Lesson Video: Classification Biology

Video Transcript

In this video, we will learn why biologists classify organisms, identify the five kingdoms of life, learn to use the binomial system to name organisms, and explain how genetic analysis has changed modern taxonomy. Then, we will try some practice questions together. Finally, we’ll review what we’ve learned.

So, let’s get started. What do we mean by “classification?” Classification is the process of organizing things based on their similarities. The study of the classification of organisms or living things is called taxonomy. But why even bother? When we group organisms based on their shared traits or characteristics, it makes them easier to study. It makes it easier to name new organisms and easier to identify organisms that we’re unfamiliar with. It even helps us to investigate the evolutionary relationships between different types of organisms.

So far, scientists have discovered about 1.3 million different species. Experts estimate that there may be a total of 8.7 million or even more species in existence. Taxonomy allows us to apply what we know about related species instead of having to approach each new discovery individually. So, how do we classify organisms? Why don’t we start with how they’re named?

The naming system that we use to identify organisms today was developed by a Swedish botanist named Carl Linnaeus. Linnaeus loved plants. But he found himself frustrated with the current naming conventions of the time. Let’s take, for example, an herb that you may be familiar with. In English, it’s called catnip. Before Linnaeus’s naming system, plants were often given long, complicated Latin names that describe various parts of their anatomy. So, scientifically, catnip used to be called Nepeta floribus interruptae spicatus pendunculus. Quite the mouthful! This system was known as polynomial nomenclature, poly- meaning many, -nomia meaning name, and nomenclature, a word that means naming system.

Polynomial names were hard to remember. And each species had to be named individually. Carl Linnaeus’s naming system is called binomial nomenclature. Each species is given a name that has only two parts. Using this system, catnip is known simply as Nepeta cataria. Not only does this system make things easier to remember, it also gives us some information about the organism.

In binomial nomenclature, the first term is the genus name. The genus is the larger group that the organism belongs to. The second word is the species name. It tells us what specific organism it is we’re dealing with. It’s also worth noting that it’s scientific conventions to capitalize the first term or genus and not the second term or the species name. Now that we’ve learned about polynomial nomenclature, let’s go ahead and take a closer look at Linnean taxonomy.

Linnean taxonomy or classification is a hierarchical system. When we say something is hierarchical, we simply mean that it’s based on rank. In biology, we often rank things based on their size and complexity. The taxonomic levels group organisms based on their similarities. From largest to smallest, the seven taxonomic levels are kingdom, phylum, class, order, family, genus, and species. Many students find it useful to develop a pneumonic device to remember the order of the taxonomic levels, commonly a simple sentence in which each word starts with the same letter as one of the taxonomic levels. One that’s commonly used is “King Philip Came Over For Good Soup.” But you might like to try coming up with a pneumonic device of your own.

Let’s try illustrating the system of classification using an example. One of my favorite animals, and an organism that you’re probably already familiar with, is a common house cat. The cat is a member of the animal kingdom because it’s multicellular, eats food to get energy instead of using sunlight to make it, can move around, and reproduces sexually. Some other members of the animal kingdom include birds, insects, lizards, fish, bears, tigers, and even humans. Cats belong to the phylum Chordata, which generally includes all animals that possess a spinal cord. So, insects do not belong to this phylum, but the rest of our example animals do.

Cats are members of the mammal class, which primarily includes animals that give birth to live young and are able to produce milk. Birds, lizards, and fish are not mammals because they lay eggs, but bears, tigers, and humans are. Cats belong to the order Carnivora, which generally includes animals that primarily eat meat. Humans do not share this trait, but bears and tigers do. The cat family is Fellidae. It includes all cats, large and small. Tigers are in. Bears are out. A cat’s genus is Felis, a classification it shares with other species of small cats. Tigers belong to the large cat genus known as Panthera. And finally, the species designation is catus, which is easy to remember.

Take a moment to recall that in binomial nomenclature, an organism’s scientific name is made up of its genus and its species designation. So, the scientific name for your common house cat is Felis catus. Also, take note that as the taxonomic levels got smaller, they included fewer different species and the organisms included had more characteristics in common. The taxonomic levels are arranged not only from largest to smallest but also from general to specific. Now that we familiarized ourselves just a little bit with the animal kingdom, let’s take a look at the five kingdoms of life.

When Linnaeus developed his system of taxonomy, he classified all life into just two kingdoms. And those were animal and plant. Today, we generally use five kingdoms into which all life on Earth can be classified. The other three kingdoms are fungi, protist, and prokaryote. Remember that kingdom is a very general classifications. And since we’re classifying organisms, we’re still grouping them based on characteristics or traits that they have in common.

The prokaryote kingdom contains all prokaryotic organisms, organisms that are unicellular and do not possess a nucleus or other membrane-bound organelles. The other four kingdoms all consist of eukaryotic organisms, or organisms made of cells that possess a nucleus in which the DNA, or genetic material, is stored. Plants and animals are multicellular, while fungi and protists can be unicellular or multicellular. Plants are autotrophic, meaning that they can make their own food using the energy in sunlight, while animals and fungi are heterotrophic, meaning that they need to consume food for energy. Protists may be autotrophic or heterotrophic.

In general, animals are organisms that move around to find food and mates. Fungi are organisms like mushrooms, molds, and yeasts that break down the material around them and absorb it for food. Plants you’re likely familiar with. They’re the green things you see growing around you, and they carry out photosynthesis. Protists tend to be organisms that don’t fit into any of the other kingdoms. Some examples you might have heard of are amoeba and paramecium. Prokaryots are primarily bacteria. Although, there are some other organisms that do fall into this category. We’ve learned a little about the most general taxonomic level, kingdom. Now, let’s take a look at the most specific, species.

So, what exactly is a species? A species is a group of closely related organisms. All humans belong to the species Homo sapiens, a term that means man the wise. Recall that in binomial nomenclature, the first term in the scientific name denotes the genus and the second tells you the species. You may remember that a house cat belongs to the species Felis catus and the common herb catnip is Nepeta cataria.

But how do we know that organisms are closely related enough to be considered the same species? Well, some closely related organisms can reproduce together, like a horse and a donkey. Their offspring is called a mule. However, you can’t cross a mule and a mule because mules are infertile. So, we can further define a species as a group of closely related organisms that can breed and produce fertile offspring.

We’ve seen this term “related” pop up a few times, but what does it actually mean? In order to answer that question, we’ll have to step beyond the world of Linnaeus into the world of genetic analysis. Recall that all organisms carry their genetic code in a molecule called DNA and that within that DNA are genes or segments of DNA that code for one particular trait or characteristic. The genetic code is carried within the sequence of the nucleotide bases. That DNA is transcribed into RNA. And that RNA is then translated into a series of amino acids, which are eventually folded into a functional protein. This process, called protein synthesis or the central dogma of biology, explains how DNA actually determines our traits or characteristics.

Organisms that are more closely related to each other will have more DNA or nucleotide bases in common with each other and for this reason will share more similar traits or characteristics. This explains why humans and chimpanzees share about 99% genetic similarity, whereas humans and cats only share about 90% genetic similarity. You might try transcribing and translating these three genetic sequences so that you can see the differences on your own. Recall that Carl Linnaeus developed his system of classification in the 1700s, while DNA wasn’t discovered until just last century.

Well, this brings us to two new terms, artificial classifications and natural classification. When Linnaeus was proposing his systems of binomial nomenclature and taxonomy, he was classifying organisms based on their physical appearance. Back then, he knew nothing about genes or even modern theories of evolution. In fact, Charles Darwin, the father of modern evolutionary theory, was born almost exactly 100 years after Carl Linnaeus. Luckily, since the traits of organisms are based on their genetic material and since Linnaeus was a shrewd botanist, he managed to get a lot of things right.

However, artificial classifications led to numerous inaccuracies. And the system has had to be modified several times since its introduction. In contrast, today, we group organisms based on natural classification. We’re able to rely directly on genetic analysis as opposed to inferences based on physical appearances, leading to much more accurate naming and classification of organisms. Thanks to natural classification, our understanding of evolutionary relationships is constantly growing.

Today, we know much more about genetics and evolution. We know that organisms that are more related share a more recent common ancestor in the same way that you and your siblings share a more recent common ancestor in your parents than you and your less closely related cousins do.

Recalling our taxonomic levels from most general to most specific, kingdom, phylum, class, order, family, genus, and species. In our evolutionary diagram, humans and chimpanzees are members of the same family, known as hominid, while the less closely related cats are members with us in a less specific group, the class mammals. This increased knowledge has led to a notable change in Linnean taxonomy.

Here are the five kingdoms we discussed a little earlier shown as a family tree. Recall that of the seven taxonomic levels we discussed, kingdom was the most general. With the more recent advent of genetic analysis, we’re able to more precisely trace the evolutionary relationships between organisms, which has led to an eighth taxonomic level that scientists call domain. Through genetic analysis, we’ve learned that prokaryotic organisms actually fall into two distinct groups, domain Bacteria and domain Archaea. While all of the other organisms in all of the other kingdoms belong to the domain Eukarya.

Now that we’ve learned about Linnean taxonomy and binomial nomenclature, the five Kingdoms and the three domains, we’re ready to try a practice question.

Which of the following is an assumption scientists make when classifying organisms based on genetic analysis? (A) The more DNA two organisms have in common, the more recently they shared a common ancestor. (B) The more DNA two organisms have in common, the less recently they shared a common ancestor. Or (C) organisms have only shared a common ancestor if they have identical DNA.

In order to answer this question, we first have to understand that when we classify organisms based on genetic analysis, we’re grouping them together based on similarities in their DNA or genes. Organisms that are more closely related will have more DNA in common. Since you and your siblings share a more recent common ancestor, your parents, you’ll have more DNA in common than you would have with your less closely related cousins, who share a less recent common ancestor, your grandparents.

The same is true when we talk about classifying organisms. The more recent the common ancestor, the more DNA two organisms will have in common. Let’s look at an example. Cats, humans, and gorillas are all members of the same class, which is mammals. However, humans and gorillas share a more recent common ancestor than the one that they share with cats. Recall our seven taxonomic levels of classification from most general to most specific. They are kingdom, phylum, class, order, family, genus, and species.

When we classify organisms based on genetic analysis, they’ll share a more specific group if they have more DNA in common. Cats, humans, and gorillas all belong to the same class, mammal, whereas humans and gorillas both belong to the same family hominid. They share a more specific classifications because they share a more recent common ancestor and have more genes in common.

Now, we’re ready to answer our question. When classifying organisms based on genetic analysis, scientists must assume that the more DNA two organisms have in common, the more recently they shared a common ancestor.

Finally, let’s take a moment to summarize what we learned in this video. We’ve learned that classification is the practice of grouping organisms based on their similarities. We’ve learned about artificial classification, which relies on the physical characteristics of organisms, and natural classification, which groups organisms based on genetic similarities. We’ve learned about the five kingdoms of life and the seven taxonomic levels and how genetic analysis has led to adjustments in these systems. We also learned about binomial nomenclature, or the scientific system of naming organisms using their genus and species classifications.