23.5 Biological classification – taxonomy

Before you read this, I suggest you read post 16.18.

I have written this post not because I know a lot about the subject but because I find some aspects of it interesting.

“The naming of cats is a difficult matter.” T.S. Elliot (American/British poet, 1888-1965) Old Possum’s Book of Practical Cats.

So let’s see how scientists name the domestic cat, like the one in the picture above. The naming process follows a hierarchical structure for the classification of all living things. I believe there is a difficulty, because I don’t believe that there is a single, simple difference between living and non-living things, for the reasons given in post 16.18. But, if we watch the behaviour of a healthy cat, we can see that it is alive – so we’ll continue to see how it is classified. The hierarchical classification method is called taxonomy.

Modern taxonomy is based on a system introduced by the Swedish biologist Carl Linnaeus (1707-1778). But there is no universal consensus on exactly what the hierarchy should be. I am going to use the hierarchy below.

Living things









Our cat belongs in the domain eukarya (eukaryotes) because its cells have a nucleus. This means that it isn’t a bacterium, or something resembling one!

It belongs in the kingdom animalia (animals) – it isn’t a plant or anything like an amoeba, diatom or slime mould.

It belongs in the phylum chordata (chordates) – any animal that has a backbone, or something resembling a backbone, at some stage of its development.

It belongs in the class mammalia (mammals) – because the female produces milk to feed her young.

It belongs to the order carnivora (carnivores) that eat meat – cats and dogs.

It belongs to the family felidae (cats).

It belongs to the genus felis (small cats – not lions and tigers etc).

And the species is called Felis catus. Note that the name of a species has two parts – the first begins with an upper-case letter (F) and the second with a lower-case letter (c). Names of species are always written in italics. If Felis catus has already been mentioned in a text, it is common to refer to it as F. catus afterwards.

How are living things assigned to a category in the hierarchy? They are placed in a category because they share taxonomic characters. Examples of taxonomic characters are

External and internal structures

Patterns of development into the adult form


Sequences of amino acids in their proteins

Sequences of bases in their DNA

Patterns of behaviour

Where and how they live

How do we define a species? The usual definition (that doesn’t apply to bacteria) is that they can interbreed to produce fertile young. This sounds sensible until you start to think about infertile individuals and their parents. How can you be sure that they belong to the species?

There is another problem – you and I belong to the genus Homo and the species Homo sapiens. All other species in this genus are extinct. But there were others – like Homo neanderthalensis. The two species were similar but, when you look at the picture above, I hope you will agree that there are real differences! For a long time it was believed that H. neanderthalensis was extinct because they were all killed by H. sapiens. More recently, DNA has been extracted from fossilised H. neanderthalensis remains. It now appears that modern H. sapiens can contain some H. neanderthalensis genes. So, it seems that H. neanderthalensis disappeared because they were assimilated into H. sapiens as a result of interbreeding.

So we have two options: (1) H. neanderthalensis and H. sapiens are not separate species or (2) our definition of a species is not reliable. I suspect that the problem lies in trying to make all living things fit into a rigid classification system. As I have written before, the aim of science is to try to understand our observations of the natural world – not to follow a set of rigid rules (see post 16.2).

Related posts

16.18 What is life?


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