Systematics, Plant

Plant systematics is a broad discipline that is often defined as the study of the kinds of organisms (both living and fossils), and of the relationships among these organisms. Thus, students and researchers in the area of systematics (termed systematists) study the diversity of all life, including bacteria, fungi, plants, and animals, with several major goals. Systematics includes the identification, naming, and classification of plants, as well as the investigation of evolutionary relationships (phylogeny) and of evolutionary processes. As such, some consider systematics the fundamental discipline upon which all other areas of biology must rely. That is, other areas of investigation all depend on a clear understanding of species names, species delimitation, and organismal relationships. Thus, systematics is a unifying discipline and operates in a highly similar fashion regardless of the major group of organisms investigated (e.g., plants, fungi, animals, or bacteria).

Research in systematics may involve the collection of organisms in the field and the study of these organisms in their natural setting, in museums, and in the laboratory; the latter may employ approaches also used in molecular biology. The sources of evidence used in systematics are also highly varied and may include morphology, chemistry (a subdiscipline often referred to as chemosystematics), paleobotany , physiology , ecology, biogeography , and various sources of deoxyribonucleic acid (DNA)/ribonucleic acid (RNA) data (an area referred to as molecular systematics ). For several reasons, computer analysis is also an important aspect of systematics. The task of classifying all life (estimated at ten to one hundred million species) is a monumental undertaking, involving the comparative analysis of many organisms; hence, systematists have made significant use of computers to analyze large data sets and to store information so that it is easily retrievable.

Because the areas of study encompassed by systematics are so diverse, the tools or methods employed are also highly varied. One aspect of systematics

involves the collection, pressing, and identification of plant specimens , herbarium management, and archiving type specimens (the actual plant specimen upon which the name of a species is based). Some systematists are heavily involved in field research. These individuals explore many natural areas of our planet seeking to describe and catalog the diversity of life; they may also be interested in discovering potential new crops or medicinal plants. This aspect of systematics may also include the discovery and description of new species. Closely associated with this aspect of systematics is developing and refining methods of identifying plants. This is also a critical area of plant systematics that includes the writing and use of identification keys and floras.

Another important aspect of plant systematics includes the collection of evidence for determining relationships among species, that is, reconstructing phylogenies (the development of an hypothesis of evolutionary history; these are depicted as phylogenetic trees). Ultimately, these phylogenetic trees are used for revising classifications. Classifying organisms in a manner that reflects evolutionary history has been a longstanding goal of systematics since the work of Charles Darwin. An evolutionary biologist of the twentieth century, Theodosius Dobzhansky, stated that "Nothing in biology makes sense except in the light of evolution." Modern systematists feel that an important corollary of this famous statement is that "things make much more sense in light of phylogeny." That is, understanding the evolutionary history of organisms and their closest relatives is central to comparative biology.

Finally, systematists are often involved in elucidating the processes of evolution. Through their study of plant diversity and natural populations , systematists may be involved in analyzing the levels and distribution of genetic variation within and among populations, estimating gene flow, analyzing isolating mechanisms and species origins, and investigating evolutionary mechanisms such as hybridization, polyploidy , and apomixis . As a result, one area of the broad discipline of systematics becomes intertwined with the field of evolutionary biology.

History of Systematics

Systematics is arguably the oldest biological discipline and has been practiced, in one form or another, for thousands of years. Prehistoric peoples knew and used almost all of the important crop plants we cultivate today, plus others specific to their geographic location. They selected plants with useful features for foods, medicines, fibers, and poisons. As civilizations developed throughout the world, people developed different ways of studying and classifying plants. Our systematics heritage traces back to early western civilizations where several men from Ancient Greece and later the Roman Empire made valuable contributions to our knowledge of plants. Theophrastus, a Greek philosopher who lived from 370 to 285 B.C.E., was a student of Aristotle and is regarded as the father of botany. Theophrastus wrote several hundred manuscripts describing and classifying plants. Many plant names used today are derived from those used by Theophrastus. In the first century A.D., Dioscorides, a Greek physician traveling with the Roman armies, wrote De Materia Medica, a book that described and classified more than five hundred species of plants based on their medicinal or other useful properties. This book served as the primary botanical text throughout Europe until the Renaissance , nearly fifteen hundred years.

Hierarchical classification systems, such as those we use today, can be traced to the late 1600s and the work of Englishman John Ray. Ray developed a classification system for eighteen thousand species and introduced the concept of placing morphologically similar species together in a larger group, the genus. The most notable contributions during the 1600s and 1700s were those of Carl von Linné, a Swedish naturalist better known as Carolus Linnaeus. Linnaeus is considered the father of taxonomy, and is best remembered for developing the binomial system of nomenclature , that is, the use of a two-part name for each species; the species name consists of the genus name and the specific epithet. Linnaeus also wrote several major books, including his two-volume catalog for plant identification, Species Plan-tarum, which was published in 1753.

All of biology, including systematics, was changed by the publication of Charles Darwin's The Origin of Species in 1859. Darwin's theory of evolution had an important message for systematists: Species are dynamic, changing entities, and classification is a way to order the products of evolution. Through efforts to reflect evolutionary history in classifications, we see the first evidence of phylogenetic classifications in the latter part of the nineteenth century—these are the roots of those systems in use today. Throughout the 1900s, improved means of data gathering and improved knowledge of the world's flora contributed to improvements in plant classifications. As noted, current efforts in phylogeny reconstruction are being incorporated into classifications, and systematics has expanded to include studies of speciation as well as phylogeny and classification.

Systematics and Classification

As the branch of biology concerned with understanding phylogeny and with organizing biological diversity, systematics also encompasses the development of classification systems for storage and retrieval of information. The major categories (ranks) of the botanical classification system still in wide use today are, in descending order:

Kingdom

Division (or Phylum)

Class

Order

Family

Genus

Species

Each organism can be placed into such a hierarchical system. Biological systematists attempt to create classifications that reflect phylogeny; that is, a group of closely related species will be classified into a genus; closely related genera are placed in a family, and so on.

Recent analytical developments in inferring phylogeny have improved our estimates of evolutionary history: now, in many groups of organisms, we can identify specific lineages , or clades , of related species. Unfortunately, our classification systems have not kept pace with our improved understanding of phylogeny. For this reason, clades and formal classifications do not always agree. For example, many textbooks follow the Five Kingdom approach to classification of life on Earth: Kingdoms Monera, Protista, Fungi, Plantae, and Animalia. However, although this approach was a vast improvement over previous Two Kingdom classifications (Plantae and Animalia), it does not accurately reflect what we know about the history of life on Earth. Other classification systems have been proposed, some recognizing as many as eight or ten kingdoms, in an attempt to include the various major lineages of life in a classification system. At present, none of these classifications adequately meets the challenge of representing current hypotheses of the phylogeny of life. Similar inconsistencies between estimates of phylogeny and classification can also be seen at other levels of the taxonomic hierarchy; thus systematists are torn between scientific reality and the tradition of classification. This inconsistency should not be viewed as a failure of systematics; instead, it should indicate that biological systematics is a dynamic area of biology, an unending synthesis that seeks to incorporate new information into our estimates of phylogeny and our classification systems that organize biological diversity. To improve the connection between our understanding of phylogeny and classification, some systematists are attempting to develop new methods of classification. One approach is to abandon the traditional Linnaean hierarchy in favor of a strictly phylogenetic system of classification.

Systematics and Society

Systematics plays a key role in benefiting human society, both directly and indirectly, and has been part of the human endeavor for millennia. To understand and appreciate the extent of human impact on either local communities or global ecosystems , it is first critical to know what species inhabit the community or area in question. Systematists also play a major role in conservation biology and in the study of invasive species, identifying those species that are endangered by human activities, as well as those being spread by humans from one part of the globe to another. Another aspect of systematics involves the careful study of relationships of domesticated plant and animal species and their nondomesticated wild relatives. For example, determination of the closest wild relatives of a particular crop may provide new sources of genetic variation for breeding programs and crop improvement. Such research has led to significant improvements in the yield and disease resistance of many of our food plants and domesticated animals. Systematists also play a critical role in the discovery of new drugs from medicinal plants through their field work and interactions with native peoples (ethnobotany ) who have long used these plants for medicinal purposes. Furthermore, the use of systematic knowledge of evolutionary relationships between related plant groups can guide chemists in choosing the best species to test for potential new drugs. In addition, systematists often serve as consultants to poison-control centers in hospitals because doctors need rapid and correct identifications of poisonous mushrooms, plants, and other potentially poisonous organisms. Systematists may also be involved in studying the evolution of diseases. For example, recent systematic studies have tracked the evolution of the AIDS virus and, in some instances, the pattern of transmission and source of infection.

Because systematics is such a large, diverse field, a distinction is often made among subdisciplines or subareas of endeavor. For example, plant nomenclature is the application of names to taxa following a strict set of published rules (the International Code of Botanical Nomenclature). Another key aspect is classification, which involves the organization of plants into groups or categories. As noted above, classification traditionally has employed the taxonomic hierarchy of categories established by Linnaeus (e.g., kingdom, division, class, etc.), but the utility of this approach to classification has recently been questioned. Some would collectively consider nomenclature and classification to represent the field of taxonomy and thus make a distinction between this and systematics, the latter focusing on the study of phylogeny and evolutionary biology. An integral part of modern systematics is phylogeny reconstruction. Phylogenetic trees showing evolutionary relationships may be reconstructed by using characters from a number of different sources, including morphological, anatomical, chemical, and palynological (pollen). Cytogenetics involves the study of chromosome morphology , as well as the investigation of chromosome pairing at meiosis . This field of systematics has enjoyed a recent revival with the application of chromosome painting techniques that facilitate the study of chromosomal evolution. Chemosystematics is the application of chemical data in a comparative fashion to study problems in systematics and to infer relationships based on the presence or absence of certain chemical compounds in the organisms studied. Recently, deoxyribonucleic acid (DNA) sequence data have been employed to reconstruct phylogeny, and at present serves as a major source of information to establish evolutionary relationships.

Systematists are often broadly trained, having not only a knowledge of field biology, but also ecology, life history, plant chemistry, population biology, speciation, phylogenetics, and molecular biology. A modern system-atist is often, therefore, a jack of all trades. The research of a systematist may involve field work and collection in the tropics, as well as extensive laboratory work involving DNA sequencing and gene cloning.

A fundamental goal of the field of systematics is understanding biological diversity and the organization of this knowledge into a classification system that reflects the evolutionary history of life. Hence, much of modern systematics is devoted to building evolutionary trees of relationships. The ultimate goal of this massive enterprise is the reconstruction of the "tree of life." Historically, most systematic research was based on morphological and anatomical similarities of organisms. Recently, however, the relative ease of DNA sequencing has provided another, very powerful approach to investigate relationships among species. DNA sequences and other molecular data not only are of enormous utility for inferring phylogenetic relationships, but also have other important applications. In much the same way that DNA markers can be used with human subjects to determine paternity, these same approaches can also be used for determining the parents of suspected plant and animal hybrids. This too is yet another aspect of the highly diverse field of systematics.

see also Biodiversity; Darwin, Charles; Flora; Herbaria; Identification of Plants; Phylogeny; Systematics, Molecular; Taxonomy.

Doug Soltis

Pam Soltis

Bibliography

Judd, W. S., C. S. Campbell, E. A. Kellogg, and P. F. Stevens. Plant Systematics—A Phylogentic Approach. Sunderland, MA: Sinauer Associates, 1999.

Kenrick, P., and P. R. Crane. The Origin and Early Diversification of Land Plants: A Cladistic Study. Washington, DC: Smithsonian Institution, 1997.

Soltis, D. E., P. S. Soltis, and J. J. Doyle, eds. Molecular Systematics of Plants II: DNA Sequencing. Boston: Kluwer, 1992.

plant classification System devised to group plants according to relationships among them. Plants are known by common names that often vary from area to area, but have only one correct scientific name. See taxonomy