Taxonomic Hierarchy

Life is complex. In biology, scientists try to organize all of the living things on Earth through taxonomy. Taxonomy is a method of arranging these living things (taxis: arrangement, -nomia: method) and we have Linnaeus to thank for it.

Taxonomy is based on a hierarchy of classification; the lower you go in the hierarchy, the more closely related the living things are. These groups, from largest to smallest are Domain, Kingdom, Phylum, Class, Order, Family, Genus, Species. When I was in high school, I learned a mnemonic to remember the order of taxonomic classification: Did King Phillip Come Over For Good Spaghetti?

To make classification simpler, let’s choose two species and follow them through the hierarchy. Let’s compare the domesticated cat (Felis catus) and humans (Homo sapiens).

The largest layer of the classification hierarchy is Domain. There are 3 domains: Archaea, Bacteria, and Eukarya. Cats and humans are in Eukarya, so we’ll follow that path.

The next layer is Kingdom. There are 6 kingdoms: Animalia, Plantae, Fungi, Protista, Archaea, and Bacteria. Cats and humans are in Animalia, so we’ll follow that path.

Classification hierarchy (via Wikimedia Commons)

Next is Phylum. The kingdom Animalia contains about 35 phyla. Cats and humans are in Chordata.

Next we have Class. The lower we go in the hierarchy the more and more choices we have. Cats and humans are in the class Mammalia.

Next is Order. Up until this point, humans and cats have fallen under the same classification. Cats are in the order Carnivora, while humans are in the order Primates. In plants, the suffix -ales denotes an order.

Next we have Family. Now the groups are getting much smaller. Cats are in the family Felidae, while humans are in the family Hominidae. The suffix -dae is used to denote a family within animals, while in plants -aceae serves the same function.

Genus is a commonly used level of classification. Cats are in the genus Felis, while humans are in the genus Homo.

Species is the most specific (pun intended) level of classification. When writing a species name, both the genus and species names are required. Cats are Felis catus and humans are Homo sapiens.

These aren’t the only levels of classification. Between levels there are superorders and suborders and tribes and subspecies and other small levels of subdivision. For example, humans are part of the tribe Homini. The use of these subdivisions is an exception, not a rule. Many species makes it all the way through the taxonomic hierarchy without using any subdivisions.

The most important thing to remember when it comes to taxonomy is that this system was entirely made up by humans to explain the world around us. We’re not perfect and sometimes things don’t fit neatly into these little boxes and the entire tree of life has to be redrawn. Regardless, taxonomy is a useful way for scientists to categorize and understand the world.

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3 thoughts on “Taxonomic Hierarchy

  1. When I was in high school, we came up with a mnemonic for taxonomy I can’t say in polite company. Yours seems to work well, too!

  2. Pingback: What Makes a Species? | Scientia and Veritas

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