King Philip came over for good spaghetti. I've heard the story many times since I was in high school, though the object of his majesty’s desire is not always the same. Most versions claim that he was looking for some sort of food:good spaghetti, garden salads, Genoa salami. Other more salacious tales allege that his royal highness came over for good sex. It turns out that this is a very simplified version of a much more complex tale.
I’m speaking, of course, of the mnemonic device taught to school kids all over the world (the English speaking parts, at least) to help them remember the levels of taxonomic classification: kingdom, phylum, class, order, family, genus, and species. These seven taxonomic levels have formed the foundation of the field of taxonomy since they were introduced by Carl Linnaeus in the 18th century. Taxonomy, by the way, is “the academic discipline of defining groups of biological organisms on the basis of shared characteristics and giving names to those groups.”
Before I go any further, I’m going to let you all in on a little secret. I hate taxonomy. It’s not that I think it’s useless, far from it. Grouping organisms into a hierarchy of increasingly focused taxa has been incredibly useful, not only for identification, but also for evaluating evolutionary relationships. I hate it because it is so inescapably useful and so fundamentally flawed. In this post I’ll look at the concept of the seven levels and discuss the benefits as well as the flaws.
Why are there seven levels?
I don’t think anyone can say for sure how Linnaeus decided on seven levels, but that’s what he used and that’s what stuck. The most inclusive groups were the kingdoms (which at the time were animal, vegetable, and mineral). Each kingdom contained several phyla; each phylum had numerous classes and so on. Allow me to give you an example using an animal we are all familiar with: the common Homo sapiens.The figure to the right shows the complete Linnaean taxonomy for H. sapiens.
On the surface, this is a great system. It places H. sapiens in clear groupings, so that we can see how we fit together with the rest of the living world. We are animals because we are multicellular, motile (capable of moving ourselves), and heterotrophic (we need to eat other organisms to get energy and nutrition). We're chordates because of our dorsal nerve chord, gills and tails (although the latter two are only present during embryonic development). Using this system, we can tell how closely related we are to a blue whale (a mammal but not a primate), a blue mussel (an animal but not a chordate), or a bluebell (not even an animal).
That sounds pretty good. So what's the problem?
The problem is that there are more than seven ways to classify an organism... a lot more. Animals, plants, fungi, and protists all have cellular organelles with membranes. That puts these four kingdoms separate from the two kingdoms of bacteria, so we’ve had to add the classification of domain above the kingdom. Now, let’s think about the chordates; some have bone surrounding their dorsal nerve cords, while others do not. That gives us the subphylum of vertebrata and two invertebrate subphyla. What about the animals that have bilateral symmetry? Chordates with four limbs? Mammals that develop with a placenta? Primates that don’t have tails? Animals that develop an anus before a mouth?
Just a quick skimming through Wikipedia (the most scientific of sources) reveals that H. sapiens belongs to at least 34 different taxa. Some have new titles like cohorts and tribes, while others are shoehorned in with clunky titles like superphylum, subspecies, infraorder, parvorder, and microphylum. Others don’t even have titles and are listed as “unranked”. Why do most biology textbooks still teach that there are only eight (most at least acknowledge the domain) levels of classification?
Now for another problem: Not only does the Linnaean system have too few levels, it also has too many. Gingko biloba is in a division (that’s the botanical equivalent of a phylum) all by itself. So there’s a class, order, family, and a genus that only contain this one species. The existence of the higher groups is rather easy to explain here. There are a number of relatives of G. biloba that are well known from the fossil record, but are now extinct. Allow me to give you a more extreme example: Limnognathia maerski. This tiny animal is the only member of phylum Micrognathozoa known to science (Pechenik 2005). When it was decided to classify this organism in a brand new phylum, it also had to get a class, order, family, and genus that don’t really serve a purpose (meaning they don't group it with other species possessing similar traits). Obviously L. maerski has evolutionary ancestors, but we will likely never know about them. Do we really need to have all these monotypic taxa?
Surely there must be a better way.
The fact that Linnaean taxonomy has endured for over two centuries means that it predates the theory of evolution (arguably the most important part of modern biology). Although the members of many taxa have evolved from common ancestors, the groups don’t necessarily reflect evolutionary relationships. Modern taxonomists now use a process called cladistics that creates taxa where all members share one or more traits with a common ancestor. Such groups are called clades and include all of the descendants of the original organism. The key difference that cladistics brings to the field is that a clade is an entire branch of the evolutionary tree; a Linnaean taxa is simply a group of organisms that share similar traits. Let’s look at a specific example. While all of the animals that we know as reptiles (snakes, lizards, turtles, dinosaurs, etc.) evolved from a common ancestor, the Linnaean class Reptilia doesn’t include two major groups of reptilian descendants: mammals and birds. A cladist would group reptiles, birds, and mammals together as amniotes (animals that produce a specialized pouch for a developing embryo).
Although cladistics is based on evolution, it is not without criticisms. The first is a matter of utility. When I mentioned reptiles, you probably immediately thought of animals that live on land, have scales, and are cold-blooded. I could talk about amniotes that don’t have either feathers or fur, but why not make a name for such a distinct group? The second (and probably more pertinent) criticism is about how the clades are formed. In order to determine if similar traits between organisms are actually inherited from a common ancestor, one has to know whether or not they actually are descended from the same species. But the only way that cladistics has to determine if they are descended from the same species is to group the organisms by their shared traits (Adrain et al. 2001). Sometimes scientists are able to agree that traits are homologous (such as the forelimb bones of mammals), but other times there is a lot of debate (like in the evolution of eyes).
I won't claim to know which is the best system. What I've presented here is only my opinion. There are no doubt people who will disagree completely with everything I've written here, and hopefully a few who will agree with me. The debate will probably never end, for if there’s one thing that taxonomists can agree on, it’s that the other guys are most certainly wrong.
Adrain, J. M., G. D. Edgecombe, and B. S. Lieberman. 2001. Fossils, phylogeny, and form: an analytical approach. Springer. pp 56-57
Pechenik, J. A. 2005. Biology of the Invertebrates. McGraw-Hill, Higher Education. p 199