Plant Systematics

Arun K. Pandey, Shruti Kasana

• Publisher: CRC Press
• Year: 2021
• ISBN:
  9781000404180
• Edition: 1
• Topic: Biological Sciences
• Subtopic: Botany
• Index: Biological Sciences

This book is designed to introduce the fundamentals of systematics in a simple, concise and balanced manner. The book aims to equip the students with the basics of plant taxonomy and at the same time also update them with the most recent advances in the field of plant systematics. The book has been organized into 21 chapters that introduce and explain different concepts in a stimulating manner. The text is supplemented with relevant illustrations and photographs. Relevant literature has been added to provide a better picture of the most recent updates in the field of plant systematics.

Note: T&F does not sell or distribute the Hardback in India, Pakistan, Nepal, Bhutan, Bangladesh and Sri Lanka.

CHAPTER-1 INTRODUCTION

Systematics is the science of organismal diversity. It involves the discovery, description, and interpretation of biological diversity, as well as the utilization of this information in the form of predictive classification systems. Systematics is therefore the study of the biological diversity that exists on earth today and its evolutionary history. Systematics also provides the framework, or classification, by which other biologists communicate information about organisms.

Systematics is essential to our understanding of the natural world and for communicating about it. The basic activities of systematics involve classification and naming which are the ancient human methods of dealing with information about the natural world. We depend on many species for food, shelter, fibre, clothing, paper, medicines, tools, dyes, etc. Knowledge of systematics guides the search for plants of potential commercial importance.

Systematics provides a reference system for the whole of biology and therefore can be seen as both the most basic and the comprehensive area of biology. Systematics is basic because organisms cannot be discussed in a scientific way until some classification has been achieved to recognize them and give them names. Systematics is most wide-ranging because it gathers together and summarizes everything that is known about the characteristics of organisms, whether geographical, morphological, physiological, genetic, ecological or molecular.

TAXONOMY

Taxonomy is that branch of science which deals with the principles and practices of classification. This biological discipline involves identification, description, classification and naming of taxa based on certain similarities and differences. The term ‘taxonomy’ was coined by de Candolle (1813) from the Greek words, taxis (meaning arrangement) and nomos (meaning law, rule). The literal meaning of taxonomy is lawful arrangement of things or arrangement by rules. Taxonomy mainly comprises of four components: Description, Identification, Nomenclature and Classification (often memorized as DINC). Often, taxonomy is considered synonymous to systematics, though the latter is much more inclusive.

THE COMPONENTS OF TAXONOMY

1. Description: Description is the written account of features or attributes of a taxon. The features are called characters. The characters can be qualitative or quantitative. Two or more forms of a character are known as character states. For example, petal color (red and white), leaf shape (ovate, elliptic, lanceolate), and fruit type (achene, capsule, berry).
2. Identification: Identification is the process of associating an unknown taxon with a known one. In other words, identification is the determination of a taxon as being identical with or similar to another and already known element. Identification is the process of finding the taxon to which a specimen belongs, like identifying the medicinal plants, edible and poisonous mushrooms. Identification is a primary function in taxonomy and by applying nomenclature it performs an essential role as a means of communication. A taxonomic key is used for identification of plants. Taxonomic keys are dichotomous, i.e., consists of a series of two contrasting statements. Each statement is a lead. Two leads constitute a couplet.
3. Nomenclature: Nomenclature is the formal naming of taxa according to prescribed rules. The naming of groups of organisms and the rules governing the application of these names together form the nomenclature. Plant nomenclature is concerned with the determination of the correct name of a known plant according to an internationally accepted system, i.e., International Code of Nomenclature for Algae, Fungi and Plants (ICNafp). Once the plant has been identified, it becomes necessary that it has a scientific name that provides universal applicability. The rules of ICNafp determine the application of name of the taxa.
4. Classification: Classification is a two-step process. Step 1: grouping objects based on similarities and differences and Step 2: ranking these groups into a hierarchy (nested series of categories) based on some criteria. Classification is thus the placement of a plant (or group of plants) in categories based on their similarities and differences. These groups are then arranged according to their levels into categories in a nested manner. Thus, similar individuals may be grouped under a “species”, similar species under a “genus”, similar genera under a “family” and so on.

Classification is the production of a logical system of categories, each containing any number of organisms, which allows easier reference to its components. Classification is the arrangement of groups of plants with particular circumscriptions by rank and position according to artificial criteria, phenetic similarities, or phylogenetic relationships.

WHAT DO WE MEAN BY “SYSTEMATICS”?

Systematics is the branch of science that includes and encompasses traditional taxonomy (description, identification, nomenclature and classification of organisms) and phylogeny (evolutionary history) (Fig. 1.1). Plant systematics is studied by acquiring, analysing, and synthesizing information about plants and plant parts.

Fig. 1.1. Systematics includes taxonomy and evolution

DEVELOPMENTS AND SCOPE

Our knowledge of the biological world has changed greatly since Linnaeus, who first published his sexual system of classification in the Systema Naturae (1735). Since the publication of Species Plantarum by Linnaeus (1753) thousands of new species have been described and named every year. The availability of new tools and techniques has helped in new discoveries.

In the past few decades, there has been a renewed focus on the discovery of new taxa, publication of checklists, revisions, monographs and Floras. More floristic and inventory activities have been initiated than at any point in the history of taxonomy. Many collaborative research programs have been developed and training programs have been organized. The past half-century has also witnessed impressive advances in floristic inventorying, incorporation of new types of comparative data, and methods of phylogeny reconstruction and classification. Molecular data, tree-building algorithms, and statistical evaluations have revolutionalized the field of systematics.

Taxonomy is extremely modern, constantly changing and adapting, yet it has strong historical roots that always keep it connected to its past. In addition to inventorying during the past half-century, new types of comparative data of utility for plant systematics have been developed. In the 1950s, cytological data, especially chromosome number and basic karyotype, were emphasized. In the 1960s, secondary plant products (especially flavonoids), numerical taxonomy or phenetics reigned supreme. The 1970s and 1980s had focus on population - level questions with use of isozymes, that still provide good answers for solving particular types of systematic problems (e.g., hybridization). The application of computer techniques, which allowed more flexibility to handling data, were introduced in the early 1980s.

A key change in the field of taxonomy occurred with the development of cladistic theory and reconstruction of phylogenies, using cladograms, which greatly help to infer the evolutionary history of taxa. But the exciting new data came with analyses of DNA sequences and fragments in the 1990s and that has revolutionized the field of systematics. Molecular systematics and the development of methods in phylogenetic analyses have revolutionized our current understanding of relationships among plants and their patterns of diversification across time, space and form.

Current taxonomy represents a body of work that has accumulated over the past ~270 years, since the introduction of the binomial naming system by Linnaeus in the 1750s. The past half-century has witnessed impressive advances in floristic inventorying, incorporation of new types of comparative data, and quantitative concepts and methods of phylogeny reconstruction and classification. Innovations in use of molecular data, tree-building algorithms, and statistical evaluations have changed the field of systematics. The excessive amount of taxonomic information is now being digitized and is made available by various global initiatives.

The systematists have added different data sources to evaluate relationships, and have analysed the data using advanced computer software. When molecular systematics began its successful ascent, computer software were rapidly refined, because much larger data sets had to be processed. The new programmes were then used in morphology with more efficiency than before. Thus, morphological cladistics too has benefited from molecular cladistics. The use of several disciplines (e.g., cladistics, phylogenetics, genomics) helps taxonomy go beyond the naming of species to understand the evolutionary processes.

Next Generation Sequencing (NGS) has revolutionized molecular systematics as well as population and conservation genetics. Only a decade ago, phylogenetic matrices of three and four genes and several hundred taxa were considered large, with current NGS technologies, phylogenetic matrices based on hundreds of genes and involving thousands of species are readily feasible.

Significance of Systematics

• Plays a crucial role in inventorization of the earth’s biota.
• Provides a means of communication about the plants.
• Helps in identification of the vast flora.
• Plays important role in distinguishing different taxa.
• It is an integral part of other fields of biology.
• Makes it easy to relate one organism with another and hence helps in classification.
• Helps in testing various evolutionary hypothesis.
• Helps in preservation of germplasm for future breeding experiments.

AIMS AND OBJECTIVES OF SYSTEMATICS

The fundamental aim of systematics is to discover all the branches of the evolutionary tree of life, document all the changes that have occurred during the evolution of these branches, and describe all species-the tips of these branches. The major objectives of systematics are:

1. To name and describe the world’s organisms, thus completing our inventory of earth’s biota.
2. To provide a classification of these organisms that expresses evolutionary relationships among them.
3. To understand the patterns and processes of evolution that have created this enormous diversity of organisms.
4. To provide an integrating and unifying focus, a means of communication, for all fields of biology by safeguarding and disseminating this knowledge.

PHASES OF TAXONOMY: ALPHA TO OMEGA

The discipline of taxonomy is often divided into four phases:

1. Alpha taxonomy: The classical or alpha taxonomy relies mainly upon morphology and is descriptive. It is concerned with the collection, identification and description of taxa. The term was coined by Turrill (1935).
2. Beta taxonomy: The beta taxonomy focuses on arrangement of taxa into taxonomic groups or categories of classification. It is concerned with the identification of natural groups based on certain similarities and differences and uses this information for the purpose of classification. The term was coined by Ernst Mayr (1969).
3. Gamma taxonomy: The gamma taxonomy includes study of intraspecific populations, speciation, and evolutionary rates and trends. The purpose of this type of study is to interpret the biological diversity.
4. Omega taxonomy: The omega or modern taxonomy uses the data obtained from various biological disciplines like cytology, palynology, phytochemistry, etc. and gives information about the relationships among organisms. It is often referred to as a ‘perfected taxonomy’ as it focuses on having a broad information base for interpreting the relationships. The term was coined by Turrill (1938).

As taxonomy makes use of data from other fields like anatomy, embryology, palynology, phytochemistry, cytology, molecular biology, it is often referred to as a synthetic science. However, every biological discipline needs some level of basic understanding about the taxonomy of organisms and the information provided by taxonomic research forms the basis for all other fields of biology. It is therefore, aptly said by May (2004) that taxonomy is the brick with which the house of biodiversity is made.

BIOSYSTEMATICS

The term biosystematics was introduced by Camp and Gilly (1943) to understand the natural relationships of plants, particularly those of the rank of genus and below. Biosystematic studies include a thorough sampling of the taxon and its populations, and counting of chromosomes of many populations within geographic races, species, and genera. Differences in chromosome number, their morphology, and behaviour at meiosis usually indicate genetic differences of taxonomic significance. Another aspect included in the biosystematic study is the determination of the ability of the different populations to hybridize which provides data on the presence or absence of breeding barriers between groups.

The major objective of the biosystematic studies is to delimit the natural biotic units and to apply to these units a system of nomenclature adequate to the task of conveying precise...