Introduction to Phylogeny: Study with Video Lessons, Practice Problems & Examples

Systematics is the scientific field that classifies and groups organisms, utilizing the Linnaean hierarchy developed by Carolus Linnaeus. This system includes domains, kingdoms, phyla, classes, orders, families, genera, and species, with each level referred to as a taxon. Modern taxonomy employs binomial nomenclature, a two-name system where the genus is capitalized and the species is lowercase, both italicized. Phylogenetics, the study of evolutionary relationships, informs taxonomy, ensuring that classifications reflect shared ancestry among organisms.

Systematics

Folks, we now want to take a look at the science of systematics and systematics. We're going to find here is a scientific field that groups and classifies organisms. Now nature is complex and messy, but we like to put things in nice little boxes and understand how everything's related to each other. Right? The science of figuring out what those boxes are in the system that we use to do that, that's systematics.

Now as part of that, we like to name organisms, and that's going to be sort of a related or subfield that we're going to call here taxonomy, That sort of systematic naming and grouping of organisms that is taxonomy. Now here we're going to look at how we do that in modern biology. What are the groups we use and how do we name things? And then going forward, we're going to spend a lot of time looking at what's sort of underneath our system. What is it that we use to group organisms in modern biology?

Now before we look at that, we are going to just introduce one more term here, this term taxon, and the plural of taxon is going to be taxa. Now taxon or many taxa, this is just any group that we come up with at any taxonomic level. Right now why we have that name and how we use it, we'll look at in just a second going forward. But first, let's introduce the system. In modern biology, we use the Linnaean hierarchy and Linnaean hierarchy is going to be this nesting classification system developed by Carolus Linnaeus.

Now Carolus Linnaeus, you may see his name anywhere from Carl von Lin to Carolus Linnaeus, depending on how Swedish or Latin he wanted to sound. Now Linnaeus was working in the 1700s. This is before our modern understanding of evolution. But he developed this system that we still largely use today just with some small modifications. All right.

Now in this system, it's a nesting system. So at the top, we have these really big groups. These really big groups that contain a lot of organisms. At the top of our modern Linnaean hierarchy, we have the groupings of domain. Domain, there are 3 domains of life: the Eukaryota, the Bacteria, and the Archaea.

Now within each domain, there will be several kingdoms. Kingdoms are smaller groups still really, really big, but a smaller group that nests within that group of domain. Now as we go down from there getting smaller and smaller, we go from kingdom down to phylum. Within a phylum is a class. Within a class are many orders.

Within an order are many families. Within a family are many genera. And within a genus are going to be many species. Alright. So this is our system going from a lot of organisms getting more and more and more specific as you go down.

Now to see how this works, let's pick an organism. I'm gonna pick a lion because everyone likes lions. The lion we call under this system Panthera leo. Now we'll look at how we name organisms using that binomial nomenclature, that two-name system in an upcoming video. Alright.

But the lion, Panthera leo, is part of a larger group. It's part of the genus Panthera. The Panthera, sometimes we call the big cats. This includes lions, tigers, jaguars, and leopards. Alright.

Panthera, though, is part of a larger group. It's part of a family that we call Felidae. Felidae are all the cats. So this now also includes cheetahs, mountain lions, bobcats, and you know, mister whiskers on the couch next to you there. Well, Felidae is part of the order Carnivora.

The carnivores. This includes wolves and dogs, bears, seals, and includes hyenas, raccoons, and weasels. Alright. Carnivora is part of a larger group, a class we call Mammalia or the mammals. Mammals are organisms that make milk or have hair.

Mammals are part of the phylum Chordata. Right? Chordata is anything with a notochord or a nerve cord running down the back. Chordata is part of the kingdom, Animalia, the animals, and animals are part of the domain Eukaryota. The eukaryotes anything with a nucleus and more complex cells with organelles.

Alright. So now we can see that any one of these levels was is a taxon. Mammalia is a taxon. Felidae is a taxon. And we have that more generic term because as we go forward, we'll say we're going to look at this taxon and compare them, and we may want to compare at different levels.

Alright. Now if you need to know this hierarchy, we have a very classic mnemonic device here, and it goes, dear King Philip came over for good soup. Alright, dear King Philip came over for good soup. We have domain, kingdom, phylum, class, order, family, genus, species. And there is no shame in bringing out dear King Philip when you need him.

I still say that to myself when I need to remember my class order family, because I always go get those flipped in my head. Alright. Well, we said that Linnaeus was working in the 1700s before our modern understanding of evolution. But today, we want to base this hierarchy on what we're going to base our groups on is evolutionary relationships.

So we now need to introduce sort of another related field we call phylogenetics. Phylogenetics, we can break down this word. Phylo comes from the Greek meaning tribe or group. And genetics, it doesn't come from the science of genetics. It comes from the word genesis, meaning origins.

So this is the study of the origins of groups. In other words, it's the study of evolutionary relationships. So in modern taxonomy and modern systematics, what we want to do is we want to make these phylogenetic trees. We're going to spend a lot of time looking at these phylogenetic trees, learning how to interpret them, and learning how to build them. Because what we're going to say here is that the lion and the tiger, leopards and jaguars, they're not part of the same genus just because they have some similarities.

We're saying they're part of the same genus because when we build a tree, they share a common ancestor. And everything descended from that ancestor, we are going to call the genus Panthera. All right. So in that way, we're going to say in modern biology, taxonomy should reflect phylogeny. Another example for what I mean here, if we look at the taxon, the mammals, the Mammalia, so this is a class.

Now I said previously mammals all make milk and have hair. But if we found a mammal that didn't make milk, we would still call it a mammal because all mammals share a common ancestor and anything descended from that common ancestor, we call a mammal. So even if something stops making milk through some evolutionary process, if it shares that common ancestor under modern taxonomy, it's still part of the class Mammalia. Alright. Now we're going to look at systematics and specifically how we name organisms our modern taxonomic system coming up, and then we're going to spend again a lot of time looking at these phylogenetic trees, understanding how we interpret and make these groups.

I think this will be a lot of fun. See you there.

Which of the following are paired correctly? 

• A. Taxonomy: study of evolutionary relationships.
• B. Taxonomy: the naming and classification of organisms.
• C. Phylogeny: the process of classifying organisms into species.
• D. Phylogeny: the process of naming organisms.

Which statement below is correct?

• A. Members of the same genus are more likely to be closely related than members of the same order.
• B. A class is made up of many phyla.
• C. An order contains more types of organisms than a class.
• D. Members of the same family are, by definition, also members of the same genus.

Binomial Nomenclature

Now we're going to take a look at how we name organisms under our modern taxonomic system, and here we use what we call binomial nomenclature. So binomial nomenclature, that literally means the two-name naming system. So this is just this two-word naming system used in Linnaean taxonomy. Now, binomial nomenclature uses Latin names or at least Latin-sounding names. We're going to use Latin spelling.

Sometimes they can be a little hard to spell or a little hard to pronounce, but with a little practice, you can get it. Now these are two-word naming systems. So the first word is always going to be the genus name, and that genus name when you write it, it should always be capitalized. The second word is going to be what we call the specific or the species name. That species or specific name, we always write in lowercase and the entire thing altogether should always be written in italics unless you're handwriting it, then you underline it.

Alright, so genus species, let's look at some examples here. First example, our friend here, the lion. Well, we've already seen this name. Its name is Panthera leo. So Panthera, that's the genus name. We can see it's capitalized. Leo, that's the specific name. We can see it's lowercase, the entire thing is written in italics. Well, let's compare this to another cat that we know here, the tiger. We've already said that these things are part of the same genus.

So we know the first half of the tiger's name then, right? It's Panthera, and it's going to be Panthera tigris. Now remember, we said that our taxonomy should reflect phylogeny. It should reflect evolutionary relationships. So by seeing that these have the same genus name, I already know something about them.

I know that they are somewhat closely related to each other because they're part of the same genus. Now we can compare this to another cat. We're going to introduce here Button. Button is a domestic cat and domestic cats, well their name is Felis catus, Right, so it's a different genus and there's other species in this genus. There's the African wildcat, the European wildcat.

There are others, but we can tell right away again because our taxonomy reflects phylogeny, that the lion and the tiger are more closely related to each other than they are to the domestic cat because they share that genus name. Alright. So now going forward, just remember always write them in italics. The first name capitalized. That's the genus name.

Second name, lowercase, that's the specific name. Examples and practice to follow. I'll see you there.

Introduction to Phylogeny Example 1

Our example tells us that three species names are given below using binomial nomenclature. Now we need to identify any errors and how these names were written. Alright, so let's go through them and see what we see. Alright, we're gonna start here with a chimpanzee, and it says here pan troglodytes. Do you see anything wrong with how pan troglodytes is written there?

Well, remember that specific name or the species name, should be written in lowercase. So, I'm going to cross out that P and write a lowercase p there. Pan troglodytes, lowercase p, that would be correct. All right, so next, we have a clownfish, and here we have the name Amphiprion ocellaris. So when you take a look at that name, do you see anything wrong with how it's written?

Well, in binomial nomenclature, we want to write these names in italics. So I'll just write italics underneath exclamation point. Write it in italics. Come on, man. Right now, again, if you hand write it, you're going to underline it.

Okay, and now our final one. This is a pika. Pikas are animals that live in the mountains or related to rabbits, and it says here, Ochotona princeps. Alright, so take a look. What do you see?

Now that genus name, that genus name should have a capital O on it. Ochotona princeps with the capital O. Alright. So remember always written in italics, the genus name capitalized, the specific or species name written in lowercase, and with that, we did it.

Henry writes the scientific name for the pigeon as columba livia. What would he have to do to format this name correctly according to the standards of binomial nomenclature?

• A. Capitalize the species and italicize the genus.
• B. Capitalize the genus and italicize the species.
• C. Capitalize the genus and italicize both the genus and species.
• D. Capitalize both the genus and species.