Fig, 1 Carolus Linneaus
Born of Necessity
During the Ages of Discovery, Exploration, and Colonialism, plants and animals from all over the world were brought back to Europe. There was an obvious need to develop some kind of order for all of the material that flooded botanical gardens, universities, and museums. Swedish botanist/zoologist Carolus Linnaeus (1707-1778) (Fig. 1) developed a hierarchical classification system in the mid-1700’s (Systema Naturae 1735) that allowed a better understanding of the relationships and connections among all the plants (and animals).
Modern Classification
The Linnaean system is still used today with modifications. However, its classifications are based on physical traits or features (morphology) which may not coincide with the evolutionary (phylogenetic) relationships. Those evolutionary relationships, found using molecular analyses of DNA, are now a goal of systematic research.
Fig. 2 Levels of Classification
What is a classification hierarchy in botany (taxonomy)? It is a system that organizes or ranks plants in descending levels of specificity. In the inverted pyramid with eight levels (taxa) (Fig. 2), the top level is the general level of Domain that encompasses everything below it, as does each subsequent level. The next level is the plant Kingdom comprising Divisions, each Division comprising Classes, each Class comprising Orders, each Order comprising Families, each Family comprising Genera (sing. Genus), and each Genus comprising Species, the most specific level (taxon) in this hierarchy. (The second part of a scientific name – a binomial (two names) – is called the “specific epithet,” which together with the genus forms the Species name.) In addition, most levels have sub-groups such as sub-class, sub-order, sub-species.
As mentioned above, relatedness is mostly based on physical traits, but much reorganization at all taxonomic levels in the last few decades has been based on DNA sequences, therefore on evolutionary relationships. For those of us who studied plants many decades ago, including to what genus and family a plant belonged, it is necessary to relearn the names of taxa. For example, the Figwort family (Scrophulariaceae) was a large family that has been divided up into several families, including the Broomrape family (Orobanchaceae), the Lopseed family (Phrymaceae), and the Plantain family (Plantaginaceae). Species have also been moved to other genera and sometimes to another family. For example, the Point Reyes bird’s-beak, formerly Cordylanthus maritimus ssp. palustris in the Figwort family, is now Chloropyron maritimum ssp. palustre in the Broomrape family, Cordylanthus being a synonym.
How to Remember It All
Remembering the hierarchy of taxonomy can be daunting. Mnemonic devices can be just as hard to remember. I have always wanted to know who the presidents on Mount Rushmore are and will try to remember “We just like Rushmore!” I do find it easy to recall the mnemonic “Kings do chess on fairly good squares.” ( Kingdom, Division, Class, Order, Family, Genus, Species). Although the International Code of Nomenclature also accepts “Phylum” (so kings can “play” chess), “Division” is preferred in botany. (You may have also heard that “Dear King Philip could only find green socks.”) As of the 1990’s, the mnemonic can become a question: “Do kings do chess on fairly good squares?” because Domain was added as the most inclusive level in the taxonomic hierarchy, indicating that the organisms, i.e., plants in our case, are Eukaryotes having cells with a nucleus containing the genetic material, DNA (Fig. 2).
Domain most general Kingdom Phylum Class Order Family Genus Species most specific Each level is called a taxon. You may want to have students draw an upside down triangle and draw 7 horizontal lines across to divide the pyramid up into 8 sections. Then have the students list the 8 taxonomic categories from top to bottom, starting with Domain at the top in the widest section and ending with species at the bottom in the smallest section.
Practical Use – A Specific Example
Fig. 3 Tiburon marposa lily – by Eva Buxton
Starting at the bottom of the inverted pyramid (tip), taxa progress from specific to broader categories. In general, the basic unit is Species – a kind of plant, for example, the Tiburon mariposa lily (Fig. 3). The next level up is Genus, a collection of closely related species such as other mariposa lilies; a Family is a group of related genera such as the Fawn lily, Fritillary, and Clintonia; an Order is a group of closely related families such as the Lily family and False-hellebore family, etc.
Applying the inverted pyramid levels, the Tiburon mariposa lily (Calochortus tiburonensis) would be classified as follows:
Domain – Eukaryota
Kingdom – Plantae
Division – Anthophyta (Angiosperms)
Class – Monocotyledonae (Monocot)
Order – Liliales
Family – Liliaceae
Genus – Calochortus
Species – Calochortus tiburonensis
Take Home
Imagine how about 320,000 known plant species in the world today would be described and information communicated about them, if a way of classifying them, using a universal language – Latin or Latinized forms of names, was not available!
When we go botanizing looking at “flowers” (Division Anthophyta), we usually only are concerned with the last three levels – family, genus, and species. However, we can sometimes differentiate between plant Classes: monocots and eudicots (formerly called dicots). If we are also looking at mosses and ferns, we are dealing with two additional Divisions (Bryophyta and Pteridophyta).
Don’t feel overwhelmed by classification. Remember that you classify every time you go to the grocery store (Order). You go to the meat department (Family) to buy chicken (Genus) and decide on breasts (specific epithet), so you buy chicken breasts (Species), maybe even marinated ones (sub-species)!
Comments: evabuxton@sbcglobal.net