In Press, Taxon Three centuries of paradigm changes in biological classification: is the end in sight? Brent D. Mishler University and Jepson Herbaria Dept. of Integrative Biology University of California, Berkeley, CA 94720 [email protected] Classification has been a centerpiece of biology ever since Linnaeus, providing a framework on which existing knowledge can be organized and predictions about unknown traits can be made. But the basis of biological classification has gone through a series of upheavals over the last three centuries, from being considered a plan in the mind of the creator, to a neutral assessment of overall similarity, to a reflection of evolutionary niches, and finally to a phylogenetic mapping of the tree of life. This paper will consider this historical process, with emphasis on phylogenetic systematics (cladistics), and also consider where we might be heading in the future. It is necessary first, however, to consider the purposes of classification itself, which have not changed much over time. Purposes for classification We need to keep firmly in mind that biological classification is a human construct, to be adopted for the uses we find most compelling in light of current understanding. Particular classifications, or general principles of classification, are not immutable or important to conserve for tradition's sake alone. Our knowledge of the biological world has changed greatly since Linneaus, we must be free to consider changing his classification system to keep pace. Perhaps all scientists would agree in principle to this point, yet because of the weight of tradition, discussions about possible changes to the Linnaean classification rapidly become emotional, even angry (Laurin, 2008). Many systematists seem threatened to the core by any suggestion to change the classification system radically. But like any other scientific product, classification is subject to revision as knowledge increases -- science should have no sacred cows. This paper will proceed in that context. There are four general categories of desirable criteria for taxonomies (Mishler, 2000): (1) practicality: names should be easy to apply, stable and clear; (2) information content: names should index an optimal summarization of what is known about the entities being classified; (3) predictivity: named groups should maximally predictive of unknown features of the entities being classified; (4) function in theories: a classification should capture entities acting in, or resulting from, major natural processes. These criteria sometimes seem contradictory, for example when debates erupt between pragmatists emphasizing criterion #1 and theoreticians emphasizing criterion #4 (e.g., debates between pheneticists and cladistists in the 1970's and 80's outlined below). Ultimately, however, these criteria should not be contradictory, and should flow from #4 to #1, in the sense that representing an important natural process in the structure of a classification will lead to high predictivity, information content, and true practicality for users ofthat classification. The key to "carving nature at its joints" is to find the joints first. Just as in chemistry, where the periodic table was chosen as the best way to classify elements even though it is not as practical as the simpler, earlier "Fire, Water, Earth, and Air" system, biology needed to model its primary classification system on the major natural processes affecting life. As it turns out, the overriding process turned out to be evolution. A brief history of biological classification The recognition of basic kinds of organisms, and arranging their properties into higher categories in classifications, was an ancient human imperative. All cultures have complex biological classifications, and it is extremely interesting to trace changes in these classification throughout history including the scientific era. Detailed treatments of this history include Raven & al. (1971), Mayr (1982), Dupuis (1984), Hull (1988), and Stevens (1994, 2000). I will only outline here the main historical stages. Early folk taxonomies came out of prehistory and were oriented towards practicality and human uses of organisms (Raven & al., 1971). Organisms were grouped by their relationship with human affairs. Fine-scale groups in the hierarchy tended to compare closely with modern scientific classifications, while higher-level groups often departed radically. The first scientific revolution in biological systematics was that provided by the ancient Greeks; as in many fields of science, they justified a new logical framework within which to view the natural world. The effect of this on systematics was nicely discussed by Hull (1965): an essentialistic approach that gripped biology for 2,000 years. In this approach taxa were viewed as defined by the possession of necessary and sufficient defining traits. In the minds of taxonomists in the Christian Era, these essential characteristics were taken as evidence of the plan of creation, with the more inclusive levels in the taxonomic hierarchy being the major elements of the plan, and species the basic elements (Agassiz, 1859). Such a view, which reached its culmination in the work of Linnaeus, became untenable as the wealth of biological diversity became known due to the explorations of the 18th and 19th centuries. It became clear that any and all characters can vary within a named group, and thus the use of defining characters became an obvious problem when a group of plants that clearly belonged together was threatened because variation was discovered in an essential character. This set the stage for the second revolution in the history of systematics, the development of the Natural System in the late 1700's and early 1800's (as discussed by Stevens, 2000). In this approach taxa were recognized by overall resemblance in many characters, which were often chosen for their "importance" in the biology of the group in question. It is important to note that this revolution in systematics preceded the Darwinian revolution, and in fact was prime evidence for Darwin to present in favor of evolution in the Origin. Several of Darwin's' friends and correspondents such as Hooker and Gray were architects of the natural system. As has been noted by many (e.g., de Queiroz, 1988), the Darwinian revolution had no fundamental impact on systematics. The switch to the natural system
View Full Document
Unlocking...