Hello, everyone. In this lesson, we are going to discuss taxonomy, and we are going to begin our discussion on phylogeny. Okay. So, we've all pretty much heard of taxonomic groups or taxonomy before in general basic bio. But let's just go over it one more time. Taxonomy is going to be the science of defining, categorizing, classifying groups of organisms based on shared characteristics that they have, and their relatedness. So I want to write and relatedness. Because that's very important. Because a lot of the work that we do now on organisms is that we look at their genetic code, we look at their evolution, and we try to better classify organisms based on how they evolve and how related they are to one another, and not just because of their shared characteristics because organisms can have very similar looking characteristics and not be related to one another whatsoever. So we're trying more to focus on relatedness, which is going to tie into our discussion phylogeny. But let's just go over taxonomy because we still do use taxonomy a lot, just not as much anymore. Basically, it's utilized for naming organisms, and naming genuses, and families, and species, and things like that. So, this is gonna be an overview or a generalized view of what we call the kind of like tree of life, or the organization of life. So all living things on our planet, you know, us humans, we gotta categorize things, we gotta give everything a name. So we have and this is going to be the naming system that we use for all the different organisms on our planet. So, if you're wondering, taxonomy, the word taxonomy comes from the 2 Greek words. The first word is taxis, which is going to mean the arrangement or organization of something. And then, nomia is going to mean method. So the arrangement method of organisms. So that is going to be what we're talking about with taxonomy. Now, phylogenetics and cladistics are going to be how organisms are related, and this is going to tie into this topic a lot. So let's look at these. We have domains, kingdoms, phylum, class, order, family, genus, and species. I know you've all probably heard of genus and species before, but all of these other ones are just as important. Now, just so you guys know, the most inclusive, is going to be the domains. And the least inclusive is going to be the species. What do I mean by that? Most Inclusive means there are more organisms in domains than there are in species, because domains are going to be a very wide, overarching group of life. In fact, there are only 3 domains of life. And this is going to be based on cell type. The type of cell that an organism has. So, we're going to have archaea, bacteria, and we're going to have Eukarya. Now, all the organisms that are in Eukarya are going to be what? They're gonna be Eukaryotes. We belong to this organism is going to be in anything that is a eukaryotic organism is going to be in this domain. There are going to be so many organisms in domains because they're the largest, overarching, most inclusive group. So these two domains, which are also very, very large, archaea and bacteria are both going to be prokaryotes. Bacteria are going to be single celled organisms that we commonly think about are gonna be found all over our bodies throughout the environment, in soil, and dirt, and things like that. And there are single celled prokaryotic organisms just going about their life. There are a 1000000 different types of them. And then archaea are also going to be prokaryotic single celled organisms, but they're generally going to be more extreme environment organisms that live in extremely hot climates, salty climates, acidic climates. They're commonly called extremophiles because they live in very extreme types of environments, and they have unique cellular structure to deal with this. So those are the 3 domains, the largest groups of life. And as you go down and down and down the taxonomic categories, you get smaller and smaller groups of organisms. So, you have a domain, and then you have a kingdom within that domain. Maybe the Plantae Kingdom inside of the Eukarya domain. So, the Plantae Kingdom inside of the Eukarya domain is gonna hold all of the plants. And then you go into the phylum, and then into the class, and order family, genus, and species. Now, genus and species is what you're normally going to hear when you hear the scientific name of an organism. The scientific name of an organism is genus and species. Now, you're probably gonna need to know this. Well, you're definitely going to need to know this. And I want you all to know how to write the scientific name of an organism. So, like I just said, first it's gonna be genus, and then it's going to be species. Now, be careful. Whenever you do the genus or species of an organism, if you're writing it on the computer it's always italicized. And if you are writing it handwritten, most teachers don't really care, but the correct way to write it is you underline it. Okay? So, the genus always has to be capitalized. So this is capitalized. And this species is lowercase. I don't know why, that's just the way it is. So, they're either italicized or underlined. And the genus is capitalized, and the species is lowercase. This is very important for all of you who are going into research, or taxonomy, or phylogeny, or the naming of organisms. It's very important to know. So, what's a great example of a a scientific name of something? Well, what about our scientific name? We have Homo, that's our genus. And we have Sapiens, that's our species. And you can see that I have capitalized homo, lowercase sapiens, and then, to be correct I would underline it. Another great example is Felis catus. Any idea what this one is? It's a domesticated cat. So, all living organisms are going to have scientific name, and that's gonna be their genus and their species. Now, you guys are probably going to have to know the order of the different taxon the taxonomic groups of life. So, there is a saying for this if you wanted to know. So, domain, kingdom, phylum, class, order, family, genus, species, you will have to know that order. So the most common one is gonna be Dear King Philip. Let me write this. Dear King Philip came over for good soup. Okay. So, now, we're gonna go down and we're going to talk a little bit about phylogeny. This is just a very short introduction to phylogeny. So, Systematics is the study of the relationships of organisms. Taxonomy was the study of classification of organisms. And phylogeny is the study of the evolutionary relationships between organisms. This is a very important field right now. It is how we are organizing all of the different organisms that we know of on our planet and that we are continuing to discover. And whenever we determine or attempt to determine the evolutionary relationships between organisms, we're gonna make a phylogenetic tree. Which is going to be a branching diagram showing the inferred evolutionary relationships between species. Inferred here is a very important word. Please understand that phylogenies, phylogenetic trees, are not set in stone. The more we learn about genetics, the more we learn about cell bio, evolutionary biology, the more we learn over time, we realize that we don't actually know how all of these organisms evolved. We are just hypothesizing upon this. So phylogenetic trees are never set in stone. They are always estimates. They are always hypotheses, but they better help us understand the world around us. This is a great example of a really cool looking circular phylogeny. Not all of them are going to be circular because these are kind of hard to read. This is going to be a phylogeny of just some very big groups in all of the life on our planet, but we will talk more about the more condensed versions, the smaller versions of phylogenies. And we'll talk about the different ways you might see phylogenetic trees in our coming lessons. Okay, everyone. Let's go on to our next.
2
concept
Homology and Homoplasy
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15m
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Video transcript
Hello, everyone. In this lesson, we are going to be talking about phylogeny. Specifically, the difference between ancestral traits, derived traits, Homology, and Analogy. Okay. So, whenever we build a phylogeny, we're specifically going to be looking at certain characters. Generally, you build a phylogeny for 1 character. Maybe it's a particular gene, maybe it's a morphological character. But these characters can be any genetic, morphological, physiological, or behavioral characteristic that is being studied. You can pretty much study anything in a phylogeny if you want to. And whenever you are studying is going to be called a character. Now, generally, we're gonna study different traits. Maybe it's having 5 fingers versus 4 fingers. Or maybe it's having 2 eyes versus 1. Some sort of trait. And you can have different versions of that trait. You can have the ancestral version, and you can have the derived version. So, an ancestral trait is a character that existed in an ancestor. It came from an ancestor. A derived trait is a modified form of the ancestral trait. So, kind of like a new twist on something. But this twist was not found in the ancestor. Now, whenever we look at these 2 different phylogenies, are they different? Well, they surely look different, don't they? But they're actually not different. The relationships between these organisms are still the same. We can see here that a and b share a more recent common ancestor at this node. So that is the ancestor to a and b. Ancestor to a and b. So a and b are more closely related to one another than they are to species c. And that is the same thing that is happening in both of these phylogenies. Now, this particular ancestor here is going to be the ancestor to a, b, and c. And that is the same over here as well. Okay, everyone? So even though these 2 phylogenies do look drastically different and a and b are arranged differently, they show us the same information. Now, let's talk about the difference between ancestral traits and derived traits in the subject of a phylogeny. So, let's say that we have this particular trait. We're gonna call it trait a. And it's found in the very first ancestor to all of these organisms. Trait a is found in the bottom ancestor down here. And let's say that organism c also has trait a, and organism a also has trait a. But then we're going to see that organism b has a new trait called trait b. It no longer has trait a. Something is different about this organism. This is the derived trait because it is a trait that is not found in the common ancestor, and trait a is the ancestral trait. Because it is found in the descendants, but it is also found in the ancestor. So, trait A is the ancestral trait because it is the same trait found in the ancestor. And trait B is the derived trait because it is not found in the ancestor, and it is a modification of the traits found in the ancestor. Now, whenever we're talking about where traits come from, whether they arise on their own or they arise via ancestry, we're gonna be talking about homology and analogy. Now, these two topics, these two concepts are very highly tested on and very important for you to understand. So let's go through them, and let's make sure that we understand what these two topics are pointing at. Okay. So, homology is going to be similarity between organisms due to a shared ancestral trait. Because these organisms share an ancestor, because they are closely related, they are similar. And those similarities are homologous. They are a form of homology. Now, if the organisms have similarities between each other, but they are not closely related, that means that these similarities arose due to Convergent Evolution. Now, do you guys remember exactly what Convergent Evolution is? Convergent Evolution is a very important topic to understand. It's very highly tested upon topic. And this is going to be when organisms who are not closely related, independently create the same trait. So convergent evolution is where these 2 organisms converge on the same idea, the same trait. But this convergence, this similarity, is not due to ancestry. In fact, these organisms are closely related whatsoever. So, this is going to be similarity that's not due to family history, but it's due to some other aspect. Maybe they live in very similar environments, so they created very similar evolutionary adaptations to combat those environments. But but but because it arose due to something else other than ancestry, we call it an analogous structure or an analogy. This is going to be caused by convergent evolution when organisms become similar, but not because of relatedness, but because of something else. So we also have this topic of homoplasy. Homoplasy is going to be analogous structures that arose independently. So these are gonna be very similar structures like we talked about that arose independently. A great example of Homoplasy is going to be winged organisms. And you guys can see these organisms right here in this first example. We're going to have what I believe is a pterosaur, some sort of, dinosaur, I guess. Let's say this is a pterosaur. If I can spell this. Pterosaur? Yeah. So, let's say that this is a pterosaur, which would be a flying reptile, or a flying dinosaur. And then the number 2 here is actually a bat, and number 3 is going to be a bird. Now, all of these organisms have powered flight. They have wings that they actively flap to keep themselves in the air. That is a similar structure. Is this similar structure due to relatedness? No. It's not. It's due to homoplasy. It's due to convergent evolution. These 2, these 3 organisms, excuse me, are similar, but it's not because they're most closely related. None of the ancestors of these organisms had wings. The ancestors to birds didn't have wings. Birds arose independently to create wings. Bats are mammals. Most mammals don't have wings. So mammals arose or, sorry. Bats arose flight independently of other mammals. I'm sorry, I keep tripping over my words there guys. And most reptiles, as we know, do not fly. So, pterosaurs arose that ability to fly independently as well. So, Pterosaurs, bats, and birds arose flight independently of one another via convergent evolution and not via ancestry. So flight is not found in the ancestors of these organisms. All of these organisms evolved flight independently of one another via convergent evolution. So this is analogy. Okay? So that is analogy. Now, this particular example, perhaps you recognize it as a very, very famous example. This is going to be a great example of homology. You'll probably see this example a lot in class whenever your teacher is going over homology. This is a very, very famous example. This is going to be the forelimbs of vertebrate tetrapods, or 4 legged organisms. And we are actually tetrapods. Even though we're not 4 legged, we have 4 limbs. So, tetrapods are 4 limbed organisms, and these are all vertebrate organisms. These are gonna be humans, dogs, birds, and this is a whale. I'm covering the the word, but it is a whale. Isn't it weird that whales have hands that look like our hands? So, this is going to be an example of homology. Now, why is this? Because these organisms, all of these organisms arose from a Tetrapod ancestor that had 4 limbs and bones of those limbs organized in the same fashion. And if you look at the bones in the limbs of all of these organisms, they have all of the same bones. So, all of the bones here in this kind of tan, orangey color are gonna be the Humerus. So, we have a Humerus, and so do dogs, and so do birds, and so do whales. And they're all in the same area, the same general location in the limb. And then we're also going to have the ulna, here in red, in all of these organisms, And then we're going to have the radius, here in white, in all of these organisms. We're going to have the metacarpals in all of the forelimbs of these organisms, it's metatarsals in the hindlimbs of these organisms. And then, we're gonna have phalanges in brown. And all of these bones, even though they are kind of positioned slightly differently in all of these organisms, or they have different lengths, as you guys can see in whales. They're all here. And they're all in the same pattern, same general location, and they all are in the forelimb. All of these structures are due to ancestry because the original ancestor to the tetrapods had these limbs and these bones in this particular order and in this particular fashion. So this is all due to ancestry. They are similar due to a common ancestor, so this is an example of homology. While flight and all these other organisms is an example of analogy. Okay, everyone? Alright. So now let's go down and talk about some other interesting topics. We're gonna talk about different types of genes. We have orthologous genes and par paralogous? You'd think I'd know how to say that, but it is hard to say. Paralogous genes. And these are gonna be different types of homologous genes. Remember, that is due to ancestry. So an orthologous gene is going to be a homologous gene, so an ancestral gene that has sequences that are separated by speciation event. So when a gene, or when 2 genes are orthologous, they diverged, they became different after a speciation event. But the ancestral version created these 2 new orthologous versions. Now, whenever you have a paralogous gene, this is going to be homologous genes as well. So, they're gonna be similar due to ancestry. But these genes are gonna be created via a gene duplication event. So, when 2 genes are paralogous, that means that the 2 genes diverged after a duplication event. So, what that's gonna look like is this, you're going to have gene A, let's call it, and then you're going to have a duplication event. And then you're going to have 2 gene A's. This happens all the time, by the way. Genes duplicate themselves via mutations all of the time, and a lot of gene families are gonna be made this way. And then you're going to have an evolutionary change. So we're gonna have evolution here. This is gonna be an evolutionary change to where we still have a gene a, because gene a is important, but that other gene a evolved and changed into a new gene called gene b. These are gonna be ortho or paralogous genes because they diverged, they changed after a gene duplication event. That's gonna be a parallogous gene. But they are similar because of ancestry. So these two genes are probably very, very similar, but slightly different. But they're similar due to ancestry, so they're homologous. Now, horizontal gene transfer is a really interesting topic. Horizontal gene transfer is where genes are transferred from one genome to another. And you may think that doesn't happen very often, does it? It actually happens a ton, just it happens a lot in prokaryotic organisms. So prokaryotic organisms have the ability to pick up genetic material from the environment or transfer genetic material between one another, And that's gonna be an example of horizontal gene transfer. So this is transmission of DNA from 1 genome to another. Prokaryotes very commonly do this. Eukaryotes sometimes do this. Now, organelles actually do this a lot. Organelles did this during the development of eukaryotic organisms. Organelles, like mitochondria, actually moved some of their DNA into the nucleus. Also, transposons are going to do this, plasmids are going to do this, and a great example of horizontal gene transfer that happens in human beings is going to be viral horizontal gene transfer. A lot of viruses actually enter your cells and incorporate their genetic material into your genetic material. So that is an example of horizontal gene transfer. This is different than Vertical Gene Transfer. Vertical Gene Transfer is when your mother and father gave their genes to you. That is Vertical Gene Transfer. Horizontal Gene Transfer is going to be 2 different organisms actually exchanging genetic material. So, this actually is not homologous. This should be analogous Because this is gonna be similar genetic material, but not due to ancestry. It's due to genetic exchange. DNA exchange via transposons, via viruses, or via simply 2 prokaryotes exchanging genetic information. So this is similarity between organisms due to exchange, not due to ancestry. So horizontal gene transfer is an analogous transfer of genetic information. Okay, everyone. Let's go on to our next topic.
3
concept
Cladistics
Video duration:
15m
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Video transcript
Hello, everyone. In this lesson, we are going to be talking about the different relationships between organisms that you can discover inside of phylogenetic trees. Okay. So, first off, we're gonna go over a generalized term called cladistics. Cladistics is going to be a field of study inside of Biology and inside of Genetics. And Cladistics is going to be a type of classification based on shared characteristics between common ancestors. And it's going to be utilized for hypothesizing evolutionary relationships between different organisms. And it is going to it is now incorporating things like genetics, shared genetic characteristics, shared genetic code, and different things like that. And this is one of the main ways that we create phylogenies. Cladistics and the shared characteristics and genetics between organisms. Now, cladistics is gonna be based off this word called clade. A clade, you are gonna need to know this. I remember being tested on what a clade was, and what the different types of groups inside of phylogenies were. And the clade is going to be a group of organisms based on a common ancestor and its descendants. So a clade is the ancestor and all of the organisms that evolved from that ancestor. And that is going to be a group, a particular group that you can find within a phylogeny. So, for example, this phylogeny that we have down here, let's say that we're looking at this particular ancestor. Well, this would be a clade Because it has the ancestor and all of the different organisms that evolved from that ancestor. So that is a clade. Now, there are going to be different types of groups that you can find within a phylogeny. And I have seen it many times, your professor might say look at this phylogeny and which of these groups is paraphyletic. And you're gonna have to know what that is. Okay? So, let's talk about monophyletic first because it is pretty much the easiest to understand. And why is that? Well, it's pretty much the same thing as a clade. A monophyletic group of organisms inside of a phylogeny is the ancestral species and all of its descendants. So, realize that that is the same thing as a clade. They are the exact same thing. A monophyletic group and a clade are gonna be the same thing. Now, how do you remember what monophyletic means? I'm not sure if this helps everyone, but this usually helps me. If I know what the words mean, if I know the basis of the words, where the words come from, it usually helps me to remember them. So, mono means what? Whenever you're thinking about numbers or you're thinking about groups, mono means only or alone. So mono means the the only ancestor and its only descendants. So, only the ancestor and its descendants. So, mono means only ancestor and descendants. I'm not sure if that helps everyone, but it does help me. So, only ancestor, plus descendants. Not anybody else, nobody else can be in this group. It's a very closed off, special group. You can only be the ancestor or the descendants to be in this group. Nobody else is allowed in. It's like a special club. So, that's gonna be monophyletic. Now, there's also paraphyletic. A paraphyletic group inside of a phylogeny is going to be the ancestral species and some, but not all of the descendants. So some of the descendants have been left out for whatever reason. Maybe the scientist was only really interested in looking at a certain number of organisms, and those organisms make up a paraphyletic group because all of the descendants were not included. So, what does para mean? Para is going to mean near. So, means near, or close to, so it's nearly complete. The group is nearly complete, but not all of the organisms, not all of the descendants are within this group. This is why this group is paraphyletic and not monophyletic. Now, which of the groups are gonna be paraphyletic? Well, a great example of a paraphyletic group is going to be this blue one. So, paraphyletic. And that is because you can see that the common ancestor is down here, and here is a descendant. So lemurs are a descendant, lorises are a descendant of that ancestor, Tarsiers are a descendant of that ancestor, but all of these guys are descendants as well, they were not included in this blue group. So this blue group is paraphyletic. Okay? So that is a great example of a paraphyletic group. Now, I didn't go over it, but just realize that this yellow group is monophyletic. Right? Or a Clade. Okay? And why is that? Because see that this is the Common Ancestor, and every single one of its descendants is included in this yellow group. So this is a great example of a clade or a monophyletic group, an ancestor and its descendants, and that's all. Nobody else, only the ancestor and the descendants. While the paraphyletic group is the ancestor and nearly all of the descendants, but not all of them. Okay? Alright. So now the last one that we have is not super commonly used, but I have seen it before. Monophyletic and paraphyletic are definitely the more important ones to know, but your professor might want you to know polyphyletic, but it's not as common. Polyphyletic is going to include variously distant related species, but not their common ancestor. So, basically, what you can think of this as is what does poly mean? Poly is the opposite of mono. Mono means 1 or only 1, poly means many. So, basically, this is going to be many different organisms that aren't really related to each other. So, many different organisms. But their ancestor is not included. So a great example of Polyphyletic is going to be right here. Polyphyletic. So, you can see that the lorises and the tarsiers do have a common ancestor way back here, but it's not included in the red group. The only thing that is included are the Lorises and the Tarsiers, which does not include their common ancestor. So this includes distantly related species, because lorises and tarsiers are pretty distantly related, but not their common ancestor. So, basically, polyphyletic means many different organisms without the common ancestor. But, again, like I said, polyphyletic isn't used as often, so you probably don't have to know that one. Definitely know clade, monophyletic, and paraphyletic. Those are very important. Okay? Alright. So now, let's go down and we're gonna talk about another, we're gonna reference another phylogeny. Sorry about that. Okay. So we're gonna talk about an out group. What is an out group? Well, an out group is gonna be something you use when you are studying the evolutionary relationships between organisms, and you need something to compare it to. An outgroup is going to be a monophyletic group. Remember what that means, the ancestor and all of its descendants. And it's going to serve as a reference point for the groups being studied. So the opposite of the out group is the in group. And the in group is going to be your group of interest. Whatever you are studying, maybe you study frogs and your out group is going to be, I don't know, some sort of fish. Okay? So you have an in group, that's gonna be the one that you are particularly interested in, the one you are studying the Evolutionary Relationships of. And then you're going to have an out group which is gonna be a monophyletting group of organisms that aren't really related, maybe very distantly related to your group of interest, and and you're gonna use that group as a reference point. Now, we're gonna talk about 2 other things that you can see or you can find inside of phylogenetic trees. And that's gonna be a synapomorphy and a symplizomorphy. Both very difficult to say. Okay. So, a synapomorphy is going to be a trait shared by taxa and their most common ancestor. This is commonly called a Derived Trait, or a shared derived trait. And this is going to be found in an ancestor and all of its descendants, but not the previous ancestor. So, this is going to be basically a trait that evolved within a group. So, for example, let's say if we're looking at We're looking at the dinosaurs right here. And let's say that the ancestor of the dinosaurs had bird hipped, or it had bird hips, which sounds funny, but they reference it right here. So, let's say it has bird like hips. The original ancestor of all of the dinosaurs had bird like hips. But then, you can see as we go up, lizard like hips evolved. So, lizard oh. I'm sorry. I'm in the way. I should probably get out of the picture so you can see. So, lizard hips evolved over time in the groups of Dinosaurs. And, it evolved in this particular ancestor. And, all of these species ended up getting lizard hips. So, all of these species have lizard like hips, and so did their common ancestor, but not the previous Ancestor. The Previous Ancestor did not have that characteristic. So that's gonna be a synapomorphy. A trait shared by taxa and their most common ancestor, which we can see right here. So, we have this common ancestor has lizard hips and all of these an all of these descendants have those lizard hips, but this is not shared by the previous ancestor which is down here. Okay? That is a synapomorphy. Now, the Symplisomorphies are a little bit different. Symplizomorphies are going to be ancestral traits that are gonna be shared by the ancestor and at least 2 or more of the descendant taxa. So I'm gonna write by 2 taxa or more. Okay? So at least 2 descendant taxa. Now, Symplizomorphy, I want you to know that it's also simply called Plysomorphy. They both mean the same thing, it just depends on which your professor prefers, Plysomorphy or symplasomorphy, they are the same thing. And that is an ancestral trait shared by 2 or more descendant taxa. So, what would that be? So like we said, if the ancestor of the dinosaurs way down here had bird hips, and these descendants did not have bird hips, They do not have a Symplisomorphy. But, notice that along this line, all of these descendants have bird like hips, which you can see right here. They have bird hips. So, there are at least 2 different taxa here that share a trait with their ancestor. Not all of them do, this group does not share that particular trait. They do not have bird like hips, but some, more than 2, of the descendants of the original Plizomorphy where the ancestral trait from the original ancestor, which is down here, is shared by 2 or more of its descendant taxa, but not all of its descendant taxa. I know that can be kind of confusing. Hopefully, that wasn't too bad. Now, I just realized that I did not give you an example of an out group. So let me show you an example of an out group. Okay. So let's say that we were studying our dinosaurs. So our dinosaurs are going to include all of these guys right here. So we have all of these organisms under Dinosaurs. And say we were studying dinosaurs, what would be our out group for our in group, which is the dinosaur group? Well, a good out group for our dinosaurs would be Pterosaurs. Why? Because it's monophyletic, remember it's one ancestor and all of its descendants, so somewhere up here we're gonna have an ancestor, and it's gonna be the Pterosaurs are gonna be a great out group. And scientists might use Pterosaurs for comparison for different evolutionary traits with Dinosaurs. So that would be a great out group. Now, what if we wanted to study all of the what? I'm not sure how you say this. Ornithidorians. What if we wanted to study all the ornithidorians? Well, that's going to include the dinosaurs and the pterosaurs. Right? So, that means that our out group would be crocs and their relatives. So, we would use crocs and their relatives as a out group for the study of the ornithidorians. I believe that is the correct way to say that. So, I hope that was helpful. Just remember that the really important things from this particular, lesson are gonna be what a clade is, what a monophyletic group, and what a paraphyletic group are inside of a phylogeny, and how to recognize those different areas or those different groups of organisms inside of a phylogeny, and be able to identify what a synapomorphy is. I think knowing what a synapomorphy is is more important than knowing what a symplasmomorphism is. But, again, it depends on what your professor wants. Okay, everyone. Let's go on to our next topic.