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The Taxonomic Level Genus: Its definition and delineation. -   Scott Thomson 

Copyright © 2003 World Chelonian Trust. All rights reserved


Related articles:

Taxonomic Confusion - Why are Some Genera and Species on the WCT Web Site Listed Under More Than One Name? - Darrell Senneke 

Introduction :

This is a short paper that I wrote a number of years ago and I am putting it here because I thought some people might find it interesting. I have added a few bits to it to give it a more general appeal. Basically it is meant to demonstrate what a genus is but it’s a little more than that in that it shows where the genus came from and hopefully will help people understand that taxonomy is an evolving science, like any other, hence names change.

The naming and classification of species has been developing for over 2000 years and the true father of biological classification is considered to be Aristotle. His important advancement was not an actual naming system, as his was inconsistent, but his development of logic and metaphysics. In addition was his recommendation that all animals be arranged into a single graded scala naturae utilising their level of perfection as the yardstick. The binomial or Linnean system of nomenclature was developed to standardise the methods of naming species and was based entirely on the Aristotlian logic. Considering the period Linnaeus assumed that he was simply revealing his "Lord's" master plan and hence subsequent taxonomists have basically dumped the entire theory behind the system whilst retaining the categories.

The binomial system can be divided into two major categories, the species and the higher orders, the later is divided into Kingdom, Phylum, Class, Order, Family and Genus. All of the categories can be subdivided by the addition of prefixes and suffixes but other levels such as division, tribe etc. are often utilised and the complete name of a species can be extremely long, although only the genus and species names are used. The naming of species using this system is subject to the International Code of Zoological Nomenclature.

The genus is the lowest of the higher orders of taxonomic classification and as the lowest comparatively tested category in a hierarchical system it is an important category for the grouping of species. As far as Linnaeus was concerned it was the most important as originally all animals were known by their genus and the following differentia specifica. The later was a specific word or words that identified the taxon, soon after this was shortened to species and made uninominal and hence we have the binomial name of genus and species together.

An interesting note is the historical connection of using Greek and Latin as the language of taxonomy. Many of the names used by Linnaeus were originally used by Aristotle. With turtles the best example is the Green Sea Turtle, Chelonia mydas. This species scientific name literally means “Turtle from the Sea” and is separated from Testudo graeca or “Tortoise of Greece”. Please note these are ancient names not necessarily completely correct. The point is Linnaeus used many of Aristotles names for the most commonly known species. Personally I think it is incredible that the Green Sea Turtle has been called Chelonia mydas for 2300 odd years.


Aristotle 384-322 BC was from a Greek colony and seaport on the coast of Thrace. In later life he became the tutor of King Phillip's 13 year old son Alexander (later world conqueror). Upon the death of Philip, Alexander succeeded to the kingship and prepared for his subsequent conquests. Aristotle then returned to Athens and set up his own school at a place called the Lyceum. For the next thirteen years he devoted his energies to his teaching and composing his philosophical treatises.


Carl Linnaeus 1707-1778, also known as Carl von Linné or Carolus Linnaeus, is often called the Father of Taxonomy. His system for naming, ranking, and classifying organisms is still in wide use today (with many changes). His ideas on classification have influenced generations of biologists during and after his own lifetime, even those opposed to the philosophical and theological roots of his work.


The species level is defined by the definitive, distinctive nature of each taxa and by the presence of isolating mechanisms, all species are equal, although the gap between them may not be, by this very nature. The genus, however, is completely arbitrary and the distinctiveness of one genus does not necessarily reflect the distinctiveness of any other. Furthermore, there is no test of the genus, by definition, and members of a genus are included by looking for similarities between species and looking for a "decided gap" between genera. The generic groupings are nothing more than a means of cataloguing species by overall similarity and pre-evolutionary users of the binomial system did this with no scientific explanation until mid last century. Hence, usage of the categories attempts to place species into monophyletic groupings.

Monophyly is one of the most important facets of the hierarchical system of nomenclature. This means that the members of the group are more closely related to each other than they are to anything else. This is what causes many changes in taxonomy because a researcher applies a new technique to a group and comes up with a new arrangement. If it this research has produced a paraphyly then taxonomic change must occur. Hence we get new genera and species all the time as our techniques for extracting phylogeny become more detailed.

With this in mind and the fact that in my research on turtles I had a couple of very distinctive lineages that were monophyletic it seemed necessary and appropriate to have some objective method for the determination of a genus. It was also important that this method be independent of tree building. What I came up with was to use character sweeps to recognise genera rather than individual characters. A character sweep can be seen as an innovation and it’s by the innovations of the lineage that we can define and delineate a genus. By using a set of characters in a way that is not used by a cladistic analysis we remove circularity from the arguments as to whether a group is a genus or not, as membership and the monophyly of the proposed genus has been determined independently.

In summary a Genus must be Monophyletic and is diagnosed by the Innovations of the lineage as determined by the presence of character sweeps.

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