So, I’m a taxonomist. Or I call myself a taxonomist. Or I’m obsessed with taxonomy. Anyway, the point is that it seems that most people are not. So here is a brief primer on the fundamentals of taxonomy if my blog occasionally confuses you. If you just want to read about the humour of taxonomy, you can laugh at taxonomists here and here.
Taxonomy is a hierarchical system of classification that allows us (scientists, naturalists, regular people even) to categorize, sort, and organize our species of interest. It is more than that, though, taxonomic hierarchies are evocative of evolutionary relatedness, trends and changes in speciation and diversification.
In the taxonomic classification system put into place by Linnaeus (which is largely unchanged today), more closely related species are put into groups. As you go higher in the hierarchy, the species groups become larger and more encompassing. Fundamentally, the hierarchy of species was based on morphological characters that allow us to separate different species. In other words, the physical characteristics of species relate to the evolutionary relatedness.
In a broad sense, most of the classifications based on morphology have held up against the test of time, but recent molecular methods have resulted in many revelations about how species are related and should be categorized. By using DNA to create molecular phylogenies, modern biologists frequently revise species accounts based on morphology.
So here is how the hierarchy works. The largest groups are the Domains.
There are three Domains: Eukarya (including everything from us to yeast), Bacteria, and Archaea. Taxonomy has already revealed something fascinating…the diversity of the vast majority of organisms that people think about on a daily basis (animals, plants, fungi) are dwarfed by the massive diversity in the prokaryotes. In fact, an entire domain of life is dominated by organisms that we know extremely little about: the Archaea.
The next level below domain is Kingdom.
(To explain, I’ll use the familiar Eukarya domain.) Kingdoms encompass broad categories like animals, plants, and fungi. But even within Eukarya, there are far more kingdoms of single celled organisms than familiar ones. Eukarya also includes slime molds, ciliates, flagellates, microsporidia, etc.
Within the kindgoms, there are Phyla.
(To explain, I’ll use the familiar Animal kingdom.) The phyla within animals include broad categories such as Arthropoda (all insects, spiders, scorpions, crustaceans, etc.), Annelida (earth worms), Cnidaria (corals, for example), and yes even our own familiar Chordata (things that have a spinal cord). So now you know that you are more closely related to things with spinal cords than things without spinal cords!
Within the phyla, there are Classes.
I think I can divide my attention here between Arthropoda and Chordata. Within the Arthropoda, classes include Insecta (yay bees, eventually!), Arachnida (spiders, mites, scorpions, etc), and Diplopoda for example (millipedes). This shows you that millipedes are about as closely related to insects as to spiders, from a taxonomic perspective. Within Chordata, classes include Mammalia (woo!), Aves, Amphibia, Reptilia, and as an example, things like Ascidiacea (sea squirts!).
The classes include Orders.
Within Insecta, orders include beetles, butterflies and moths, bees wasps and ants, and true bugs. Within Mammalia, orders include things like Monotremes (platypus!), and here’s where things get a little hairy*…placental mammals are not an order of their own, but an “infraclass”, which basically just means less than a class but more than an order. This kind of weird fuzziness is more common than you’d think in taxonomy. However, Primates represent and order all their own, because we’re all biased toward humans.
Within Orders, there are Families.
As we get down deeper into these levels of hierarchy, it becomes easier and easier to see the relatedness between species. For example, for plant identification, it is generally pretty easy to get to the family level of identification, just by looking at the flowers. Within the order of ants, bees, and wasps, families include things like mining bees, mason bees, and sweat bees. Within primates, the families include Tarsiidae (tarsiers!), Cebidae (Capuchins and squirrel monkeys), and yes, even Hominidae (that’s us!). Our family includes six other species…but we’ll get there.
Within Families, there are Genera (singular: Genus).
Species within genera are very closely related, and it is pretty evident by their morphology. These sister species are sometimes able to interbreed. For example, two species within a genus of thistles might be able to hybridize. Within Hominidae, there are four genera: chimpanzees, gorillas, humans, and orangutans. We are the only extant (still alive) member of our genus.
Finally! Genera includes Species.
Species are the individual units that most macrobiologists study. However, the lines between species can be very fuzzy and in fact there is a constant dialogue about what actually constitutes a species. I call this the taxonomist’s fallacy (and you can read further about my thoughts on it here). Our species is fondly known as Homo sapiens. (The proper way to write it is to capitalize the genus, undercase** the species, and italicize the whole thing.)
Unfortunately, there are complications to the basic hierarchy presented here. There are subphyla and superorders and subgenera and infraclasses. Mostly, you can intuit where those groups fall (between two others usually).
There are many mnemonics to remember this basic hierarchy. The letters are D K P C O F G S…my high school biology teacher used “King Philip Came Over For Good Soup”, but if you can come up with a better one, let me know. Wikipedia has this weird graphic:
Which I suppose is supposed to show how each group is nested in the one above, but I think I like this one better because it shows how animals are in the same broad categories but drop out as you get to the lower categories.
**Is that a word?