Which is the smallest category in the hierarchical taxonomic system of classification

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Taxonomy is the practice of categorizing and naming of species. The official "scientific name" of an organism consists of its Genus and its Species Identifier in a naming system called binomial nomenclature.

The Work of Carolus Linnaeus

The current taxonomic system gets its roots from the work of Carolus Linnaeus in the early 1700s. Before Linnaeus set up the rules of the two-word naming system, species had long and unwieldy Latin polynomials that were inconsistent and inconvenient for scientists when communicating with each other or even the public.

While Linnaeus's original system had many fewer levels than the modern system has today, it was still an excellent place to start to organize all of life into similar categories for easier classification. He used the structure and function of body parts, mostly, to classify the organisms. Thanks to advances in technology and understanding the evolutionary relationships among species, we have been able to update the practice to get the most accurate classification system possible.

The Taxonomic Classification System

The modern taxonomic classification system has eight main levels (from most inclusive to most exclusive): Domain, Kingdom, Phylum, Class, Order, Family, Genus, Species Identifier. Every different species has a unique species identifier and the more closely a species is related to it on the evolutionary tree of life, it will be included in a more inclusive group with the species being classified.

(Note: An easier way to remember the order of these levels is to use a mnemonic device to remember the first letter of each word in order. The one we use is "Do Keep Pond Clean Or Fish Get Sick")

Domain

A domain is the most inclusive of the levels (meaning it has the most number of individuals in the group). Domains are used to distinguish between the cell types and, in the case of prokaryotes, where they are found and what the cell walls are made of. The current system recognizes three domains: Bacteria, Archaea, and Eukarya.

Kingdom

Domains are further broken into Kingdoms. The current system recognizes six Kingdoms: Eubacteria, Archaebacteria, Plantae, Animalia, Fungi, and Protista.

Phylum

The next division would be the phylum.

Class

Several related classes make up a phylum.

Order

Classes are further divided into Orders.

Family

The next level of classification that orders are divided into are Families.

Genus

A genus is a group of closely related species. The genus name is the first part of the scientific name of an organism.

Species Identifier

Each species has a unique identifier that describes only that species. It is the second word in the two-word naming system of the scientific name of a species.

In this explainer, we will learn how to recall the levels in the taxonomic hierarchy and apply this knowledge to example organisms.

Since the first life appeared on Earth over 3.8billion years ago, new species have continually evolved and gone extinct. The study of classification, also called taxonomy, determines how we group and organize the diversity of life on Earth in the past, present, and—eventually—future.

Definition: Taxonomy

Taxonomy is the study of classifying and naming organisms.

The current system of biological classification has been heavily influenced by the 18th century work of Swedish botanist Carl Linnaeus. Linnaeus is credited with developing the system of binomial nomenclature as well as popularizing the system of taxonomic hierarchy, both of which we still use today.

Taxonomy was initially based on physical characteristics. Scientists of the past did not have access to the genetic technology we possess today. When we group species based on physical similarities alone, we call it artificial classification. With improvements in genetic science, today we try to group species based on evolutionary relationships. The classification of organisms based on genetic and evolutionary analysis is called natural classification, or phylogenetic classification.

We classify organisms by arranging them into groups and subgroups on the basis of similarities and differences, and then we place them into a hierarchy that brings out these relationships. This hierarchy consists of several ranked levels, called taxonomic levels, that progress from general to specific, as shown in Figure 1. The 7 basic taxonomic levels are kingdom, phylum, class, order, family, genus, and species.

Key Term: Biological Classification

Biological classification is the arrangement of organisms into groups and subgroups on the basis of meaningful similarities and differences. Those groups are then ranked by size.

Key Term: Taxonomic Hierarchy

Taxonomic hierarchy is the ranking of the different groups, or taxa, that organisms are classified into. These taxa are ranked from large/general (i.e., kingdom) to small/specific (i.e., species).

Students commonly use a mnemonic device to help them to remember the order of the taxonomic ranks. A mnemonic device is a phrase where the first letter of each word corresponds to the letters in a sequence you are trying to remember. For example, King Phillip came over for good soup is a popular mnemonic device for kingdom, phylum, class, order, family, genus, and species. You may find it easy to use this example or come up with one of your own!

Example 1: Recalling the Correct Order of the Taxonomic Hierarchy

Which of the following is the correct order of taxonomic hierarchy, from largest to smallest?

1. Kingdom → phylum → class → order → family → genus → species
2. Phylum → class → kingdom → order → family → genus → species
3. Kingdom → class → order → family → phylum → genus → species
4. Kingdom → family → genus → phylum → class → order → species
5. Phylum → class → kingdom → order → family → species → genus

Answer

We classify organisms using a system that can be referred to as the taxonomic hierarchy. In this system, living things are divided into groups based on their features and/or genetic relationships. These groups are then divided into smaller subgroups, and the groups themselves are ordered in a meaningful way—generally, from largest to smallest or from most general to most specific. The taxonomic hierarchy is the ranking of these groups. Students often find it useful to use a mnemonic device to remember the order of the taxonomic rankings. A popular mnemonic device is King Phillip came over for good soup. The first letter of each word in that sentence reminds us that the correct order of the taxonomic hierarchy from largest to smallest is as follows: kin gdomphylumclassorderfamilygenusspecies ⟶⟶⟶⟶⟶⟶.

Let’s look at an example. We will examine the taxonomic hierarchy of the common house cat. Domestic cats have many varied features, like fur color, face shape, and tail length, but they are all members of the same species.

• The species of the common house cat is Felis catus. The scientific name of a species actually includes both the genus name (in this case, Felis) and the species name (catus).
• This tells us that cats belong to the genus Felis, along with other small cats like sand cats, jungle cats, and mountain cats.
• The Felis genus is a part of the Felidae family, which includes all cats, large and small. The Felidae family also includes lions, tigers, leopards, bobcats, and more.
• The Felidae family is a part of the carnivora order. You may be able to tell by the name that the order carnivora includes animals that are adapted to a diet of meat primarily.
• The Carnivora order is a part of the class Mammalia. Mammals are organisms that typically give birth to live young and produce milk for their offspring.
• The mammal class is in the phylum Chordata. This means that mammals all possess what we would think of as a spinal column.
• Finally, the phylum Chordata is a part of the animal kingdom. Animals are typically organisms that move around to find food and mates.

So, with this information, we can deduce that some features of the common house cat are that it moves around to find food and mates, has a spinal column, nurses its kittens, and eats meat. Each of these taxonomic groups tells us about the features of the organisms within it. Figure 2 shows a summary of the taxonomic ranks that the domestic cat belongs to.

We can also observe that more closely related species have more physical characteristics in common. Cats and lions are both members of the Felidae family and resemble each other in certain ways. Cats and grizzly bears are both members of the carnivora order, but they have fewer features in common. Organisms that are members of the same family are more closely related than those that are only members of the same order because a family is a more specific taxonomic rank than order.

Example 2: Determining the Missing Ranks in the Taxonomy of a Wolf

The taxonomic hierarchy for a gray wolf is provided in the diagram.

1. Which taxonomic rank is represented by X?
2. Which taxonomic rank is represented by Y?

Answer

Part 1

Scientists classify organisms using a system that can be referred to as the taxonomic hierarchy. In this system, organisms belong to certain groups based on their features and/or genetic relationships. The smallest of these groups is called species. Several related species are grouped into a genus, and several related genera are called a class. These groups, or taxa, are ranked from largest to smallest or from most general to most specific. The taxonomic hierarchy is the ranking of these groups. Students often find it useful to use a mnemonic device to remember the order of the taxonomic rankings. A popular mnemonic device is King Phillip came over for good soup. The first letter of each word in that sentence reminds us that the taxonomic ranks, in order from largest to smallest, are kingdom → phylum → class → order → family → genus → species. In the diagram provided, we can see that the kingdom is at the top, so these ranks must also be listed from largest to smallest.

With this information, we can conclude that the taxonomic rank represented by X is class.

Part 2

The second part of the question is asking about the taxonomic rank represented by Y. From the previous part, we know that taxonomic rank is arranged in a hierarchy from the most general to the most specific. We can use the mnemonic device King Phillip came over for good soup to remember the order of ranks in the taxonomic hierarchy. The kingdom is the most general group and it is at the top of the flowchart above. Since the most specific group is the species, we can deduce that it would be at the bottom of the flowchart, where Y is.

Therefore, we can conclude that the taxonomic rank represented by Y is species.

A species is commonly defined as a group of closely related organisms that can reproduce and bear fertile offspring. This means that their offspring can have offspring. Domestic dogs and wild wolves are able to reproduce and their offspring are fertile, so we know that dogs and wolves are actually members of the same species. On the other hand, a horse and a donkey can reproduce and their offspring is called a mule; however, mules cannot produce offspring of their own because they are infertile, as shown in Figure 3. So, we know that horses and donkeys are not members of the same species.

Definition: Species

Species is a group of organisms with similar characteristics that can breed together to produce fertile offspring.

Example 3: Determining the Characteristics of the Offspring of a Lion and a Tiger

A female tiger and a male lion are mated in a zoo. Which of the following is correct about the cub they produce?

1. The cub will be able to reproduce successfully with both lions and tigers.
2. The cub will be classified in a different genus than both of its parents.
3. The cub is likely to be sterile and unable to have offspring of its own.
4. The cub will not survive longer than 4–5 years.

Answer

A species is the most specific of the common taxonomic ranks. The members of a species are all closely genetically related. Another feature of a species is that the organisms within a species can mate and produce fertile offspring. A lion and a tiger are not the same species even though they belong to the same genus. The species a lion belongs to is called Panthera leo and the species a tiger belongs to is called Panthera tigris. If a lion mates with a lion, they will produce more lions of the same species. The same is true if a tiger mates with a tiger. Tigers and lions are closely related enough that they can mate and produce an offspring, sometimes called a liger. However, since lions and tigers are different species, their offspring will be infertile (unable to reproduce).

So, the correct answer is that the cub will be sterile and unable to have offspring of its own.

Many related species that share similar characteristics can be grouped into a genus. Horses and donkeys from our previous example are two different species within the genus Equus. Several related genera with traits in common can be grouped into a family. Following this pattern, several related families are an order, several related orders are a class, several related classes are phylum, and similar phyla are what constitute a kingdom.

Definition: Genus

A genus is a taxonomic rank that includes several related species.

Definition: Phylum

A phylum is a taxonomic rank that includes several related classes.

Definition: Kingdom

Kingdom is one of the most general taxonomic ranks. A kingdom includes many different species that are divided into phyla, classes, orders, families, and genera.

Table 1 illustrates a few important points about the taxonomic hierarchy:

• Moving from left to right, the taxonomic ranks become more specific. As the taxonomic ranks become more specific, they also become smaller, involving fewer and fewer different species. We can see that, of the 8 species listed, 7 belong to the same kingdom, while only 3 belong to the same family.
• More closely related species have more taxonomic ranks in common. For example, humans are more closely related to chimpanzees, with which we share 5 taxa, than we are to goldfish, with which we only share 2.
• The scientific name of a species in binomial nomenclature consists of both the genus name and the species designation. Sometimes the genus name is shortened to just an initial, so bonobos could be written as P. paniscus and chimpanzees as P. troglodytes. Binomial names are usually printed in italics.

Example 4: Determining the Genus and the Species of a Robin

An incomplete taxonomic hierarchy of a robin (Erithacus rubecula) is provided in the diagram.

1. What should replace Y?
2. What should replace Z?

Answer

Part 1

The question provides us with the scientific name of this robin species, Erithacus rubecula. The scientific name of an organism is determined using a system called binomial nomenclature. According to binomial nomenclature, the scientific name of a species is made up of the genus name first, then the species name second. This tells us that the genus this species of robin belongs to is Erithacus. Since in the diagram, Y is the genus name, we can conclude that Erithacus should replace Y.

Part 2

In the previous part, we were able to determine the name of the genus of the robin from its binomial name. Let’s use the binomial or scientific name of the robin to determine the name of the species of the robin. Remember that scientific names are binomial names where the first word corresponds to the genus and the second corresponds to the species. Looking at the second word of the robin’s scientific name, we will find that the robin’s species is called rubecula. Therefore, the answer will be that Z would be replaced by rubecula.

The taxonomic hierarchy helps us to effectively share information about different species and their relationships. We can use it to classify organisms that we are already familiar with, those that have recently been identified, and even organisms that are extinct.

Key Points

• A species is a group of similar organisms that are able to breed to produce fertile offspring.
• The levels in the taxonomic hierarchy, from most general to most specific, are kingdom, phylum, class, order, family, genus, and species.
• The binomial naming system gives each organism a scientific name consisting of its genus and species names.