Ramblings on Plant Taxonomy

For a long time, I've had a lot of thoughts about plant taxonomy. It's something I think about on a daily basis, given the amount of time I spend around plants, looking at them and pondering them. Taxonomy is interesting because here, we're trying to draw neat little human boxes around things that are, at the best, on a spectrum. Trying to figure out exactly the best way to do this and the best practices for it are a very interesting topic!

What is Taxonomy?

Taxonomy is the naming and classifying of life. A huge part of it, at least for plants, is the description of species by the act of typification. Basically, taxonomists take a pressed, dried plant in an herbarium and declare it to be the quinnessential example of a speices: any plant that looks more like this type specimen than any other type specimens is that species.

Photo of an herbarium sheet (though not a type specimen)

What's the point in doing this? Well, it gives us a valuable method to communicate to each other about different organisms. Imagine how clunky it would be if I were talking about a population of plants without any concept of what a species is. "They're those trees, with the weeping branches and small, awl-like leaves closely appressed to the twigs, that grow in the Sierra Madre of Mexico as far north as the Chisos Mountains of Big Bend National Park" is a lot clunkier than Juniperus flaccida. Even this binomial name is telling us something about the plant: a) it's a juniper (Juniperus) and b) it's weeping (flaccida). But, if there's not a type specimen, how are we supposed to know exactly which plant we're talking about? For all I know, there's some other juniper in some other part of the world that also has drooping twigs, like J. flaccida does (for instance, J. recurva).

So, we're giving names to different things, now, right? And these names are based upon physical specimens of the organisms they're referring to, right? But what, actually, is a species? I could go and pick types of slightly different forms of a plant that grow in the same range, and give them names. Many plants have an large amount of phenotypic diversity across their range.

(WEB LINK) A page that mentions variability in alligator juniper

So what, exactly, is a species? If I were to go and give a name to every little variation of a plant across its range, it rapidly becomes too much to deal with and no longer serves to be a useful basis for communication. Now, sometimes, that's not a problem. Lots of plants don't pose the problem of variability across their range. Take Salvia roemeriana, cedar sage, as an example. Sure, some are a little bigger, some are a little smaller, some only make cleistogamous flowers in the fall, some make chasmogamous flowers in the fall, but you don't have to be well versed in botany to tell that they're all very similar.

One commonly used definition of a species is that of Ernst Mayr, which is likely what you learned about in school:

A group of actually or potentially interbreeding populations, which are reproductively isolated from other such groups.

This makes a lot of sense. If populations of organisms are interbreeding with one another, how can they maintain differences sufficient for us to tell that they're different? Plants, though, don't like to fit in neat boxes like that. Some of our plant "species" cannot consist of interbreeding populations, considering that there can be diversity in the number of chromosomes they have!

A great example of this is American persimmon, Diospyros virginiana. Some populations, mostly in the Deep South, have four sets of 15 chromosomes, for a total of 60. Outside of the deep south, they have six sets of 15 chromosomes, for a total of 90. Because of the difference in chromosome counts, it is impossible for these groups to produce fertile offspring.

(WEB LINK) A paper covering cytotypes of American Persimmon

There's loads of examples like that all over the plant kingdom. And there's also plenty of examples of two clearly distinct species that are not reproductively isolated from one another. In Central Texas, for instance, Texas red oak (Quercus buckleyi) comes into contact with Shumard oak (Q. shumardii). When you look at these individually, they're quite different: Q. buckleyi is a short-ish, often multi-trunked tree with dark, deeply furrowed bark as it ages, with deeply lobed leaves, that grows on extremely shallow, droughty soils overlying limestone. Q. shumardii is a large, single-trunked tree with modestly lobed leaves and rather smooth, grey bark that grows on deep, sticky clay soils. But, all along the I35 corridor in Texas, the red oaks show characteristics of both Q. buckleyi and Q. shumardii in varying amounts. The two forms to the west and to the east are clearly distinct from each other (and are considered different species), but they do blend together in the middle. Compared to the ranges where they're distinct, the intermediate zone is relatively narrow -- probably under 50 miles wide -- so it's not a huge deal. But it's a great example of how we can't necessarily fit plants into clean little boxes.

Higher Levels

OK, so enough about species. What about higher taxa? We all learned in school that "Kings Play Chess On Funny Green Squares": Kingdom, Phylum, Class, Order, Family, Genus, Species. But what, exactly, are these higher levels representing?

In modern biology, there is a very big focus on assigning names to clades. What is a clade? Simply put, it's a group that consists of every organism descended from the same common ancestor. As an example, you and your siblings form a clade. This clade is everyone descended from your parents. An important thing to note is that clades nest within each other. Going back a generation, you, your siblings, your cousins, your parents, and your aunts/uncles on the same side of the family all form another clade -- everyone descended from one set of your grandparents.

If a name is given to a group of organisms that are all in the same clade, we call it "monophyletic". That's what taxonomists strive for -- every name should represent a real evolutionary group. So Homo sapiens -- humans -- are a clade. The great apes, Hominidae, are a clade that consists of us, chimpanzees (including bonobos), gorillas, and orangutans. It also contains every extinct species descended from the most recent common ancestor of humans, gorillas, orangutans, and chimpanzees. The great apes for a clade with the gibbons. That group forms a clade with the rest of the old world monkeys, which, in turn, forms a clade with the new world monkeys. All these monkeys form a clade with other primates, and so on and so forth. You can draw these clades to be arbitrarily small or arbitrarily large: all life on the planet forms a clade, as does one yeast cell that buds into two daughter cells.

For plants, there's been lots of work to make things like families and orders line up nicely with real clades. This can be controversial -- for instance, within the monocotes, the family Asparagaceae (the asparagus family) has been enlargened to include the agaves and kin, the hyacinths and kin, Dracaenas, Nolina, and more. Some people say that that's throwing away actually useful groups that you can tell apart from one another and putting them into such a morphologically messy family it becomes useless.

(Large PDF) This poster shows how the different families and orders of flowering plants, the angiosperms, relate to one another.

When you include the gymnosperms, the ferns, and all the bryophytes, determining exactly what clades are can be very challenging because these groups of plants are so distantly related to one another. There's still some debate around where the gymnosperms called the Gnetophytes (Welwischia, Ephedra, and Gnetum) fit in the family tree -- do they make a clade with angiosperms? Do they make a clade with the conifers? Do they make a clade with the pine family? Do they make a clade with the cypress family? Is it something else?

Why?

Why am I rambling on and on about this? Well, I think plant taxonomy is absolutely fascinating and there's interesting and valuable things to be drawn from figuring out where all the lines are. I think spending a lot of time figuring out exactly where we want to draw lines gives you a much better appreciation for the immense and beautiful diversity of the plant kingdom. Seeing the blurriness and the messiness of what actually is a species and how challenging it is to define is a wonderful demonstration of how so many things do not and will not ever be easy to fit in a neat little box, however much we want it to. Biology is messy and life exists on a spectrum. Just because we like it to be all orderly, doesn't mean that it is.

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