Chapter 1
The living world is truly amazing! The wide variety of living things and the unique habitats they occupy—whether in cold mountains, deciduous forests, oceans, freshwater lakes, deserts, or hot springs—leave us speechless. The grace of a galloping horse, the journey of migrating birds, the beauty of a valley full of flowers, or the power of an attacking shark fill us with wonder. The ecological conflict and cooperation among members of a population and populations of a community or even the molecular traffic inside a cell make us deeply reflect on – what Indeed is life? This question has two implicit questions within it. The first is a technical one and seeks to answer the question of what living is as opposed to non-living. and the second is a philosophical one and seeks an answer to the purpose of life. As scientists, we shall not attempt to answer the second question. We will try to reflect on – what is living?
Diversity in the Living World
If you observe the world around you, you’ll notice a wide variety of living organisms—plants, insects, birds, pets, and other animals. There are also countless microscopic organisms that are present but not visible to the naked eye. As you expand the area of observation, the diversity of organisms increases. For instance, a dense forest would reveal an even greater variety of life forms. Each unique type of plant, animal, or organism represents a species. There are currently between 1.7 and 1.8 million identified species on Earth, a number that contributes to biodiversity. As new areas are explored and familiar ones are revisited, new species continue to be discovered.
With millions of species in the world, people often know local plants and animals by their regional names. These names differ from place to place, even within the same country, leading to potential confusion. Therefore, a standardized system is needed for naming organisms so that the same name is recognized worldwide. This process is called nomenclature, which requires accurate identification of the organism.
To streamline the study of organisms, scientists have established procedures to assign scientific names to every known species. These names are universally accepted by biologists. For plants, the International Code for Botanical Nomenclature (ICBN) governs the naming process, while for animals, the International Code of Zoological Nomenclature (ICZN) is used. Scientific names ensure that each organism has a unique name, which cannot be duplicated for any other known species.
Biologists follow universally accepted rules for assigning scientific names, which consist of two parts: the generic name and the specific epithet. This system, known as binomial nomenclature, was introduced by Carolus Linnaeus and is used worldwide. For example, the scientific name for the mango is Mangifera indica. In this case, Mangifera represents the genus, and indica is the specific epithet, referring to the species. This two-part naming system is not only practical but also universally understood.
- Biological names are usually in Latin and written in italics. They are either Latin or made to look like Latin, no matter where they come from.
- The first part of a biological name represents the genus, while the second part indicates the species.
- When written by hand, both words in a biological name are underlined separately. When printed, they are written in italics to show their Latin roots.
- The first word, which shows the genus, starts with a capital letter, while the second word, showing the species, begins with a lowercase letter. For example, Mangifera indica.
Name of the author appears after the specific epithet, 1.e., at the end of the biological name and is written in an abbreviated form, e.g… Mangifera indica Linn. It indicates that this species was first described by Linnaeus. Since it is nearly impossible to study all 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 characteristics. For example, we easily recognise groups such as plants or animals 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? 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 a taxa. ‘Wheat’ is also a taxa. Similarly, ‘animals’, ‘mammals’, and ‘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. External and internal structure, along with the structure of cells, 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 about their 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, for a 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 a 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 CATEGORIES
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 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 recognisable 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 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 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 human beings, is written as Homo sapiens.
Genus
The genus comprises a group of related species that have more characteristics 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 is family. has a group of related genera with still less number of similarities as compared to genus and species. Families are characterized based on 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, the genus Panthera, comprising lion, tiger, and leopard is put along with the 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 fewer in number as compared to different genera included in a family. Plant families like Convolvulaceae and 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 which includes animals like tiger, cat, and dog. Class Mammalia has other orders also.
Phylum
Classes comprising animals like fishes, amphibians, reptiles, and birds along with 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 kingdom have been shown in ascending order starting with species in Figure . These are broad categories. However, taxonomists have also developed sub-categories in this hierarchy to facilitate more sound and scientific placement of various taxa.
Look at the hierarchy in Figure 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. The lower the taxa, the more 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 indicates the taxonomic categories to which some common organisms like houseflies, man, mango, and wheat belong.
Summary
The living world is full of diverse plants and animals. While millions of species have been identified, many are still unknown. The wide variety of organisms, differing in size, color, habitat, and other features, makes it important to define what makes something living. To help study and understand this diversity, biologists have developed rules and principles for identifying, naming, and classifying organisms. This field of study is called taxonomy. Taxonomy is useful in agriculture, forestry, industry, and helps us understand and manage our biological resources. The basic processes of taxonomy—identification, naming, and classification—follow international rules. Every organism is given a scientific name based on the binomial naming system, and it is placed in a specific category in the classification system. These categories, called taxonomic categories or taxa, form a hierarchy.
Exercises
- Why do we classify living organisms?
- Why do classification systems keep changing?
- What criteria would you use to classify people you frequently meet?
- What do we learn from identifying individuals and populations?
- Below are two scientific names for mango. Identify the correctly written one. Mangifera Indica Mangifera indica
- What is a taxon? Give examples of taxa at different levels in the classification hierarchy.
- Can you identify the correct order of taxonomic categories?
Species Order Phylum Kingdom Genus Species Order Kingdom Species Genus Order Phylum - Try to collect all the currently accepted meanings for the word ‘species’.
- Define and understand the following terms: i) Phylum ii) Class iii) Family iv) Order v) Genus
- Explain the taxonomic classification of a plant and an animal with clear examples.