The Development of Categories of Organisms (Kingdom, Phylum, Class, Order, Family and Genus)

Although the contribution of Species Plantarum by Linnaeus was important to biology, its usefulness would have been limited if there was not a means by which unknown plants could be identified without the aide of a teacher. Thus, in addition to cataloging the species name, Linnaeus divided the genera of plants into 24 "classes", based on the number of floral stamens. Plants that lacked flowers and seeds, such as mosses, ferns and even fungi, were classified in their own class. By placing plants in categories, in this fashion, Linnaeus provided, for the first time, a means of identifying unknown plants.

The two earlier classification schemes that were discussed earlier were based on medicinal usage of plants and general usage of plants, in herbals and herbalists, respectively. In these schemes, the identity of a plant could not be identified without the aide of a teacher. Afterall, how would you determine the usage of a plant if you do not know its identify? Theophrastus' classification of plants into trees, schrubs and herbs with the added use of leaf characteristics was a more sensible means of classifying plants and is more comparable to the features that we use, today, to identify and classify plants.

Although the classification scheme of Linnaeus enabled students of botany to identify plants, it did not classify related plants in the same groups. Ferns, mosses and fungi, for example, were placed in the same class even though they are obviously not related to one another, and cone bearing, e.g. pines and firs, and flowering plants were classified together, as well. Another words, Linnaeus grouped plants according superficial similarities that they may have in common or features that were absent. If we continued to use a system like that of Linnaeus we would be grouping whales and dolphins with fishes and bats with birds because, superficially, they would appear to belong to those groups. However, whales and dolphins are not fishes, and bats are not birds. These animals are all mammals. This type of classification scheme is not desirable and even Linnaeus admitted that this scheme was composed for convenience rather than according to their natural relationships such as in a natural system of classification. Although the need for such a system was known even before Linnaeus, its universal usage in science has only come about recently.

There are many classification schemes, each one can be argued to be more natural, in one respect, than another. It seems that as long as people will continue to make up such systems, there will always be disagreements. One point, which has been agreed upon, is the categories in which plants are classified. Presently, closely related species are arranged into a genus, genera are arranged into families, families into orders, orders into classes, classes into divisions and divisions into kingdoms. Using this classification hierarchy, we can place any fungal species into the different categories or taxa in various classification schemes. For example, the complete classification of Agaricus bisporus, the super market mushroom, in three different systems of classification has been reproduced below:

Classification Categories of Fungi Moore-Landecker (1993) Alexopoulos, Mims & Blackwell (1996) Hawksworth, Sutton & Ainsworth (1983)
Kingdom Fungi Fungi Fungi
Division Basidiomycota Basidiomycota Eumycota
Subdivision* Basidiomycotina
Class Basidiomycetes Hymenomycetes Hymenomycetes
Family Agaricaceae Agaricaceae Agaricaceae
Genus Agaricus Agaricus Agaricus 
Specific Epithet bisporus bisporus bisporus

*Each category in the classification scheme may have a subcategories such as "subdivision" or not, as can be seen in Moore-Landecker's and Alexopoulous, Mims & Blackwell's classifications where it is absent.

Even with the progress that have occurred in molecular biology, there is still disagreement even at the broadest categories of our classification schemes. Intuitively, kingdoms would appear to straightforward, but this is not the case. As more is learned from molecular studies of various organisms, this has only caused more disagreement as to the classification scheme to use at the kingdom level.

The Concepts of Kingdoms

The kingdom is our broadest category and in the earliest classification, only two were recognized, Plants and Animals. Plants were then characterized as organisms that lack motility, and do not consume food while animals have the ability to move and eat their food. Even today, the lay public still categorize life in the two kingdoms, and for most organisms that we are in daily contact with this is a workable system. We know that pines, ferns and mosses are different types of plants and that dogs, birds and fishes are different categories of animals. However, most microscopic organisms do not fit comfortably into either kingdoms. Fungi, for example, were placed into the Plant Kingdom because they lacked motility and their cells were surrounded by a rigid cell wall, but unlike plants they cannot photosynthesize. The genus Euglena (Fig. 3.)is an example that has been classified as both a plant and an animal. It is a unicellular organism found in fresh water and can swim through the water with a "hair-like tail" called a flagellum (plural: flagella). A groove or a gullet is present in the cell that allows it to ingest food. However, Euglena also has chloroplasts and, like plants, can make its own food through photosynthesis. Because Euglena possess both plant and animal characteristics, it was classified as both a plant and an animal.  Despite these problems, this two kingdom system was used until Whitaker (1969) proposed that organisms be classified into five kingdoms: Monera (=Bacteria), Protista (=Mostly Algae and Protozoans), Plantae (=Plants), Mycetae (=Fungi) and Animalia (=Animals). Whitaker's five kingdoms have since been the accepted scheme, in textbooks, for classification of organisms. 

Euglena.GIF (15317 bytes)
Fig. 3: Euglena is neither plant nor animal.

Although Whitaker was credited with this system of classification, most of the additional kingdoms that he recognized did not originate with him. It had long been accepted, in biology, that many organisms do not fit comfortably into a two kingdom system. A third kingdom, Protoctista, was first proposed by the German biologists J. Hogg and Ernst Haeckel, in 1860. This kingdom included organisms such as fungi, bacteria, algae and protozoans that were characterized by having simple cellular organization and not producing complex tissues, and also often had a combination of plant and animal characteristics. However, it was obvious that these organisms were not closely related and did not form a natural grouping. A modification of this system was proposed by Herbert Copeland, in 1938. He removed bacteria from Protoctista and erected a fourth kingdom that he called Monera. From here, the modification from Copeland's classification was not a big leap to Whitaker's, which differed only in the removal of the fungi from Protoctista and placing them in their own kingdom (=Mycetae). It should also be noted that while a five kingdom system of classifying organisms has been used in most text books, for more than 25 years, the concepts of Protoctista, now Protista in most textbooks, and Mycetae have changed during this period of time. With these changes in concepts, other classification schemes have been proposed. Some schemes have expanded the number of kingdoms to six, and eight and one has even reduced the number to three. Thus, there is still not agreement among scientist as to the number of kingdoms that should be recognized in the classification of organisms, and it is probable that there never will be.

The lack of agreement on a single classification scheme does not mean that one system is necessarily better than another. Only that they are different. One point to keep in mind concerning the various classification schemes is that they are man-made and are only our concepts of how organisms should be classified. The organisms, themselves, have not been changed one iota as a result of these changes in classification, even though this is the impression that we are occasionally left with. One example that might amuse you is the change in policy of the Hawai‘i State Plant Quarantine Department, with respect to a group of organisms commonly referred to as "blue-green algae". Plant quarantine's import policy of this group of "algae" was to treat them as algae. However, a few years ago, they learned that blue-green algae were reclassified and now belong to the Kingdom Monera, which means that they are more closely related to bacteria than algae. All of a sudden, the import policy regarding bluegreen algae also changed and are now treated as bacteria. Blue-green algae have probably undergone relatively few changes in over a billion years, but yet the change in policy by the State of Hawai‘i would seem to indicate that there was a drastic change in these organisms simply because they have been reclassified into a different kingdom.

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