Classification: Reconciling Old & New Systems

Traditional classification

In the 18th century Carolus Linnaeus revolutionized the field of natural history by introducing a formalized system of naming organisms, what we call a taxonomic nomenclature. He divided the natural world into 3 kingdoms and used five ranks: class, order, genus, species, and variety. He also introduced the system of binomial nomenclature, in which every species has an internationally recognized two-part name.

Since Linneaus’ time, other ranks have been added to the taxonomic nomenclature system. The major taxonomic ranks are Kingdom, Phylum, Class, Order, Family, Genus, Species. These ranks have been used to describe and understand major animal groups for a long time, and many people are taught about animal natural history through these traditional ranks. We grow up referring birds to “Class Aves,” snakes to “Class Reptilia,” etc.

New understanding of relationships among organisms

Scientific understanding of relationships among organisms has changed dramatically since the time of Linnaeus and classical taxonomy. Scientists now understand that major animal groups are related in ways not anticipated by classical taxonomists. So, for example, we now understand that the bird lineage (Class Aves) shares a more recent ancestor with some modern reptiles (crocodiles) than with others (snakes). Yet both snakes and crocodiles are part of Class Reptilia. Modern taxonomy seeks to represent animal groups in a system that reflects an understanding of their evolutionary relationships – so “Class” Aves comes to sit within “Class” Reptilia.

Reconciling traditional classification and new understanding

The many changes in our understanding of evolutionary relationships among animals have resulted in confusing and conflicting relationships among animal groups defined using ranks. Many scientists agree that ranks are not especially useful concepts in animal taxonomy any longer; instead, they argue for a “rank free” classification system. However, ranks remain commonly used and widely recognized “placeholders” in natural history. Even the field of zoology retains a recognition of these classical ranks in the major fields of specialization: ornithology (birds), herpetology (reptiles and amphibians), etc. For this reason, we continue to display them in our classification, while at the same time acknowledging that their usefulness is limited and that they may disappear in the future.

The Animal Diversity Web strives to represent taxonomies that reflect current understanding of evolutionary relationships among animals (see nested hierarchies in the classification tab). In the interest of usability and ease of navigation for our diverse audience, we also retain certain rank labels. As a result, you may find areas of our animal taxonomy where ranks seem to be inconsistently used or contradictory. Please understand this as the unfortunate result of merging two widely used but incompatible systems of classification.


Tanya Dewey (author).