Plant taxonomy is the branch of botanical science dealing with the identification, naming, and classification of plants. A principal goal for the modern plant taxonomist is to improve and refine classification systems with the ultimate goal being to develop a system that reflects evolutionary relationships. Since we will never know the true evolutionary relationships within a large group of organisms like Bromeliaceae, we will never know the single, true classification for that group. Consequently, all classifications represent hypotheses of evolutionary relationships, for example, that the species within genus "X" are more closely related to each other than they are to the species in the genus "Y." In the case of competing or alternative hypotheses, the best classification is the one that is most natural (monophyletic); that is, the hypothesis that best reflects evolutionary or genealogical relationships. In short, the modern plant taxonomist strives to identify natural taxonomic units including species, genera, subfamilies, and families and their relationships.
What sorts of data are used to generate and support classification hypotheses? The answer is: the characters of the taxonomic units being considered. Never before has the taxonomic community had such a diversity of research tools and types of data, for example, morphological, anatomical, chromosomal, chemical, developmental, molecular, genetic, ecological, and biogeographical, for use in reevaluating classifications, or for formulating new and improved hypotheses.
What is the role of nomenclature in all this? Nomenclature is the mechanism through which a classification is communicated. For example, "Bromeliaceae" communicates a plant group that, according to all available data, appears to be natural. Consequently, as classifications are improved to more accurately reflect genealogical relationships, nomenclatures frequently require corresponding change. The reverse is true as well. It is change in nomenclature and, specifically, change unsupported by convincing data, that this commentary is about.
There are formal rules describing how changes in nomenclature must be made in order to be considered legal. However, names of plants that are legitimately described are not absolute laws in themselves. We do not have to use them. For example, changes in taxonomic rank, such as removing a group of species from one genus to make a new genus, or sinking an old, smaller genus into a larger one, can be accepted or rejected by botanists and horticulturists regardless of how legally the names were published. But if the names are legitimately published, they cannot be ignored even if based on faulty taxonomic judgment. For this reason, new names should not be published in the absence of convincing scientific evidence.
The monumental monograph by Smith and Downs (1974, 1977, 1979) stands as an important synthesis of the taxonomy of Bromeliaceae. New insights, however, gained from current investigations of character distributions have made us recognize that there are some errors in the classification and nomenclature proposed in that work. They must be corrected after thorough scientific investigation.
Recent attempts to establish more natural groups from several obviously artificial taxa accepted in the Smith and Downs monograph have produced major nomenclatural (and thus classification) changes. For example, Smith and Kress (1989, 1990) believing that the genus Aechmea was an unnatural taxon, elevated eight subgenera to generic rank in order to provide smaller natural groups to indicate the evolution of the subfamily. They stated that this seemed, "...not only logical but also convenient...." Elton Leme (1992), however, pointed out that these nomenclatural changes were not supported by published data, or as he said, by "convincing, fresh arguments." Even though some of these new genera may be good groups, the nomenclatural changes proposed by Smith and Kress did not advance our knowledge of Bromeliaceae but certainly did increase the amount of taxonomic bookkeeping required to keep track of all the names. To complicate matters, those authors transferred several taxa misplaced at the subgeneric level within Aechmea to the wrong, newly created genus. We stress that proposed changes in classification must be accompanied by a thorough scientific reevaluation of all affected taxa.
A second example also concerns changes in the genus Aechmea. Smith and Spencer (1992) placed the genus Streptocalyx into synonymy under Aechmea because they decided that "...petal appendages have proven unacceptable as a delimiting generic character in Bromeliaceae..." Again, a major nomenclatural change not supported by a thorough taxonomic analysis. In the case of Streptocalyx, although petal appendages may not be a valid character on which to base a genus, the Smith and Spencer paper failed to establish this as a fact. The work documenting questionable reliability of petal appendages as a generic-level character, as currently used in the Bromeliaceae, was done by Brown and Terry (1992). It did not prove that petal appendages are unacceptable as a delimiting generic character in all bromeliad genera but suggested only that this might be the case. Furthermore, Smith and Spencer did not provide data or valid arguments why Streptocalyx should be placed with Aechmea.
Again, the sinking of Streptocalyx was not preceded by a critical investigation of the species involved. For example, Streptocalyx colombianus var. laxus, a synonym of Aechmea hoppii, was superfluous from the beginning and should not have been transferred to Aechmea.
Explicit data and sound, scientific justifications to support proposed changes are essential. Bromeliad taxonomists must make concerted efforts to reevaluate characters and character correlations, identify new or under-utilized characters, and generate new data sets in an effort to produce better (i.e. natural) classifications. Unfortunately, this was not done in these examples. We must resist the urge to act simply on intuition and wait until a sound scientific study based on many characters is carried out. Only in this way can we improve on the Smith and Downs monograph.
Brown, G.K.; Terry, R.G. 1992. Petal appendages in Bromeliaceae. Am. J. Bot. 79:1051-1071.
Leme, E.M.C. 1992. Aechmea vanhoutteana and its synonyms. J. Brom. Soc. 42:103-108.
Smith, L.B.; Downs, R.J. 1974, 1977, 1979. Bromeliaceae. Flora Neotropica. Monograph, 14, pts. 1-3.
________; Kress, W.J. 1989. New or restored genera of Bromeliaceae. Phytologia 66:70-79.
________; ________.1990. New genera of Bromeliaceae. Phytologia 69:271-274.
________; Spencer, M.A. 1992. Reduction of Streptocalyx (Bromeliaceae: Bromelioideae). Phytologia 71:96-98.
1 Dept. of Botany, University of Wyoming, Laramie.
2 M.B. Foster Bromeliad Identification Center, Marie Selby Botanical Gardens, Sarasota, Florida.
3 Dept. of Botany, Smithsonian Institution, Washington, D.C.