Taxonomy is one of the more important concepts in biology, and one that is actually not too hard to understand. This article presents some ideas about how it can be taught at a basic level.
Taxonomy is about the groups that different kinds of living organisms fall into. The first problem is finding a name for these groups. The word group is both vague and confusing; it suggests an actual group of individual organisms. One term is systematic unit, which is not even a single word. Taxon is short and sweet. The only problem is that it has a bizarre irregular plural: taxa. (Well, not so irregular, c.f. "phenomena" and "criteria"; but these words are more common.)
Maybe we should regularise the plural to taxons so that the irregular grammar does not distract from the discussion. This would be consistent with a basic rule of grammar which states that the inflected forms of any word that you have never heard of before are all regular. (For example, hardly anyone says "virii" anymore, instead we just say "viruses".) So I will do just that.
Some taxons are so easy that we already know them. I think the best one is the birds. Birds have wings which they use to fly and they have feathers and beaks and two legs. They lay eggs with hard shells. OK, so some of them don't fly, but we'll deal with that issue later. We see a lot of birds without knowing what particular kind of bird they are, and we just think of them as being birds.
Mammals are a bit harder, because "mammal" is not such a common word. Very rarely do we say something like, "Look at that mammal over there!". Our natural level of identification is usually more precise, like "Look at that cat!", or "Look at that kangaroo!", or "Look at that elephant!". If we don't have a clue what something is, it will probably be called an "animal".
So we have to define what a mammal is. Traditionally this is taught with a definition similar to the following:
Mammals are animals that have hair, feed milk to their babies, and are warm-blooded.
The intention of this definition is to name features that are found in all mammals and which clearly distinguish them from those organisms that are not mammals. At this point the students learn useful facts like: even elephants and dolphins have hair, so we can know that they are mammals.
This type of definition is consistent with the idea that taxonomy is about classification – there is a set of membership criteria for each taxon, and if you satisfy them all you belong and if you don't then you don't belong.
But it leaves unanswered the question of why such a set of criteria should be used to define a taxon. What students need to learn is that the taxons themselves are natural phenomena.
There are some simple laws that taxons follow:
The fact that not every member of a taxon has every feature from the taxon's feature list creates an unsettling degree of uncertainty – we have lost the idea of a precise criterion of membership. At the same time it means we can put many more features in the list, and in fact we can go as far as stating that if a taxon does not have a decent list of features that define it then it is not a very good taxon. To drive the point home, we should name as many features as possible for each taxon that we define.
Furthermore, a taxon does not exist as an isolated entity – it has super-taxons that contain it, and sub-taxons contained within it. Questions of membership can often be settled by referring to the super-taxons and sub-taxons.
For example, to describe mammals, we name the super-taxons that contain it, for example the four-legged vertebrates, and the sub-taxons contained within it, e.g. placental mammals, marsupials and monotremes. We name features that are typical of mammals, and note those that derive from the super-taxons, and which ones may not apply to all the sub-taxons.
I'm not an expert in mammal physiology, but here is my attempt at a feature list for mammals, and for those features which are really features of a super-taxon or a sub-taxon, showing that super- or sub-taxon in brackets:
As in the case of toes on each foot, specific number values are often maximum values. There seems to be an evolutionary rule that countable things can disappear but they cannot increase in number.
These range from tbe obvious to the subtle:
The concept of a taxon as being defined by a large features list enables us to justify the concept of a largest taxon that contains all living things. Not just because we want there to be a largest taxon that has everything else in it, but because there is quite a long list of features that are found in all living things, including DNA, RNA, proteins, ribosomes and the universal code that translates from DNA to RNA to protein.
The smallest well-defined taxon is the species, which is defined according by a criterion of inter-breeding: members of a species interbreed with each other, either directly, or indirectly (in the case of species dispersed over a large region), whereas members of different species do not interbreed with each other. Sub-species can often be identified by certain characteristics defined on a regional basis within a species, but whether an individual organism belongs to one sub-species or another is not necessarily as sharply determined as membership of different species or higher level taxons.
We can use the power of computers to play a taxonomy game. The computer does not have to be used to play the game, but it provides an easy way to generate the game data. The game is played as follows:
It may be necessary to adjust the alphabet length and the rate of "evolution" to make sure that players have enough information to successfully reconstruct all or most of the taxonomy, but at the same time not make the answers so obvious that it is too easy to find them.