Taxonomy

Narrowly defined, plant taxonomy is that aspect of the study of plants having to do with taxa—that is, the naming of plant names (nomenclature) and the determination of the hierarchical relationships used to identify them or understand their relationships (identification and classification). More broadly defined, plant taxonomy includes areas of investigation from a variety of disciplines, including classification, nomenclature, phylogeny reconstruction, and investigation of evolutionary processes. Under this broader definition, the goal of taxonomy is to use comparative data to assess relationships between plants, define taxonomic boundaries between species and other ranks, provide means of identifying and communicating about the organisms, and understanding the patterns (and even the processes) in the evolution of the plants. Seen in this way, plant taxonomy overlaps and grades into the disciplines of plant systematics, phylogenetics , and evolution.

Distinguishing, classifying, and naming plant taxa is basic to any study of nature and for making use of plants. To have the name of a plant (or taxon) is the prerequisite to get further information about it. Taxonomy, therefore, is inevitably necessary for all further studies of plants in order to understand their structures, life histories, and distributions, as well as for making use of them for the benefit of humans (i.e., applied botany, comprising pharmacy [medicinal plants], crops and food production [agriculture],

forestry, production of plant raw material [fibers, gum, etc.], horticulture [ornamental plants], and conservation).

Taxonomy, therefore, is one of the oldest branches within the life sciences. But taxonomy is a synthetic science, as it uses and relies on data from all other fields of botany, including that from structural botany (cytology and anatomy, palynology and ultrastructure), morphology (including embryology), genetics (heredity), physiology , phytochemistry, ecology (habitats), and phytogeography (distribution of taxa).

Plant Systems: Artificial and Natural

Variation among plants is not continuous—that is, there are discontinuities separating one type of plant from another. We distinguish different individuals of the same kind, and we distinguish different kinds of oaks, roses, dandelions, violets, and so on. Variations among taxa are of different quantity and quality, and as a result, they can be arranged in a hierarchical system. Those plants being almost alike and interbreeding belong to the same species. All similar species together form a genus, similar genera are put together as a family, and so forth. The tasks of taxonomy, therefore, are to describe the plants thoroughly (phytography) and to compare them in order to discover natural groups, and to classify (classification) and name them (nomenclature).

Other questions examined in evolutionary botany and phylogenetics include: How did the differences between taxa come about? And what are the reasons for their mutual hierarchical connections? To some authors, all these studies together are called (biological) systematics, and taxonomy is restricted to dealing only with classification.

The Swedish botanist Carolus Linnaeus (1707-1778) was an excellent analytical observer, and his talent for methodical recording, concise summarizing, and systematic thinking enabled him to produce a synthesis, the Linnaean System, which was in use for almost one hundred years. It was easy to handle because it was mainly based on very few numerical flower characters (number of stamens and styles). A system using only one or very few characters is considered an artificial one, and the Linnaean System for the most part did not reflect natural (evolutionary) relationships.

Botanists who followed Linnaeus elaborated a natural system of classification, considering all organs of the plant. Important contributors included the Frenchmen M. Adanson, A. L. de Jussieu, and Jean Baptiste de Lamarck; their Swiss colleagues A. P. and Augustin de Candolle; the Scot Robert Brown; the Austrian S. Endlicher; and the Britons G. Bentham and J. D. Hooker. Their system was based on comparative morphology, a discipline arising in the beginning of the nineteenth century. It made use of the concept of homology and of crucial discoveries like the alternation of generations by the German W. Hofmeister. Their "natural system"—though subsequently modified in many ways—essentially persisted up to modern times, although it aimed at reflecting the natural order they believed to have been established by God the Creator rather than attempting to reveal evolutionary relationships.

Biological Kinship: Phylogenetic, Phenetic, and Evolutionary Systems

A revolution in biology was the discovery of evolution and its mechanisms by the English naturalists Charles Darwin and A. R. Wallace in the middle of the nineteenth century, presenting common descent and gradual changes in geological time scale as an explanation for the taxonomic hierarchy already established. No essential changes of the previous morphologically based system, therefore, proved necessary. However, the acceptance of evolution as a unifying concept in biology turned the attention of taxonomists to questions of ancestry and descent.

Toward the end of the nineteenth century, the connections among taxa were more carefully studied in order to answer the questions: Which are the older (i.e., more primitive) characters and taxa, and which are the younger and more advanced ones? How did specific adaptations evolve? A typical and important question concerned the evolution of pollination mechanisms. Are unisexual wind-pollinated flowers primitive (original) or advanced (derived, secondary) or vice versa?

The first attempts to answer these questions were by A. Engler and R. V. Wettstein and followers in the late nineteenth century. They considered the Fagales (beech and birch families), Juglandaceae (hickories), and Salicaceae (wil-lows and cottonwoods) as the most primitive angiosperms because of their small, wind-pollinated, unisexual flowers arranged in catkins , thus forming a presumptive link between the more ancestral gymnosperms (including conifers like pines, spruces, firs, cedars, etc.) and the angiosperms , which often have showy, insect-pollinated, hermaphroditic flowers. However, subsequent research in the early part of the twentieth century, especially by C. E. Bessey, showed that the phylogenetic relations are just the other way around: angiosperms appear to be primarily hermaphroditic and insect-pollinated, these traits being responsible for one of their major advantages (more effective pollination) in contrast to gymnosperms . The catkin-bearing families are judged to have evolved later, adapting to habitats in which wind pollination is advantageous.

In the beginning of the twentieth century, the new field of genetics strongly improved our understanding of evolution. Many phylogenists, however, were skeptical because of the large number of empirically ill-based, speculative phylogenetic trees produced at the turn of the century. Toward the middle of the twentieth century, taxonomy at the specific and infraspecific (below species) levels was changed into the New Systematics, Biosystematics, and Experimental Systematics, strongly accelerated by knowledge from genetics, population genetics, breeding systems, cytogenetics, karyology (chromosome number), and ecology in combination with traditional data from morphology, embryology, anatomy, and distribution.

Despite these stimulating and largely successful efforts to combine understanding of evolutionary processes with the needs and challenges of taxonomy, the study of phylogeny (trees of evolutionary descent) and of systematics above the species level (macrosystematics) was not markedly improved because no data were available for establishing scientifically sound hypotheses. A pragmatic method was developed, therefore, that tried to overcome the difficulty of having classification based on phylogeny, but these were often burdened with intuitive and speculative hypotheses. Phenetic systems were designed in the late 1950s to resolve these problems.

Phenetics

Phenetics is a strictly empirical method. It avoids evaluating character states as ancestral versus derived and thus avoids any phylogenetic hypotheses. All characters are treated as equal (unweighted). The use of newly developed mathematical methods and of electronic data processing allowed calculation of large amounts of character data and classification based exclusively on overall similarity. The resulting phenetic system does not attempt to reveal kinship (relationships based on descent). As it is not biased by phylogenetic judgements, it can be considered, in some way, a multipurpose system allowing a maximum of predictability. Convergences (i.e., similarities caused by evolutionary adaptation to the same environmental factors), however, may not become apparent in a phenetic system.

Not all workers were satisfied with phenetics, however, and starting in the late 1960s the method called cladistics was developed for handling taxonomic characters and exploring relationships between taxa. Cladistics, which was strongly opposed to phenetics, developed rapidly and successfully.

Cladistics

Using and developing the ideas and concepts of the German entomologist Willi Hennig, cladistics is an effective tool for studying and describing phylogenetic relationships and reconstructing a truly phylogenetic system. It constructs conceptually strict phylogenetic branching patterns by evaluating character states as ancestral (plesiomorphic) or derived (apomorphic). The basic assumption is that speciation takes place mainly by bifurcation: one ancestral species splits into two sister species, which are to be treated as having equal taxonomic rank, both characterized by newly developed (apomorphic) characters.

The big advantage of cladistics is the use of clear assumptions and procedures, in contrast to intuitive phylogenetic trees devised by earlier generations of phylogenetic taxonomists. An important conceptual change is the more restrictive definition of monophyly, meaning a group containing all descendants of a certain taxon. But if, say, one or more descendants are excluded, the remaining group forms a paraphyletic group because it does not contain all descendants from the common ancestor. In pre-cladistic phylogenetic systems, however, a paraphyletic group has been accepted as being monophyletic because all its members share a common ancestor. This, however, has not been accepted by cladistic practitioners.

These simple concepts, seemingly formalistic and of no empirical value, have, however, major effects on defining phylogenetic taxa and constructing a phylogeny-based system. Through this type of work, the understanding of relationships in the plant kingdom has recently undergone a major revision.

There is much evidence, accumulated for many decades and including modern molecular investigations, that all plants in the strict sense (the so-called Embryophyta, the bryophytes and the vascular plants) are offspring of the green algae (Chlorophyta s.l.) because they share a combination of common features that would have been unlikely to have evolved several times independently. Both together form a single big branch (clade ) of the phylogenetic tree and, therefore, are united into one monophyletic taxon, the Chlorobionta. This big and highly heterogenous taxon, comprising unicellular algae together with all the land plants, has the same rank in the phylo-genetic system as the red algae, comprising a number of seaweeds only. In a cladistic sense, you cannot unite the green algae with the red algae (and other algal groups) to form the taxon algae. Such a group, which mixes organisms from different branches, is called a polyphyletic group.

see also Algae; Bessey, Charles; Candolle, Augustin de; Family; Flora; Herbaria; Hooker, Joseph Dalton; Identification of Plants; Linnaeus, Carolus; Phylogeny; Species; Systematics, Molecular; Systematics, Plant; Taxonomic Keys; Taxonomist.

Manfred A. Fischer

Bibliography

Greuter, W., et al., eds. International Code of Botanical Nomenclature. Berlin: Regnum Veg, 1988.

Stuessy, T. F. Plant Taxonomy. New York: Columbia University Press. 1990.

INTERNATIONAL CODE OF BOTANICAL NOMENCLATURE (ICBN)

In order to standardize terminology and promote effective communication among plant scientists, plant taxonomists have agreed to a set of formal guidelines to be used when naming plants. Known as the International Code of Botanical Nomenclature (ICBN), these guidelines were adopted in 1867 and serve as the official reference rules with which nomenclatural decisions about plants must be made. The Code is updated and modified periodically by an international committee on plant nomenclature and serves as the basis for all naming of plant ranks from infraspecific taxa (variety, subspecies, etc.) to species and to taxa of higher ranks (genus, family, order, etc.). There is also a separate code for the naming of cultivated plants, which has nomenclatural conventions very similar to that of the ICBN.

At its founding, the ICBN agreed that plant nomenclature would officially begin with the work by Linnaeus, Species Plan-tarum, published in 1753. To avoid unnecessary duplication of names for the same species or to stabilize the way plant names are applied to the various taxonomic hierarchies, a system of date priority was also instituted, so that the earliest correctly published name for a taxon must be used if the plants concerned are found to be the same. Taxonomic publications describing new species or proposing new genera, families, or other formal ranks are expected to follow the rules included in the ICBN. If these requirements are not met, the name proposed by the botanist may be rejected.