Animals are lifeforms within the kingdom Animalia. From there, the classification of animals gets more specific, going through various other classes and orders. Let’s take a look at the ways animals are classified.
“When I look into the eyes of an animal I do not see an animal. I see a living being. I see a friend. I see a soul.” — A. D. Williams
All animals are multicellular organisms, are composed of multiple cells. These cells have various forms, shapes, and functions and they combine together to create the animal. Most animals are capable of moving independently, and animals consume other living creatures for food and energy. All animals are part of the kingdom Animalia, which covers many different living creatures, from insects to humans.
(As a quick aside, the other kingdoms of life are: Fungi, Plantae, Protista, Archaebacteria/Archaea, and Bacteria/Eubacteria.)
Vertebrates and Invertebrates
Invertebrates don’t have a backbone, while vertebrate animals do possess a backbone. There are more than 800,000 known species of animals within the Animalia kingdom and most of these animals are invertebrates. These invertebrates mainly belong to the phylum known as Arthropod. A phylum is the next level of classification for the Animalia kingdom.
Photo: Peter Halasz via Wikimedia Commons, Public Domain
Each level of classification gets more specific, running down to the specific species that the animal is. The classification order goes like this: Kingdom, Phylum, Class, Order, Family, Genus, Species. (There actually is a Domain level of classification above kingdom, sometimes referred to as an empire or superkingdom. Not all classification systems use it, but the classification systems that do break down domains into three separate groups: Eukarya, Bacteria, and Archaea.)
In terms of phyla or phylum, there are 33 recognized different phylum in the animal kingdom. Some of the more notable phyla include:
- Echinodermata – example: Starfish
- Porifera – example: Sponges
- Cnidaria – example: Jellyfish
- Arthropoda – example: Insects
- Annelida – example: common worms
Many phyla are various forms of worms. Humans and other mammals belong to the phylum Chordata.
Speaking of mammals, mammals are one of the classes found within the animal kingdom. The kingdom is usually broken down into four different classes. Mammals are found in the class dubbed Mammalia, amphibians in the class amphibia, reptiles found in the class reptilia, and birds found in the class Aves.
In general, classes are somewhat malleable with no exact agreement upon what counts as a class. However, for well-known groups of animals, there is usually a consensus. Classes are rarely used when classifying plants, as most plants are classified with orders and informal clades.
Order, Family, Genus, Species
From here, the classification of animals is broken down into order, family, genus, and species.
Much like classes, the order an organism belongs to is determined by individual taxonomists and not always agreed upon. Certain suffixes are used to note certain orders, for example, the suffix “-formes” is often used to describe fishes and birds, though not invertebrates and mammals. Families are often divided into smaller subfamilies, which make up an intermediate group between family and genus. Much like order and class, there is disagreement over how families are defined.
“Until one has loved an animal, a part of one’s soul remains unawakened.” — Anatole France
A genus comes above the species taxonomy, and like the others, there are no strictly codified standards for classification into a genus. There are general practices used to create a genus, however, including the idea that all those in an ancestral taxon should be grouped together and that that should be distinct with regards to things like their morphology, ecology, and DNA sequences.
The classification of animals becomes quite vast at the order level and becomes even more vast when going into family and genus then finally species. As a result, it is impossible to show every instance and every example of these categories, but what follows are a few of the more common ones.
The orders found within the kingdom Animalia include:
- Chiroptera (bats)
- Proboscidea (elephants)
- Primates (humans and our close relatives), Rodentia (rodents).
Examples of families include Hylobitadae (gibbons) and Hominidae (humans and the other great apes).
Carnivora is another order that is subdivided into classes like Canidae (dogs), Felidae (cats) and Ursidae (bears).
When looking at the levels of the genus, Felis covers domestic cats, while Panthera covers animals like the tiger and lion.
Meanwhile, Hominidae is family found under the primate genus which includes gorillas, Pan (chimpanzees) and Homo (humans).
History of Classification
A phylogenetic tree that shows relationships between individual species. The pink region represents eukaryota while the blue region represents bacteria and the green region represents archea. Photo: Ivica Letunic: Iletunic via Wikimedia Commons, Public Domain
People have been trying to classify animals throughout much of history, going as far back as Aristotle, when he first classified organisms into groups with certain attributes, like having blood or having four legs. During the Enlightenment, more scientists began to take an interest in classifying animals, and taxonomies became more complicated and ambitious. The individual usually created with giving rise to the modern era of taxonomy is Carl Linnaeus, who created standardized naming systems for species.
The classification of animals has undergone many changes since the idea of a class was originally created by Carl Linnaeus. While Linnaeus’ work was groundbreaking and set the stage for the work done by more modern taxonomists, many of Linnaeus’ ideas have been abandoned by modern taxonomists. As an example, Linnaeus was of the opinion that bats were related to birds, while modern taxonomy places them within entirely separate classes.
Furthermore, the paleontological record – the study and classification of organisms based upon fossil remains – has changed significantly since the 1950s. The late 1950s saw the introduction of the idea of “clades” to taxonomy, which tries to group organisms together based upon their most recent shared ancestor. Cladistic classification uses concepts like homologous structures, along with other forms of evidence (like DNA sequences), to pinpoint probable common ancestors between organisms. This is in keeping with the Darwinian theory of common descent.
“Science is the systematic classification of experience.” — George Henry Lewes
Modern biological classification is eclectic in nature, combining methods and observations from fields like biological systematics and phylogenetics.
In the process of creating a taxonomy, modern taxonomists use databases to structure, store, and organize documents relating to their taxonomy. There are various databases, as taxonomists frequently create their own databases and organize them according to their own preferred methods. However, some databases are more comprehensive than others. One of the most comprehensive databases is the Catalogue of Life. The Catalogue of Life attempts to create a database of every known, documented species on Earth. The catalog currently lists around 1.8 million out of the Earth’s 1.9 million known species.