Lesson video

Hello, today's lesson is from the unit, "Classification in Modern Biology" and the lesson title is "Classification and Reclassification: Genetic Evidence." Hello, my name is Dr.

Pugh and I'll be taking you through today's lesson.

So by the end of today's lesson, you should be able to describe the classification and sometimes reclassification of organisms based on evidence from their genomes.

To do that, you're gonna need a few keywords which are displayed on the slide now.

The keywords are DNA, evolutionary trees, and gene sequence.

I'm not gonna go through with those with you now, but if you want to pause the video and copy them down or make notes, do so now.

Okay, so the lesson outline for today, the lesson is gonna come in two parts.

The first part of the lesson is concerning evolutionary trees.

The second part of the lesson is concerning gene sequencing and how this is being used to reclassify organisms. So let's look at evolutionary trees.

Now, what Linnaeus did in the 18th century was to use superficial similarities in the phenotype, or physical appearance of organisms, to group organisms that fit together with similar characteristics.

So if we look at the five members of the Hominidae family, the chimpanzee, the bonobo, the human, the gorilla, and the orangutan, if we look at the skeletons, for example, of these different hominids, we can use those similarities to group these organisms and to put them into evolutionary trees.

So the classification system provides information about evolutionary relationships.

As we descend through each level of classification here, the number of species in the group decreases.

Now let's focus on the genus.

So we've got two species here.

The species are the Santiago tortoise and the Wolf Volcano tortoise, both tortoises that you'll find on different islands in the Galapagos.

And because they share the same genus name, that means they probably share a common ancestor in evolutionary history.

So let's look at the evolutionary trees.

Now, evolutionary trees are diagrams which show the evolutionary relationships between organisms, and it shows how species have evolved over time from a common ancestor.

So if you look at the timeline on the left-hand side of the slide, we've got 30-million-year timeline there, from 30 million years to the present day.

And we can see that in the evolutionary tree, there are branch points.

So if we go from chimpanzee and bonobo to the branch points, that branch point represents the common ancestor to both chimpanzees and bonobos.

If we go back to the next branch points, so we're going in time, we can see that the common ancestor here is common to chimpanzees, bonobos, and humans.

If we go further back, then we have a common ancestor that chimpanzee, bonobos, humans share with gorillas, and even further back in time, about 10 million years ago, where there is a common ancestor to all of the hominids in this group.

So let's do a quick check.

The model showing the diagram below is an evolutionary what? So pause the video, write down your answer, and we'll go through the correct answer in a moment.

Okay, hopefully you got the answer of evolutionary trees.

So quite a simple one there.

Now let's look at the branch points of the evolutionary tree.

So which letter represents the common ancestor of chimpanzees, bonobos, and humans? Is it a, is it b, is it c, or is it d? So pauses the video and I'll give you the correct answer in a moment.

Okay, so hopefully you put down the correct answer of b.

We can see that if we go back around about six million years, there was a common a ancestor to chimpanzees, bonobos, and humans.

Next question, which two hominids share the most recent common ancestor? Pause the video and we'll feedback in a moment.

Okay, so the answer to that is chimpanzees and bonobos.

Right, let's do a practise task.

So we've got a different evolutionary tree here, looking at finches in the Galapagos Islands.

We have a timeline on the left-hand side from one million years ago to the present day.

So let's answer the following questions based on this tree.

So the questions I'd like you to answer are as follows, which finch is least closely related to all the others? Which two finches share the most recent common ancestor? And finally, which letter represents the common ancestor to all the ground and tree finches? So pause the video and I'll give you the correct answers in a moment.

Okay, so let's go through those answers now.

Question one, which finch is least closely related to all the others? The answer being the vegetarian finch.

Which two finches share the most recent common ancestor? Small ground finch and large ground finch.

And finally, which letter represents the common ancestor to all the ground and tree finches? The answer being P.

Okay, so that's evolutionary trees covered.

Now we're gonna move on to the second part of today's lesson, which is the use of gene sequencing to reclassify organisms which are difficult to classify based on comparisons of their physical characteristics.

So let's see how we can use DNA sequences to determine the evolutionary relationship between these different mammals.

And the three mammals we've selected here are baleen whale, toothed whale, and cow.

So the first thing we need to do is to look at the similarities in the sequences.

And if we do so, we can see we've got the following similarities.

But we also have differences as well.

So by looking at gene sequences, we can start to identify evolutionary relationships.

And this has helped us to reclassify organisms that we didn't previously know that were related.

So let's look at the sequences here.

We've got baleen whale, we've got toothed whale, we've got cow, we've got hippo.

And from these gene sequences, we can see that cows and hippos are related to each other, but surprisingly, they're also related to whales.

So using this information, we can start to draw evolutionary trees.

And you can see one that's been started at the bottom of the page where we can see that cow and hippo are related, they do share a common ancestor, and they're also related to whales.

Now let's do a quick knowledge check using these sequences.

So how many differences are there between the gene sequences of the whale and the cow? Is it two, three, 12 or 15? So pause the video, make your selection, and I'll go through the answers in a moment.

Okay, the answer is there are three differences in the gene sequences between whales and cows.

Next question, how many differences are there between the gene sequences of the cow and the hippo? Is it two, is it three, is it 12, is it 15? Pause the video, make your selection, and I'll give you the answers in a moment.

Okay, the answer is two.

How many differences are there between the gene sequences of the whales and the hippo? Is it two, three, 12 or 15? Pause the video, I'll give you the answers in a moment.

Okay, the answer is there are three differences.

Okay, so we're gonna do more an extensive task now in this practise part of the lesson.

So we have all of the following mammals listed here, baleen whale, toothed whale, cow, tapir, hippo, and zebra, and we're comparing the DNA sequences of one gene.

I would like you now to highlight the differences between the whales and the other mammals.

Pause the video, highlight the differences, and I'll go through those differences in a moment.

Okay, so let's look at those differences.

There we go, so we can see there's the differences between the cow and the whales, differences between the tapir, hippo, and the zebra.

And we can see that some organisms have got more similarities and some organisms have got more differences.

So the cow and the hippo have got fewer differences to the whales, but tapir and zebra have got more differences.

Okay, let's do some practise questions.

So what I'd like you to do, using that information, that DNA sequence information, is firstly, find two mammals with the same DNA sequence.

Secondly, what does this information tell us about these pairs of mammals? So you know the drill, pause the video, and we'll feedback in a moment.

Okay, so question two on the board here, find two mammals with the same DNA sequence.

And surprisingly, well, not surprisingly for the first one, the whale and the toothed whale have got the same DNA sequence, but the zebra and the tapir have the same DNA sequence.

Now, if you look at the physical characteristic of zebra and tapir, there are some similarities, but they're not terribly obvious.

So this shows the power of DNA sequence comparisons to determine these evolutionary relationships.

So what does information tell us about these pair of mammals? The whales are more closely related, they are to each other than to other mammals, and the zebra and the tapir are more closely related to each other than they are to the other mammals.

Okay, so now we're going to count the number of differences in the DNA sequences between the whales and other mammals.

So look at the sequence, count the number of differences and enter them in the table now.

Pause the video and I'll feedback with the correct answers in a moment.

Okay, so let's look at those differences.

So interestingly, the hippo and the cow, there are three differences between the DNA sequence in that gene and with the gene of the whales.

Tapir and zebra both have seven differences.

So a couple more practise questions for you.

Looking at the number of DNA differences, which mammal species are most closely related to whales? Artiodactyla are hoofed mammals with an even number of toes on each foot, and Perissodactyla have odd numbers, odd numbers of toes.

What does the DNA evidence suggest about the evolutionary relationship of whales to these mammals? Pause the video, write down your answers and we'll go through them in a tick.

Okay, question five, looking at the number of DNA differences, which mammal species are most closely related to whales? The hippo and the cow.

Artiodactyla are hoofed mammals with an even number of toes on each foot, Perissodactyla, odd number of toes.

What does this DNA evidence suggest about the evolutionary relationship of whales to these mammals? It tells us that whales evolved from an ancestor with an even number of toes on each hoof.

So the ancestor of modern whales used to be a hoofed animal with even number of toes on each of those hooves.

So that's a really surprising result from looking at these DNA differences in DNA sequences.

Okay, so another practise question.

Let's see which of these evolutionary trees best fit the DNA evidence.

So have a look at the evolutionary trees, make your selection, and I will feedback in a moment.

Okay, so let's look at the answer.

So which of these evolutionary trees best fits the DNA evidence? Hopefully, you've picked this tree here where we can see that the tapir and zebra are more closely related, and they share a common ancestor further back in time because they've got seven differences in the DNA sequence.

Cow and Hippo share a common ancestor with the baleen whale and the toothed whale, more recently back in time, because they only have three differences in their DNA sequences.

So let's summarise today's lesson.

In the past, classifications relied on using superficial similarities in the appearance of organisms as a way of putting similar organisms into groups and naming them.

Scientists have used these similarities and differences to construct evolutionary trees, which suggest how species have evolved over time from a common ancestor.

Now, one of the limitations of the Linnaean system of classification is that some species look very similar but are distantly related, whereas some are very different and are closely related.

Now scientists are using DNA evidence to classify organisms. Gene sequencing technologies have enabled scientists to establish the genetic relatedness of organisms by comparing similarities and differences in their DNA sequences.

And hopefully, the brief exercise that we've done today have helped you understand how scientists are using these DNA sequences to determine these relationships.

So I hope you enjoyed today's lesson, and I hope to see you again soon, goodbye.