In this entry I make reference to taxa (or taxon in the singular). These terms simply mean a biological grouping. It could be a species, a kingdom or whatever. I won’t attempt to define it here, in fact biologists don’t agree on the exact definition of the term, but just assume the term means a group of living thing. The word Taxonomic means pertaining to taxa.
If the labels attached to these animals don’t make sense to you, have a read of this quick explanation of the Linnaean classification system. It was pioneered by Carl Linneaus, an eighteenth century Swedish Naturalist who pioneered the System. It was system that separated the natural world at seven levels. The idea followed on from Aristotle’s genus and species concept. The genus was supposed to sort things which share a common form, and the species differentiated things within the genus. Linnaeus realised that different genera (the plural of genus) shared characteristics with other which could group them together into what he called families, thus creating a third level. In turn families could be lumped together in higher taxonomic levels called ‘Orders’ and so on. Overall, he recognised seven Taxonomic levels. These seven levels in order from highest to lowest are:
These seven levels are still the backbone of biological classification today. Of course originally Linnaeus erected three kingdoms, Animals, Plants and Minerals, so the system was not just set up to classify biological organisms; however the modern system is only used for biological classification. At each level certain taxa were separated from the rest by certain characteristics, for example mammals have fur and they lactate, as opposed to all other thing with a vertebral column.
So let’s examine an example, and why not use our own species Homo sapiens. Our classification runs like this:
Species: Homo sapiens
When reporting the species, you include the Genus name (Homo) and the specific epithet (sapiens). The genus name is often abbreviated the first letter of the Generic name with a full stop (i.e. H. sapiens). Generic and species names are always italicised, but no other taxon is. The first letter of the generic name or any higher taxon is always capitalised, but the specific epithet is all lower case. There are many more rules with regard to naming and citing species, but they are not the same between Zoology and Botany. If you are very interested in these you can read up on their respective codes online, which have been formalised.
Over time several other levels have been added, some of which are more universally recognised than others. This is mostly for convenience sake; there are families of insects with tens of thousands of species and hundreds of genera. It is impractical to divide these up in such a way that makes it easy to identify all the genera and species based on unique characteristics without erecting several levels in between. The original seven levels are compulsory when classifying any organism, however various levels in between are optional, and in many groups they only designate the original seven. Here is a list of some modern term you might come across. I’m not sure if list is exhaustive, and I certainly won’t use all these levels.
Subspecies (often, but by no means universally considered not to be a valid taxon)
Various unranked taxa also exist, but we will discuss that when I write my explanation of phylogeny.
The aim was probably more than just a convenient way to categorise natural things, Linnaeus probably thought that these categories reflected objective realities, which inexactly corresponded to Aristotelian ideas of forms. A testament to this is the fact that he chose the Animals, Plants and minerals as his original three kingdoms, corresponding to Aristotle’s three types of souls, animal, vegetable and mineral. This idea that the categories reflect objective realities went somewhat out of fashion with most biologists for a long time, but it made a comeback with the modern theory of evolution, particularly after Willi Hennig.
You can see how this system could have had a profound effect on the development of the theory of evolution. It is no coincidence that mammals have hair and lactate, and nothing else has hair (that is biologically the same as mammal hair) or lactates. This is because lactation evolved in a lineage with had hair, or vice versa. There are literally million examples of that sort of thing throughout the tree of life, and biologists quite rightly noticed it. There is not biological reason why you could not have an organism with hair which does not lactate or an organism which lactates but does not have hair, the fact that if an organism has one you will find that it has the other suggests a common origin. Having said that it was a long time between Charles Darwin’s origin of species and when evolutionary theory began to have a substantial effect of biological classification.
Most modern biologists consider both convenience and objective reality when classifying taxa. We try to erect groups which have obvious characters which separate them from their close relatives, but we also try to take evolutionary history into consideration. Most biologists will agree that taxa should be ‘natural’ or monophyletic groups. This means that the group must include ALL the descendants of the most recent common ancestor of any two internal taxa, and nothing else. It can make sense to think of it this way: the descendants of taxon A cannot be in a different taxon of the same level, they are just a type of taxon. To give examples, strictly speaking it is illogical to say that birds evolved from dinosaurs, they are dinosaurs. A bird is more closely related to a Tyrannosaurus rex than either the bird or the T. rex is to a Triceratops. It does not make sense to call both a Triceratops and a Tyrannosaurus rex a dinosaur, while excluding birds from that category. Birds are a type of dinosaurs, and the dinosaurs are not extinct, although most dinosaurs are. This point will be further developed when I write my explanation of Phylogenetics.
My website tags animals of the day on the basis of their classification. They all come with the disclaimer that not all biologists will necessarily agree with the classifications I use, and some of these classifications will be the subject of intense and vicious debate. I will do my best to try to consider what the best classification is, but as there are about 2 million species in the world (that we have described), and I am much more familiar with some groups than others. If you disagree with my classification, please e-mail me with arguments, I will consider them as best I can.