Ralph E. Taggart

Plant Biology

Geological Sciences

Michigan State University

A system of biological classification attempts to arrange the vast array of biodiversity in some sort of meaningful order. Back in 1758, when Linnaeus presented his Systema Naturae, the goal was simply to bring some sort of order out of the chaos of known living forms. Linnaeus' system was an artificial system of classification in that it assigned organisms on the basis of any convenient characteristics. Remember, this was a century before Darwin's Origin of Species and no one was dealing with the problem of classification in evolutionary terms. Today, we want to create phylogenetic systems of classification, where the placement of organisms in different categories reflect their evolutionary relationships. Despite the lack of an evolutionary basis, many of the categories of the Linnaean system are still used and most organisms, are, in general, placed in many of the same categories that seemed appropriate over 250 years ago. This is because most of the characters used by Linnaeus, despite the fact they were chosen for their convenience, are characters that are shared by virtue of evolutionary descent! Shown below are sample classifications for two organisms - maize (corn) and the common house fly:

The scheme above uses only the most common categories, for it is possible to recognize sub-classes, super-families, and a host of other intermediate categories when dealing with specific organisms. It is important to realize that the only "real" category in the scheme of biological classification is the species - individual populations of organisms that are reproductively isolated from other such populations. Two species may be very similar, by virtue of recent evolutionary descent from a common ancestor, causing us to group them into a single genus, but the concept of genus is a human creation. The same can be said of all the other larger categories in the hierarchy of classification.

Historically, comparative morphology has provided the basis for placement in the structure of plant and animal classification. Since plants, for example, are profoundly different, in terms of morphological characters, from animals, the criteria for what constitutes a genus, or any other category, must be different for plants and animals. Criteria that seem to work for insects, for example, may not be applicable to mammals, and so on across the scope of biodiversity.

In recent decades, new approaches to classification have emerged, including cladistics, which represents a numerical approach to assessing relationships, without the assumptions inherent in comparative morphology. Cladistic approaches, using morphological data, biochemical criteria, and even DNA analysis, generally confirm classification decisions based on classical morphology, but often suggest new insights into the timing of evolutionary divergence.

Ralph E. Taggart (taggart@pilot.msu.edu)