An Introduction to Zoology: Chapter 1

Hierarchical Classification:

Different species can be grouped into more inclusive taxa, based upon characteristics they share as a result of common ancestry. This means that there is a hierarchy of classification, from the least-inclusive taxon (the species) up to the most-inclusive taxon (the domain).

A group of related species makes up a genus. A group of related genera makes up a family. A group of related families makes up an order. A group of related orders makes up a class. A group of related classes makes up a phylum (or division*). A group of related phyla makes up a kingdom. Sometimes it’s useful to include an even higher level of classification, the domain.

There are times when it’s useful to subdivide taxa even further. For example, a taxon that’s more inclusive than the class but less inclusive than the phylum is the “superclass.” A taxon that’s less inclusive than the class but more inclusive than the order is the “infraclass.” And so forth.

Still, for the most part, the traditional seven taxa are sufficient: Kingdom, Phylum (or Division), Class, Order, Family, Genus, and Species. There are all sorts of mnemonics out there that can help you to remember the proper order. One I frequently teach my students is: “King Phillip Chooses Oranges For Green Salads.” The students in one of my classes decided that “King Phillip Came Over For Good Sex” was more easily-remembered, though.

Classification of three representative eukaryotic species.



Rhododendron catawbiense, a member of the Kingdom Plantae.



Homo sapiens, a member of the Kingdom Animalia.



Morchella esculenta, a member of the Kingdom Fungi.

*The terms “Phylum” and “Division” mean exactly the same thing, for practical purposes. The difference is that Botanists (those who study plants) and Mycologists (those who study fungi) have traditionally used the term “Division,” whereas Zoologists have traditionally used the term “Phylum.” Old habits die hard, I suppose, and this is still the way things are done.

Up until fairly recently, it was thought that plants and fungi were closely-related, so the fields of botany and mycology have generally been fairly closely linked. This is one reason why botanists and mycologists tend to use the same terminologies. Recently, molecular analyses have shown quite conclusively that fungi are, in fact, much more closely related to animals than they are to plants. The old habits persist, nonetheless.