Organismal classification - evolutionary relationships and ranks

The diversity of living organisms on earth is truly astounding, almost overwhelming. Humans have come up with ways of organizing, or classifying, biological diversity throughout human history. Organisms can be classified according to any number of criteria, including overall similarities, colors, ecological functions, etc. However, it is generally agreed that the most useful way for scientists to organize biological diversity is to group organisms according to shared evolutionary history. This way the grouping not only results in an organized classification, it also contains and conveys information about our understanding of the evolutionary history of these groups.

Although our understanding of evolutionary relationships among organisms has greatly improved in the last century, it is by no means complete. Relationships among organisms, and groups of organisms, continues to be revised as new data becomes available. The rate of such revisions has increased in recent years primarily as a result of the huge amount of new molecular data (such as DNA sequences) that has been brought to bear on tests of evolutionary relationships. This means that nearly all taxonomies (systems of nomenclature) based on evolutionary relationships among organisms are being revised, sometimes radically so. Traditional ideas about how organisms are related, and in which groups they belong, often prove inaccurate.

Traditional, biological classification schemes included the idea of “ranks,” such as species, genus, family, order, class, etc. In this system (the Linnean system), for example, there is a Class Reptilia and a Class Aves. However, the bulk of evidence supports, and the majority of scientists now agree, that the group Aves belongs within the larger group Reptilia (birds share a most recent common ancestor with crocodiles, which are generally included in the Class Reptilia). Within a traditional, Linnean system of classification this means that either the Class Aves is demoted to something below a class, or that a class (Aves) exists within another class (Reptilia). Problems such as this have prompted many scientists to propose that a system of naming and classification of biological diversity be rank-free. Classification systems then only indicate the hierarchical structure of groups according to the current understanding of their evolutionary history, leaving out rank labels.

The Animal Diversity Web prefers a rank-free classification, and uses such a format on our classification pages. However, because rank labels are still used extensively in biological education, we retain rank labels at certain levels, including class, order, and family. In doing so we hope to provide an opportunity for educators to discuss the issues of ranks, classification systems, and our understanding of the evolutionary history of organisms in their classrooms. Please consider the rank labels retained on the ADW as convenient place-markers to indicate the correspondence of current, phylogenetic classification schemes with traditional classification systems.

More on Animal Diversity Web taxonomic information

Glossary of terms related to classification and naming of organisms:

Classification – a system of naming objects or entities by common characteristics. In a biological sense, classification is the systematic grouping of organisms based on structural or functional similarities or evolutionary history. A process of establishing, defining, and ranking taxa within hierarchical series of groups.

Taxonomy – the classification of organisms into a system that indicates natural relationships (evolutionary relationships); the theory and practice of describing, naming, and classifying organisms.

Systematics – the systematic classification of organisms and the evolutionary relationships among them; taxonomy.

Phylogeny – the evolutionary history of a group or lineage.

Nomenclature – the system of scientific names applied to taxa (groups of organisms).

Definitions taken from and from Lincoln, R., G. Boxshall, and P. Clark. 2001. A dictionary of ecology, evolution, and systematics. Cambridge University Press, Cambridge, 360 pp.


Tanya Dewey (author).