Plant Taxonomy

The Systematic Evaluation of Comparative Data

Tod Stuessy

• Publisher: Columbia University Press
• Year: 2009
• ISBN:
  9780231518642
• Edition: 2
• Topic: Biological Sciences
• Subtopic: Botany
• Index: Biological Sciences

The field of plant taxonomy has transformed rapidly over the past fifteen years, especially with regard to improvements in cladistic analysis and the use of new molecular data. The second edition of this popular resource reflects these far-reaching and dramatic developments with more than 3,000 new references and many new figures. Synthesizing current research and trends, Plant Taxonomy now provides the most up-to-date overview in relation to monographic, biodiversity, and evolutionary studies, and continues to be an essential resource for students and scholars.

This text is divided into two parts: Part 1 explains the principles of taxonomy, including the importance of systematics, characters, concepts of categories, and different approaches to biological classification. Part 2 outlines the different types of data used in plant taxonomic studies with suggestions on their efficacy and modes of presentation and evaluation. This section also lists the equipment and financial resources required for gathering each type of data. References throughout the book illuminate the historical development of taxonomic terminology and philosophy while citations offer further study. Plant Taxonomy is also a personal story of what it means to be a practicing taxonomist and to view these activities within a meaningful conceptual framework. Tod F. Stuessy recalls the progression of his own work and shares his belief that the most creative taxonomy is done by those who have a strong conceptual grasp of their own research.

P A R T O N E

Principles of Taxonomy

SECTION 1

The Meaning of Classification

Taxonomy is dynamic, beautiful, frustrating, and challenging all at the same time (fig. 1.1). It is demanding philosophically and technically, yet it offers intellectual rewards to the able scholar and scientist. It can be manifested in works of incredible detail as well as in logical and philosophical conceptualizations about the general order of things. It has strong implications for interpreting the reality of the world as we can ever hope to know it.

Because taxonomy has deep historical roots, the past is never escaped. This places an increasing burden upon practitioners to understand old and new material. The past must be dealt with for older results, and every new discovery must be digested and incorporated. As Constance aptly put it, “My ideal taxonomist, therefore, must be very versatile indeed, and should preferably be something of a two-headed [i.e., two-faced] Janus, so that one set of eyes can look back upon and draw from the experience of the past, and the other pair can be focused upon deriving as much of value as possible from developments on the present scene” (1951:230).

Taxonomy is a synthetic science, drawing upon data from such diverse fields as morphology, anatomy, cytology, genetics, cytogenetics, chemistry, and molecular biology. It has no data of its own. Every new technical development in these other areas of science offers promise for improved portrayal of relationships of organisms. This is a demanding aspect of taxonomy for a practicing worker, because it is virtually impossible to understand completely all of these different data-gathering methods, yet highly desirable to be able to master as many as possible. Furthermore, the accumulation of data and their interpretation never cease. Not only do new techniques of data-gathering provide more information that must be brought to bear on understanding relationships, but also these new interpretations reveal new taxonomic groups that must be understood and utilized. These are some of the reasons why taxonomy (and its parent discipline, systematics) has rightly been called “an unending synthesis” (Constance 1964), “an unachieved synthesis” (Merxmüller 1972), or even more poetically, “the stone of Sisyphus” (Heywood 1974).

FIGURE 1.1 An example of the challenges facing the plant taxonomist is shown dramatically by this bizarre landscape, which could represent an obscure area of the earth or perhaps even another planet, with completely new and different plant forms. If this scene were on earth, we would have considerable biological information on plants in general, e.g., modes of reproduction, structures, and functions, and a good background of ideas on how to proceed with classification of these groups based upon historical classificatory records. If on another planet, however, attempting a predictive classification of these forms would be unbelievably difficult, with nothing known about modes of reproduction, structures and their functions, mechanisms of evolution, or even what is an individual or population. This same type of overwhelming challenge was faced by plant taxonomists on this planet approximately 500 years ago. (From Lionni 1977, frontispiece)

CHAPTER 1

A Few Definitions

Classification, Taxonomy, and Systematics

Taxonomy has had various meanings over the past 150 years, and particular confusion with systematics has prevailed. Systematics no doubt was used very early as “a casual self-evident term” (Mason 1950:194) to refer to the ordering of organisms into rudimentary classifications. This activity has occurred ever since people have lived on earth (Raven, Berlin, and Breedlove 1971). The early documented use of the term systematics (as systematic botany) can be traced at least as far back as Linnaeus (1737a, 1751, 1754), and it has persisted to the present day although in modified form. Linnaeus (1737a:3) stated that “we reject all the names assigned to plants by anyone, unless they have been either invented by the Systematists or confirmed by them.” In 1751, he used the term (as “botanico-systematici,” p. 17) to refer to workers who “carefully distinguish the powers of drugs (in plants) according to natural classes.” He made the definition of a Systematic Botanist even more clear in his preface to the fifth edition of the Genera Plantarum:

The use of some Botanic System I need not recommend even to beginners, since without system there can be no certainty in Botany. Let two enquirers, one a Systematic, and the other an Empiric, enter a garden fill’d with exotic and unknown plants, and at the same time furnish’d with the best Botanic Library; the former will easily reduce the plants by studying the letters [i.e., features of diagnostic value] inscribed on the fructification, to their Class, Order, and Genus; after which there remains but to distinguish a few species. The latter will be necessitated to turn over all the books, to read all the descriptions, to inspect all the figures with infinite labor; nor unless by accident can be certain of his plant” (1754:xiii, 1787:lxxvi).

Books using the term systematic botany appeared thereafter (e.g., Smith’s An Introduction to Physiological and Systematical Botany 1809 and Nuttall’s An Introduction to Systematic and Physiological Botany 1827). Mason, although admitting the difficulty of establishing the place of its first use, ventured the opinion that systematics “might possibly have even preceded it” [i.e., the use of taxonomy] (1950:194) and gave Lindley (1830b) as the earliest reference.

A biologist interested in relationships during this early period mostly studied morphological features and accordingly grouped organisms into units. This ordering of organisms into groups based on similarities and/or differences was (and still is) called classification. This is a very old term going back to Theophrastus in the third century B.C. (see 1916 translation). The Swiss botanist, Candolle (1813), in the herbarium at Geneva, coined taxonomy (as taxonomie)¹ to refer to the theory of plant classification. It later became more generally used for the methods and principles of classification of any group of organisms and is still used basically in this way (e.g., Simpson 1961). From this point to the publication of the theory of evolution by means of natural selection by Darwin (1859), the two words, taxonomy and systematics, were regarded as synonyms, although the latter was used much more frequently. During this time, classifications were believed to reflect the plan of natural order created specially by God, and man was simply rediscovering the Divine Plan. Biologists engaged in these activities of classification were called interchangeably either taxonomists or systematists. Since Darwin’s time, systematists have not only continued their interest in classification, but also have attempted to understand evolutionary relationships among the groups so ordered. Furthermore, some systematists have become interested in the process of evolution itself, that is, in the mechanisms that produce the diversity. Consequently, a systematist today may study many different aspects of evolutionary biology that are far removed from the morphological investigations of a century ago. For a useful overview of themes and progress in plant systematics during the past half-century, see Stevens (2000a).

The basic methodology of modern systematics is outlined in table 1.1. Data are gathered from organisms and their interactions with the environment and used to answer questions about classification, phylogeny, and the process of evolution. Specific examples of systematic studies might be analyzing the patterns of adaptive radiation within a particular group of species, comparing DNA sequences for reconstruction of phylogeny, or investigating patterns of intra- and interpopulational genetic variation. A similar and equally legitimate viewpoint was presented by Blackwelder and Boyden (1952), who indicated three steps: (1) recording of data; (2) analysis of the data for making classifications; and (3) synthesis of these generalizations for insights on phylogeny and evolution. Wilson aptly pointed out:

most systematists by choice are not problem solvers in their method of working. It might be said that the perfect experimental biologist selects a problem first and then seeks the organism ideally suited to its solution. The systematist does the reverse. He selects the organism first (for reasons that are highly individual) and only secondarily chances upon phenomena of general significance. The chief value of his discoveries is that they are typically of the kind that would not be made otherwise. If the systematist has an ideal program, it runs refreshingly counter to the conventional wisdom: select the organism first, as a kind of totem animal (or plant) if you wish, then actively seek the problem for the solution of which the organism is ideally suited (1968:1113).

TABLE 1.1 Outline of Methodology of Systematics

I. Accumulation of Comparative Data

A. From the Organism

1. Structures

2. Processes (interactions among structures)

B. From the Organism-Environment Interactions

1. Distributions^a

2. Ecology

II. Use of Comparative Data to Answer Specific Questions

A. Classification (most predictive system of classification at all levels)

1. Method and result of grouping of individuals

2. Level in the taxonomic hierarchy at which the groups should be ranked

B. Process of Evolution

1. Nature and origin of individual variation

2. Organization of genetic variation within populations

3. Differentiation of populations

4. Nature of reproductive isolation and modes of speciation

5. Hybridization

C. Phylogeny (divergence and/or development of all groups)

1. Mode

2. Time

3. Place

^a Floristics, or the documentation of what plants grow in particular regions, is deliberately not listed in this table as a separate question, nor does it find a specific place in the areas of systematics in figure 1.2. Determining where particular plants grow is a very legitimate and valuable activity within systematics, but it is essentially data-gathering of distributions of plant groups that have already been classified. Some floristic projects, however, especially of poorly known regions (e.g., Rzedowski and McVaugh 1966; McVaugh 1972a, b) involve considerable amounts of classification as well as original historical scholarship. To this extent, they become more revisionary, and less floristic, in character (for these and other distinctions, see Stuessy 1975). Many innovations in floristic work are presently occurring, especially using Web-based technologies (see symposium introduced by Kress and Krupnick 2006).

This is a very important perspective. Were all biologists interested only in seeking answers to basic biological processes, then all would work with model organisms such as Drosophila or Arabidopsis, both easily manipulated in the laboratory. This is certainly one approach to biological science, but if we do not continue to investigate other organisms, we will miss out on other interesting and useful processes (Greene 2005). Where would molecular biology (and molecular systematics) be without polymerases that work at high temperatures, which were found in the course of systematic studies on bacteria of thermal pools (Saiki et al. 1988). Even model-organism researchers themselves recognize the importance of adding other representatives of different life forms (Fields and Johnston 2005).

Because of the diverse nature of these studies that were spawned by evolutionary theory, a collective term was needed to designate these different activities and the people so engaged, and another term also was needed for describing the more traditional activities of classification. As a result, the term systematics (or systematic biology) has come to have a meaning different from and broader than taxonomy. The definition used by most people and preferred here is that of Simpson: “the scientific study of the kinds and diversity of organisms and of any and all relationships among them” (1961:7). A slightly simpler way of defining systematics is “the study of the diversity of organisms” (Mayr 1969c:2; cf. Wilson 1985, for his similar definition “the study of biological diversity”). Diversity is such an important concept in systematics that a useful and delightful perspective on this was provided by Constance:

Much as I respect the giant strides that have been made in clarifying basic principles and processes of wide applicability, I have chosen to celebrate diversity. It is well enough to know that all music can be reduced to a relatively few notes and a minimum of ways of evoking and receiving them in the human ear. This does not suggest to the music lover that symphonies, sonatas, and operas are redundant because their parts and processes can be analyzed. All literature, after all, is merely spun out of words. Human beings are a lot alike, but it does not necessarily follow that there is no point in knowing more than one of them (1971:22).

Myers provided a slightly more general definition of systematics: “the study of the nature and origin of the natural populations of living organisms, both present and past” (1952:106). The term biodiversity is now often used (e.g., Fosberg 1986; Wilson 1988; Minnis and Elisens 2000) to refer to the total biotic diversity on earth.

Some biologists have equated taxonomy with systematics (Lawrence 1951; Crowson 1970;² Radford et al. 1974; Ross 1974; Jones and Luchsinger l986; Stace 1980; Minelli 1993; Singh 2004);³ Wilson echoed this viewpoint and talked about a pure systematist, who “can be defined as a biologist who works on such a large number of species that he has only enough time to consider classification and phylogeny. If he narrows his focus, his unique knowledge provides him with a good chance to make discoveries in genetics, ecology, behavior, and physiology, as well as in taxonomy. But then we come to know him as a geneticist, or an ecologist, or a behaviorist, or a physiologist” (1968:1113). Swingle (1946) made a distinction between systematics and taxonomy with the latter regarded as dealing with phylogenetic classification, and the former being broader to include taxonomy and nomenclature. Mason (1950; and followed by Porter 1967) took a different approach and regarded systematics (specifically systematic botany) as the data-gathering aspect (e.g., as Systematic Anatomy, Systematic Cytology, Systematic Genetics) and taxonomy as the interpretive phase in constructing classifications and in revealing evolutionary relationships. Wheeler offered a cladistic definition of systematics as “the production of cladograms that link taxa through their observed variation” (2005:71), a much too limited definition in my view. Smith defined systematics as “that branch of science which investigates the philosophy that underlies a classification” (1969:5), which would be closer to the definition of taxonomy used in this book. Schuh followed a similar definition describing systematics as “the science of biological classification” (2000:3). Wägele talked about biological systematics as the “science of the systematization of organisms and the description of their genetic and phenotypical diversity (= biodiversity)” (2005:10). For general agreement with the definitions used here, see Danser (1950), Davis and Heywood (1963), Blackwelder (1967a), Sylvester-Bradley (1968), Mayr (1969c), Darlington (1971), Stuessy (1978b), Woodland (2000), and Judd et al. (2002). For additional discussion of these (and related) concepts, see Small (1989). The term synthetic biology has emerged recently (e.g., Benner and Sismour 2005), but this deals with creating artificial life and using different units of the natural world to form new functioning systems. There also exists the term syntaxonomy, but this refers to classification of plant communities (e.g., Fremstad 1996). The relationships among terms as used in this book are shown in fig. 1.2.

Because most fields have a set of principles, and because taxonomy is the main focus of this book, I herewith list the six points that to me seem significant for our field (from Stuessy 2006):

1. All known life originated on earth during the past 3.5 billion years.

2. Due to evolutionary processes (e.g., speciation, hybridization, extinction), life-forms show natural patterns of relationship to one another.

3. Using selected features of organisms (characters and states), we determine patterns of relationship that we assume reflect these evolutionary processes.

4. Humans need hierarchical systems of information storage and retrieval to live and survive, including dealing with the living world.

5. The assessed patterns of organismal relationship are used to construct hierarchical classifications of coordinate and subordinate groups that are information-rich and have high predictive efficacy; these are the taxonomic hypotheses that change with new information and new modes o...

Table of contents

• Cover 
• Half title
• Title
• Copyright
• Dedication
• Contents 
• Preface to the First Edition
• Preface to the Second Edition
• Acknowledgments for the First Edition
• Acknowledgments for the Second Edition
• Part One. Principles of Taxonomy
• Part Two. Taxonomic Data
• Epilogue
• Literature Cited
• Author Index
• Taxon Index
• Subject Index