Biology - Systematics of Living Organisms

Introduction :

There is great diversity of organisms around us. Since time immemorial, we humans have been exploiting this wealth for our own benefit. During this process man tried to differentiate between and identify the organisms. Eventually this evolved into a branch of biology known as systematics or classification. The methods of classification dates back to ancient time when Indian, Greek and Roman philosophers have contributed their might to systematise science. 

Systematics :

‘‘Systematics is the study of kinds and diversity of organisms and their comparative and evolutionary relationship’’(G. Simpson, 1961).

Taxonomy :

Taxonomy means classification following certain rules or principles. Word Taxonomy comes from two Greek words, taxis – meaning arrangement and nomous meaning law or rule. The term taxonomy was first introduced by A. P. de Candolle (Swiss Botanist) [1778-1841].

Classification :

It is the arrangement of organisms or groups of organisms in distinct categories in accordance with a particular and well established plan. This classification is based on similarities and dissimilarities among the organisms. 

Artificial : 

It is the classification that is based on few easily observable and non-evolutionary featurs such as habit, colour, form, etc.; often irrespective of their affinity (relationship) with other organisms. e.g. Linnaeus system of classification. 

Natural : 

It is the classification which is based on objectively significant rather than being selected for convenience like artificial system of classification e.g. Bentham and Hooker's system of classification.

Phylogenetic : 

It is the classification based on common evolutionary descent. e.g. Engler and prantles classification. In the system of classification the terms like ‘taxa’ and ‘categories’ are often used. Each category is referred to as a unit of classification. In fact, it represents a rank and is commonly termed as taxon.

Phylogenetic descent                          

Three domains of life :

It is believed that the life originated on earth in its very simple form. Constant struggle of the early living beings gave rise to more and more perfect forms of life. This struggle and progress is evolution which led to formation of diverse life forms. Carl Woese in 1990 proposed three domains of life to classify life forms. They are Archaea, Bacteria and Eukarya. Domain is an unit larger than Kingdom in the system of classification.

Bacteria and Archaea both have prokaryotic cells where as Eukarya have eukaryotic cell. All the three domains have very unique ribosomal RNA (rRNA). Archaea are known for their survival in very extreme conditions like high tempreature, salinity, acidic conditions, etc. Bacteria, though are prokaryotes differ from Archaea in structure of cell wall.

Chemotaxonomy :

It is method of biological classification based on similarities and differences in structure of certain compounds present among the organisms being classified. In short, it is the classification based on chemical constituents of organisms. e.g. Archaea cell wall is without peptidoglycan and that of Prokarya is with peptidoglycan. Among Eukarya, fungi have chitinous cell wall while plants have cellulosic cell wall. 

Numerical taxonomy : 

The system is based on quantification of characters and develops an algorithm for classification. The basic aim of this taxonomy was to create a taxonomy using numeric algorithms like cluster analysis rather than using subjective evaluation of their propertise. This system was first proposed by Sokel and Sneath in 1963.

Cladogram :

It is a typical branching pattern. As shown on previous page, a diagram of three domains of life is a cladogram. It represents a hypothetical relationship denoting a comparison of organisms and their common ancestors. 

Phylogeny :

It is evolutionary relationship of organism. It is an important tool in classification as it takes into account not merely the morphological status but also the relationship of one group of organism with other groups of life. The system helps to understand the evolution and also focuses on the similarities of their metabolic functioning. Woese’s three domain concept as well as Whittakar’s five kingdom system are very good examples of phylogenetic relationship.

DNA barcoding :

DNA barcoding, is a new method for the identification of any species based on its DNA sequence from a tiny tissue sample of the organism under study. It helps to study newly identified species as well as understanding ecological and evolutionary relationships between living beings. The process of DNA barcoding includes two basic steps: (a) collecting DNA barcode data of known species and (b) matching the barcode sequence of the unknown sample against the barcode library for identification. DNA barcoding has many applications. A few to mention are, protection of endangered species, preservation of natural resources, pest control in agriculture, identifying disease vectors, authentication of natural health products and identification of medicinal plants.

Taxonomic Categories :

Classification is not a single step process but involves hierarchy of steps in which each step represents a rank or category. Since the category is a part of overall taxonomic arrangement it is called taxonomic category and all categories together constitute the taxonomic hierarchy. Kingdom, division, class, order, family, genus, species are the categories in hirarchial sequence. These are compulsory categories. Besides, there are some facultative categories like sub-order, sub-family, etc. to be used as per need.

Taxonomic Hierarchy :

Taxon : 

A taxon is the taxonomic group of any rank in the system of classification (H.J. Lam 1948) e.g. in plant kingdom each one of the following such as Angiosperms, Dicotyledonae, Polypetalae, Malvaceae represents a taxonomic group i.e. a taxon.

Classification of China-rose and Cobra 

Units of Classification :

• Species : 

Species is the principal natural taxonomic unit, ranking below a genus and denoted by latin binomial (considered as the basic) unit of classification. It is a group of organisms that can interbreed under natural condition to produce fertile offspring. It was thought to be an indivisible, stable and static unit. However in the modern taxonomy, sub￾division of species such as sub-species, varities and populations are seen and given more importance.

• Genus : 

Genus is a taxonomic rank or category larger than species used in the biological classification of living and fossil organisms. Genus is a group of species bearing close resemblance to one another in their morphological characters but they do not interbreed. e.g. Tiger, Leopard, Lion all three belong to same genus Panthera. They have common characters yet are different from each other because their genus is same but species is different. Another example is genus Solanum. Brinjal and potato both belong to this genus.

• Family : 

It is one of the major hierarchial taxonomic rank. A family represents a group of closely related genera. e.g. genera like H ibiscus, Gossypium, Sida, Bombax are included in same family Malvaceae. Cat also belongs to family of leopards, tigers and lions, family Felidae but dog belongs to different family Canidae.

• Cohort / Order : 

It is taxonomic rank used in the classification of organisms and recognised by nomenclature codes. An order is a group of closely related families showing definite affinities. Order thus is a step above family in taxonomic hierarchy. Members belonging to same order but different families may show very few dis similarities. e.g. family - Papavaraceae, Brassicaceae, Capparidaceae, etc with parietal plancentation are grouped in order Parietales. Families of dogs and cats though are different, they belong to same order Carnivora. 

• Class : 

The class is the distinct taxonomic rank of biological classification having its own distinctive name. A group of higher taxonomic rank than order. Class is the assemblage of closely allied orders. Orders Carnivora and order Primates belong to class Mammalia. Thus monkeys, gorillas, gibbons (Primates) and dogs, cats, tigers (Carnivora) belong to same class.

• Division / Phylom : 

The division is a category composed of related classes e.g. division. Angiospermae includes two classes - Dicotyledonae and Monocotyledonae (In animal classification division is a sub-unit of Category / Phylum).

• Sub-kingdom : 

Different divisions having some similarities form sub-kingdom. e.g. The divisions Angiospermae and Gymnospermae will the sub-kingdom Phanerogams or Spermatophyta.

• Kingdom : 

It is the highest taxonomic category composed of different sub￾kingdoms. e.g. sub-kingdom Phanerogams and Cryptogams form the Plant kingdom or Plantae which includes all the plants while all animals are included in kindom Animalia.  The taxonomic categories we have considered so far are broad categories. Scientists have added sub-categories to these in order to place organisms in more scientific manner. You will observe that as we go higher in taxonomical ladder, number of common characters go on decreasing. 

If we are comparing two organisms that are related to each other only at division or phylum level, their classification may become difficult. 

Nomenclature : 

Any object that becomes known to human intelligence must possess a name. It may not be possible or convenient to describe it in order to communicate ideas about it. The art of naming the objects is in fact, a science called nomenclature. All living organisms are known by a particular name. 

1. Vernacular / Local names / Common names :

Widely distributed organisms have a large number of common names. Pansy (V iola tricolor L.) grown in most European and American gardens has about 50 common English names. In a multilingual country like India, almost all useful plants have local names which differ from language to language and even from dialect to dialect. As in Ayurveda,mango (Mangif era indica L.) is known by over 50 different names, all in the Sanskrit language. Hence the common names obviously have limited usage and for universal applications, a unique name for a particular individual is very much essential.

2. Scientific Names : 

To overcome the difficulties raised by common names, scientists have given scientific names to all the known organisms. These are systematic, thus provide means for international communication. Initially the polynomial system was used but Carl Linnaeus used binomial system of nomenclature. He introduced this system in his book ‘‘Species Plantarum’’ published in 1753. International Code of Botanical Nomenclature (ICBN) has been set up to confirm the scientific names.

According to this system the scientific name of sunflower is H elianthus annus. In the above H elianthus indicates name of the genus (generic name) and second word annus denotes name of the species.

The Binomial Nomenclature system follows certain rules : 

• Name of organism is composed of two Latin / Greek words. 

• Generic name is a simple noun. It should come first and begin with capital letter. 

• Specific name is the descriptive adjective which should come later and begin with small letter. 

• Scientific names must be underlined separately if hand written and must be printed in italics. 

• The generic and specific name should not have less than three letters and more than thirteen letters.  

• Usually the name of the author who names a plant or animal is also written in full or abbreviated form after scientific name. e.g. Mangif era indica L. Where L stands for Linnaeus.