that have really distant common ancestors, like these organisms we see here, and they still share traits as embryos. Alright, our final one here is going to be molecular homology.

And really, in modern biology, this is the big one. Right? If you see evolutionary trees today, if it's not based on fossils, then almost certainly it's based on molecular homology. And this is just the idea that DNA and protein sequences are more similar in related organisms. Alright.

So we have some DNA sequences here. We have the sequences of a lion, a tiger, and a house cat. But we know the lion and the tiger are more closely related, and we can look at the DNA sequences. Their DNA sequences are more similar. Why?

Well, we have these stretches of DNA here that are identical between these two cats, the lion and the tiger, because that is almost certainly the sequence that was in their common ancestor. Now the cat, there's some parts that you see that are similar because these things do have a common ancestor if you go back far enough, but there's more differences because, sent with modification, it's been longer since these things shared a common ancestor. Alright. So with homologies, remember, homologies instruct us as to how things are related to each other evolutionarily. And there's lots of different ways that we can see homology.

Alright. With that, I'll see you in the next video.

Evidence of Evolution Example 2

Our example tells us that below are the DNA and amino acid sequences for 3 different but related species. We want to use the sequences to answer the following questions. So here are the sequences. We have the DNA sequences on the top. So we see the sequences there for species 1, 2, and 3, and those nucleotides look to be broken up into codons.

And then below that, we have the amino acid sequence. So we have 3 amino acid sequences, again for the same species, 1, 2, and 3. We see we're using the 3-letter abbreviations here, and you can see that they're sort of lined up underneath the codons. Alright. So our first question is based on the DNA sequence, which two organisms do you expect to be more closely related?

Alright. So take a look. How would you know and well, which ones do you think are more closely related? Well, we're using this idea of homology, and the homology is the idea that, well, things that are inherited from a common ancestor, those are the same and those show that things are more closely related to each other. So as I look at the DNA sequences, the sequences that are more like each other that show more homology, they're probably that way because they were inherited from the same common ancestor.

So I'm going to just start by going through and highlighting differences I see. So in species 2 here, that nucleotide's unique. Here's a difference. Here's a difference. Here's a difference in species 1.

Here's a difference in species 2, and another one there. Alright. So as I look at that, species 2 seems to have a lot of places that have a unique nucleotide different from the other ones. That means that species 1 and 3 show more homology. If they show more homology, I assume that's because they inherited that from the same common ancestor.

So I believe that species 1 and 3 are more closely related for that reason. Alright. Now, B, based on only these data, do you expect to see differences in the traits of these organisms? Why or why not? Right?

The traits, the phenotypes of these organisms. Well, look here. How would you know, and what do you expect to see? Well, for the traits, the phenotypes, I'm going to look at the amino acid sequence. Right?

Because the proteins are things that actually go out and do things in the cell. And as I look at the amino acid sequences here, well, they're identical in all three organisms. So I just based on these data, I don't expect to see any difference in the phenotypes. So I'm going to say no difference in the traits. Why? Well, the amino acid sequence is the same in all three organisms.

Next, it asks us to circle the regions of the DNA and amino acid sequence that show homology in all three organisms. Alright. What are we going to circle?

Well, we're going to circle the places that are the same because we're going to assume that they're the same because they were the same in that common ancestor. So in all three organisms, well, it's going to be these two nucleotides, these two nucleotides. Really, the first two nucleotides in every codon here. There's a difference in the third one, and then as I look at the amino acid sequences, well, they're all identical. So I'm going to circle the whole thing.

Our next question then is, did you circle the same regions for both sets of sequences? Well, obviously, no. Now but then we want to explain why or why not. So I'll write no.

So how is it that the entire amino acid sequence could show homology even when the DNA sequence that codes for that amino acid sequence doesn't show complete homology? Well, I'm going to note that those nucleotides that don't show homology are in the 3rd position. Remember the 3rd position in a codon is often a silent site. So I'm going to say differences in DNA are in silent sites. Alright.

So DNA holds a lot of information when we're trying to make evolutionary relationships and using homology to do it. And that's why we use DNA so much in modern biology today. You know, every protein has typically thousands of nucleotides that code for it. And so there's just a ton of data that you can use to look for homologies. Every nucleotide could be a possible homology that you can use to establish evolutionary relationships, and that's a lot of data.

So that's why today molecular homology is what scientists try to use if they can to build phylogenetic relationships. Alright. With that, we have more practice problems after this and I'll see you there.

Different alleles of the MC1R gene have been associated with hair/coat color variation in many different mammals, including mice, jaguars, cows, and humans. In humans, specific mutations of the MC1R gene are associated with red hair. Using the concept of homology, explain why the same gene would affect similar traits in multiple organisms.

• A. Because the MC1R gene affects hair/fur color variation, you can expect that the gene will change often as organisms adapt to different environments.
• B. Since hair color is not a trait that confers a survival or reproductive advantage in humans, the MC1R gene can be considered an example of a vestigial trait.
• C. The MC1R gene was likely involved in coat color determination in the common ancestor of these organisms and maintains that role in each of these lineages.
• D. It is likely that the MC1R gene evolved to control hair/fur color separately in each lineage, as the presence of hair/fur is a homologous trait in mammals.

Which of the following statements describes a vestigial structure?

• A. While cats have much better balance than humans, the inner ear structures responsible for balance are largely the same in both cats and humans.
• B. Owls’ arteries are located much closer to the spine than in other birds, allowing for extreme neck flexibility without diminished blood flow.
• C. A peacock’s tail is large and brightly ornamented to advertise its fitness to females.
• D. Cave salamanders live in complete darkness but still develop eyes with limited functionality.

Fossils

As we think about the evidence for evolution, how we know that species have changed over time, one of the clearest pieces of evidence that we have is going to be fossils. And fossils are the remains of organisms that lived in the past that we find in layers of rock or sediment in the Earth. This happens because as Earth goes through time, sediment gets laid down, and organisms that die get trapped in that sediment. And then that sediment gets compressed, and it forms these layers of rock that we can then go dig in and find the remains of those organisms. Now, this means that the geologic layers provide evidence for broad patterns in evolution.

So when you dig into these sorts of layers of the Earth, you're going to create a timeline for earth, where the deeper layers are older. And when we do this, when we look through these layers, we see this very clear pattern. The deeper, or we could say the older layers, have very different organisms than we see on Earth today. Alright. So we have this sort of simplified version here that we're going to go through.

We can imagine this as like, you know, a cliff wall or a canyon or something where we can see these exposed layers. Well, if we went digging here, near the surface, you might expect to find fossils, but near the surface, you're going to expect those fossils to be similar to things that you see today. Maybe you find something like this woolly mammoth skeleton. Right? Woolly mammoths, they're not around anymore.

That's an extinct organism, but they're very similar to the elephants that we see on Earth today. Now, if you go in deeper sediments, you're not going to find woolly mammoth skeletons anymore. Right? When you go deeper, you might find something like dinosaur skeletons. Now dinosaur skeletons, you don't find in those higher-up sediment, but you know there are layers where you find these dinosaur skeletons all over the earth, so it's very clear that these things were once common, living at a very specific time in Earth's history.

Now you go deeper, you don't see these dinosaur skeletons anymore. Maybe you see something like trilobites. Hundreds of millions of years ago, trilobites lived all over the ocean, and then they didn't. Right? They are definitely not around anymore, and they haven't been for a very long time.

So these layers in the fossils that we see demonstrate both extinctions, right? Things that were once alive aren't alive anymore. And we can also track these sorts of broad evolutionary changes. But we can also use fossils to track very specific evolutionary changes. So here we're going to say that fossils demonstrate specific evolutionary histories and transitions.

For this, we're going to use whales as an example, and whales are mammals. And the idea is that whales evolved from 4-legged animals that lived on land. Alright. So we're going to start with that as our hypothesis. Whales evolved from 4-legged, we're going to say here, terrestrial.

Terrestrial just means living on land. Terrestrial ancestors. Well, if that's the case, when we go digging and looking for fossils, we should be able to find fossils that show whale-like fossils with, we're going to say, intermediate intermediate leg structures. Alright. So we can test that hypothesis.

Right? So if you look at modern whales, modern whales have been around for basically the last 34 million years. Now they haven't looked exactly like this for the last 34 million years, but a characteristic in modern whales is this vestigial pelvis. Modern whales have a pelvis where you would expect the legs to connect to, but they don't have any legs, and only a couple of muscles connect to that pelvis. It's really not doing a lot for the whale.

Well, if we look back in time, if we look down in the layers and dig for fossils, what do we find? Well, in layers from about 40 million years ago, you find this organism called Darudin. Now, Darudin looks a lot like a whale, but it's got all its leg bones. Now, this is clearly a vestigial leg. It's not using this to walk on.

It's quite diminished from something that you could use to walk, but all the bones are still there. Now you go deeper. Well, you find this animal, Rodicetus, that was alive about 47 million years ago. Now, this is an organism that swam a lot, but it has a full leg. This is a swimming leg with probably webbed feet.

You can sort of imagine this maybe swimming kind of like a very large otter or something like that. Well, you go even further into the past, deeper in the layers, from 50 million years ago, you find this organism Indohyas. And Indohyus clearly has a leg that was used for walking mostly on land. Now we know that these things are related to each other from all the different homologies that they share, But we can see this transition in leg structure over time. Now it's important to remember, Inohayas wasn't trying to become a modern whale.

None of these organisms are trying to evolve in that direction. But if we play this sort of back in the other direction, we can see how it could happen. Indohyas, people think, lived by the water's edge, maybe hunted in the water. And so maybe it does a little bit of swimming, and then over time, variation that makes swimming easier becomes more common. You end up with this swimming leg in the Rodicetus.

This organism is spending more and more time in the water. It's swimming more and more with its tail. It ends up really not using its leg. It's not going on land anymore. The leg becomes even more diminished until eventually variation that doesn't have any leg bones, that seems to work the best for swimming.

That's what you find in the modern whale. All right. So remember, fossils can show us those very broad evolutionary patterns, but we can also test hypotheses for how species have changed through time. All right. Like always, there's examples and practice problems to follow.

You should give them a try.

Sedimentary rock creates distinct layers that paleontologists can use to help interpret their findings. Use the image below to determine which of the following statements is true about fossils and sedimentary rock.

• A. The fossils found in layer B will be older than the fossils in layer A.
• B. More fossils are likely found in layer C than in layer B.
• C. Fossils are most likely to be found in the transitions between layers.
• D. Fossils in layer C are likely to be the ancestors of fossils in layer D.

Dinosaurs went extinct approximately 65 million years ago (mya). Fossils of the organism Archaeopteryx have been found in strata dating to approximately 150 mya. These fossils show a dinosaur-like skeleton with a bony tail, teeth, and claws, as well as clear impressions of feathers similar to those found in flying birds. It is thought that Archaeopteryx belongs to a group of dinosaurs from which birds evolved. Which statement below is consistent with the evidence presented above?

• A. Because Archaeopteryx had more dinosaur-like traits than bird-like traits, it should not be considered a transitional form.
• B. Flight is an analogous trait found in both dinosaurs and birds.
• C. Archaeopteryx should be considered a bird and not a dinosaur due to the presence of feathers.
• D. At least some traits common in modern birds evolved long before dinosaurs went extinct.