Taxonomy of the Animal Kingdom: The Principle Taxonomic Ranks (Part I)

Updated: Sep 3, 2020

The Kingdom Animalia, or better known as the Animal Kingdom, consists of all known animals in our world. With there being simply *too* many types of animals out there in this vast world of ours, how do we differentiate and classify them?

(This article was written in collaboration with Rick Tan for Wix Reads)

Taxonomy: The Practice of Classification

In this gigantic, far-reaching world of ours, we have evolved to the point where everyday, progress is being made. Inventions are made, concepts are taught and created, new technologies are emerging, cures and medicines are tested and trialed in laboratories, new things are discovered and found, and so much more. With so much at hand, and a seemingly ample one at that, how do we categorize and classify it all into neat little pieces for our own convenient reference? This is where taxonomy comes in.

Taxonomy is the practice of the classification of things or concepts, as well as the principles itself that fall under the category of such classification. It basically takes up a compressed, jam-packed load of information and divides it nicely into various classifications, each having their own 'criteria' and regulations to correctly classify new things/concepts found in the future.

Taxonomy has its applications in a multitude of major fields, such as in the natural sciences, safety and health care, business and economics, education and computing. Where it concerns us today however, is in the natural sciences. To be just a little more specific, it is in the field of biology.

Taxonomy in Biology: Biological Classification

*Don't let all these 'taxonomic' terms confuse you. The word 'taxonomic' or anything of the form is only added because this is the field guessed it, taxonomy!

Taxonomy in the field of biology is defined as a branch of science that encompasses the description, identification, nomenclature and classification of biological organisms, on the basis of shared characteristics under a set 'criteria'.

So, how does it work? There are simply so many different species out there, it almost seems impossible to break them down. Let's start from the basics.

Firstly, of course, you have to group them into groups of a population to form an easy-to-refer unit. This is called a taxon, taxa for plural, if you would.

A group of one or more populations of an organism is first identified as a unit, known as a taxon.

Once you have your group/taxon, they are by then usually already assigned a specific name, unique only to that taxon. This particular taxon is then assigned a given rank/taxonomic rank based on a taxonomic hierarchy.

The taxon is given a unique name and assigned a taxonomic rank based on a taxonomic hierarchy.

A taxonomic rank is the relative level of a taxon/group in a taxonomic hierarchy, and is assigned that particular rank based on its shared characteristics. Typically, there are 8 main taxonomic ranks that are in use today: Domain, Kingdom, Phylum, Class, Order, Family, Genus and Species.

Within a taxonomic hierarchy, 8 principle ranks are typically used. A taxon will be assigned a taxonomic rank based on its shared characteristics.

The Principle Taxonomic Ranks

As mentioned above, there are 8 principle taxonomic ranks in use today. Think of it as a flipped pyramid with 8 sections, with each one cut out horizontally. You could say that the ranks go in a descending order; with each rank being more specific as you go down the pyramid. The first rank being the most general and largest category of specification, and the last rank being the most specific description of a given organism.

Still with me? Let's dive into the principle ranks, one at a time!

1. Domain

A domain is considered as the highest taxonomic rank in the taxonomic hierarchy. This is pretty much the most general description of a given (Very, very, very large) group of organisms. According to the three-domain system of taxonomy used today, there are 3 categories (As you can tell) under this rank: Archaea, Bacteria and Eukarya. Of importance to us today is the domain Eukarya, as it concerns the animal kingdom.


They are a domain of single-celled organisms that lack cell nuclei and are therefore considered as prokaryotic microorganisms.


They are also a domain of single-celled, prokaryotic microorganisms. While they share similar characteristics in certain aspects with the domain Archaea, they still differ in many ways, such as the composition of their cell membrane layers.


This domain comprises of all life that has a cell nucleus enclosed within a nuclear envelope/membrane, together with eukaryotic membrane-bound organelles. You should be familiar with this term, as it encompasses most of the life that you can see with your own two eyes: Animals, plants, fungi and us humans!

2. Kingdom

A kingdom is the second-highest taxonomic rank in the taxonomic hierarchy. The purpose of the various kingdoms is to further categorize and accommodate the growing number of recognized species in our world. They managed to do this due to the clear differences between organisms that are included within a domain.

The next two paragraphs here are just for your own knowledge, it really isn't that important!

A concept that has developed throughout the years are the use of the traditional kingdom-style groups. As of now, the widely-accepted concept regarding the kingdom-level classification is still out in the open. Throughout the years, many scientists and taxonomists have come up with different concepts, some with 4, 5, 6, 7 and even 8 kingdoms!

There has also been a new concept drawn up by two taxonomists, Simpson and Roger, that stated that the domain Eukarya could be broken down into eukaryotic supergroups instead of the traditional kingdom concepts.

That's a bit too complicated to explain however, and would take ages to explain it all! If you would want to check it out, do refer to the link here!

(Simpson AG, Roger AJ. The real 'kingdoms' of eukaryotes.)

3. Phylum

A phylum is the third-highest taxonomic rank in the taxonomic hierarchy. The idea of a phylum-rank group came up when a zoologist, Ernst Haeckel discovered the fact that species were constantly evolving into new species, but they still retained a few features that were consistent among themselves, even through evolution. This allowed him to trace these consistent features and identify them as a group, thus the term phylum!

A phylum gets a little more specific as it goes from a kingdom, more than when a domain goes to a kingdom. There are two main distinctions that should be made between each group in a phylum: Genetic relations (How related organisms are to each other within a phylum) and similarities in an organism's body plan.

4. Class

A class is the fourth rank in the taxonomic hierarchy. There are many ways a taxonomist can judge how a taxon fits into a class, but a generally accepted one is that a class is meant to classify organisms of a phylum into various categories based on their grade of organization and their type of construction.

A grade of organization can be said as a measure of the level of complexity of how differentiated an organism's organ systems are. A type of construction is how an organism's layout of its organ systems are like. Generally, taxonomists will use these two defining measures to classify taxa into different classes.

5. Order

An order is the fifth rank in the taxonomic hierarchy. It is purely used as a means to further classify/distinguish organisms from the rank above. There is not much in terms of criteria/measures that are taken by a taxonomist to decide what can and cannot fall under an 'order'. A taxonomist can choose the parameters to fall under a certain 'order', and can even completely disregard the 'order-rank' as a whole.

At the end of the day, it really depends on the taxonomist. *shrugs* It really is all up to them to decide :P

Wait..... what? That's it? Weren't there 8 of them? What happened to the rest?Hello? Anyone?

We'll Be Back.......

Welp, so much for that. Honestly though, this isn't over yet, we're not even halfway through! After conducting some extra research, I've decided to extend this topic beyond a single article, as it simply takes too much time to explain it all in one article!

As a conclusion...

So far, we've learned taxonomy as a whole and how it applies in the field of biology. We then moved on to how it could be used and applied in classifying biological organisms.

Within the taxonomy of biology, we have learned 3 important points that introduced us into how the system works:

A group of one or more populations of an organism is first identified as a unit, known as a taxon.

The taxon is given a unique name and assigned a taxonomic rank based on a taxonomic hierarchy.

Within a taxonomic hierarchy, 8 principle ranks are typically used. A taxon will be assigned a taxonomic rank based on its shared characteristics.

These three points have set us up nicely to go into the eight principle taxonomic ranks in use today. So far, we have only covered five of them: Domain, Kingdom, Phylum, Class and Order.

What's Next?

The remaining three (which are covered in the next article): Family, Genus and Species, are far more important and meaningful in the nomenclature and taxonomy of the animal kingdom as a whole. This is because from the 'Family' rank onward, the organisms get more specific, and this is where the nomenclature comes in to sort them all out. Doesn't sound too complicated? Well, there are a whole lot of standards and guidelines when it comes to naming biological organisms, and that's when it starts to get real messy.

P.S: Don't be intimidated by the complexity of it all however, as we'll go through this together! The next article will cover the rest of the principle taxonomic ranks and give a small insight into the code that regulates it all.