Classification
Since it is nearly impossible to study all the living organisms, it is necessary to devise some means to make this possible. This process is classification. Classification is the process by which anything is grouped into convenient categories based on some easily observable characters. For example, we easily recognise groups such as plants or animals or dogs, cats or insects. The moment we use any of these terms, we associate certain characters with the organism in that group. What image do you see when you think of a dog? Obviously, each one of us will see ‘dogs’ and not ‘cats’. Now, if we were to think of ‘Alsatians’ we know what we are talking about.
Similarly, suppose we were to say ‘mammals’, you would, of course, think of animals with external ears and body hair. Likewise, in plants, if we try to talk of ‘Wheat’, the picture in each of our minds will be of wheat plants, not of rice or any other plant. Hence, all these - ‘Dogs’, ‘Cats’, ‘Mammals’, ‘Wheat’, ‘Rice’, ‘Plants’, ‘Animals’, etc., are convenient categories we use to study organisms. The scientific term for these categories is taxa. Here you must recognise that taxa can indicate categories at very different levels.‘Plants’ – also form taxa. ‘Wheat’ is also a taxa. Similarly, ‘animals’, ‘mammals’, ‘dogs’ are all taxa – but you know that a dog is a mammal and mammals are animals. Therefore, ‘animals’, ‘mammals’ and ‘dogs’ represent taxa at different levels.
Hence, based on characteristics, all living organisms can be classified into different taxa. This process of classification is taxonomy. The external and internal structure along with the structure of cell, development process, and ecological information of organisms are essential and form the basis of modern taxonomic studies.
Hence, characterization, identification, classification, and nomenclature are the processes that are basic to taxonomy.
Taxonomy is not something new. Human beings have always been interested in knowing more and more about the various kinds of organisms, particularly with reference to their own use. In the early days, human beings needed to find sources for their basic needs of food, clothing, and shelter. Hence, the earliest classifications were based on the ‘uses’ of various organisms.
Human beings were, since long, not only interested in knowing more about different kinds of organisms and their diversities, but also the relationships among them. This branch of study was referred to as systematics. The word systematics is derived from the Latin word ‘systema’ which means the systematic arrangement of organisms. Linnaeus used Systema Naturae as the title of his publication. The scope of systematics was later enlarged to include identification, nomenclature, and classification. Systematics takes into account evolutionary relationships between organisms.
Taxonomic Category
Classification is not a single-step process but involves a hierarchy of steps in which each step represents a rank or category. Since the category is a part of the overall taxonomic arrangement, it is called the taxonomic category and all categories together constitute the taxonomic hierarchy. Each category, referred to as a unit of classification, in fact, represents a rank and is commonly termed as a taxon (pl.: taxa).
Taxonomic categories and hierarchy can be illustrated by an example. Insects represent a group of organisms sharing common features like three pairs of jointed legs. It means insects are recognizable concrete objects which can be classified, and thus were given a rank or category. Can you name other such groups of organisms? Remember, groups, represent categories. Category further denotes rank. Each rank or taxon, in fact, represents a unit of classification. These taxonomic groups/categories are distinct biological entities and not merely morphological aggregates.
Taxonomical studies of all known organisms have led to the development of common categories such as kingdom, phylum or division (for plants), class, order, family, genus, and species. All organisms, including those in the plant and animal kingdoms, have species as the lowest category. Now the question you may ask is, how to place an organism in various categories? The basic requirement is the knowledge of the characters of an individual or group of organisms. This helps in identifying similarities and dissimilarities among the individuals of the same kind of organisms as well as of other kinds of organisms.
Species
Taxonomic studies consider a group of individual organisms with fundamental similarities as a species. One should be able to distinguish one species from the other closely related species based on the distinct morphological differences. Let us consider Mangifera indica, Solanum tuberosum (potato), and Panthera leo (lion). All the three names, indica, tuberosum and leo, represent the specific epithets, while the first words Mangifera, Solanum, and Panthera are genera and represent another higher level of taxon or category. Each genus may have one or more than one specific epithets representing different organisms, but having morphological similarities. For example, Panthera has another specific epithet called tigris and Solanum includes species like nigrum and melongena. Human beings belong to the species sapiens which is grouped in the genus Homo. The scientific name thus, for the human being, is written as Homo sapiens.
Genus
Genus comprises a group of related species that has more characters in common in comparison to species of other genera. We can say that genera are aggregates of closely related species. For example, potato and brinjal are two different species but both belong to the genus Solanum. Lion (Panthera leo), leopard (P. pardus), and tiger (P. tigris) with several common features are all species of the genus Panthera. This genus differs from another genus Felis which includes cats.
Family
The next category, Family, has a group of related genera with still fewer similarities as compared to genus and species. Families are characterized on the basis of both vegetative and reproductive features of plant species. Among plants, for example, three different genera Solanum, Petunia, and Datura are placed in the family Solanaceae. Among animals, for example, genus Panthera, comprising lion, tiger, leopard is put along with genus, Felis (cats) in the family Felidae. Similarly, if you observe the features of a cat and a dog, you will find some similarities and some differences as well. They are separated into two different families – Felidae and Canidae, respectively.
Order
You have seen earlier that categories like species, genus, and families are based on a number of similar characters. Generally, order and other higher taxonomic categories are identified based on the aggregates of characters. Order being a higher category is the assemblage of families that exhibit a few similar characters. The similar characters are less in number as compared to different genera included in a family. Plant families like Convolvulaceae, Solanaceae are included in the order Polymoniales mainly based on the floral characters. The animal order, Carnivora, includes families like Felidae and Canidae.
Class
This category includes related orders. For example, order Primata comprising monkey, gorilla, and gibbon is placed in class Mammalia along with order Carnivora that includes animals like tiger, cat, and dog. Class Mammalia has other orders also.
Phylum
Classes comprising animals like fishes, amphibians, reptiles, birds along mammals constitute the next higher category called Phylum. All these, based on the common features like the presence of notochord and dorsal hollow neural system, are included in phylum Chordata. In the case of plants, classes with a few similar characters are assigned to a higher category called Division.
Kingdom
All animals belonging to various phyla are assigned to the highest category called Kingdom Animalia in the classification system of animals. The Kingdom Plantae, on the other hand, is distinct and comprises all plants from various divisions. Henceforth, we will refer to these two groups as animal and plant kingdoms.
The taxonomic categories from species to the kingdom have been shown in ascending order starting with species in Figure 1.1. These are broad categories. However, taxonomists have also developed subcategories in this hierarchy to facilitate more sound and scientific placement of various taxa.
Look at the hierarchy in Figure 1.1.
Figure 1.1 Taxonomic categories showing the hierarchical arrangement
Can you recall the basis of the arrangement? Say, for example, as we go higher from species to kingdom, the number of common characteristics goes on decreasing. Lower the taxa, more are the characteristics that the members within the taxon share. The higher the category, the greater is the difficulty of determining the relationship to other taxa at the same level. Hence, the problem of classification becomes more complex.
Table 1.1 indicates the taxonomic categories to which some common organisms like a housefly, man, mango and wheat belong.