Taxonomy | Definition, Classification & Example

The process of classification of living organisms is not a single step process but it involves a number of steps, i.e., 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. Therefore, each category is referred to as a unit of classification. It also represents a rank in taxonomic hierarchy.

Taxon (plural: taxa) represents any level of grouping of organisms based on certain easily observable common characteristics like insects represent a group of organisms which share some common features like three palirs of jointed legs, hence insects can be uniquely recognised and classified. So, they were given a deinite rank or category in taxonomic hierarchy. Likewise, birds represent that group of organisms which have feathers, beak, hollow bones, etc., due to which they are kept in separate rank or category in taxonomic hierarchy.

We can memorise this, by remembering that each rank or taxon represents a group of organisms which share some unique features among members of their group or category which are distinct from organisms kept in various other ranks or taxon. Hence, these taxonomic groups or categories form distinct biological entities and should not be thought as random aggregates of animals based on their morphological features.

Diverse kinds of organisms known to scientists are studied from taxonomical aspect which led to the development of some definite categories. These definite categories or ranks in classification of plants, animals and other organisms are :

1. Kingdom
2. Phylum (for animals) or division (for plants)
3. Class
4. Order
5. Family
6. Genus
7. Species

These seven obligate categories in which all living organisms are classified are arranged in a descending sequence starting from kingdom upto species or in an ascending order from species to kingdom. The number of similar characters of categories decreases from lowest rank, i.e., species; to highest rank, i.e., kingdom.

Specificity decreases when we go from species to kingdom, i.e., the higher the category, lesser will be the number of similar characteristics of organisms belonging to that category, while when we go from kingdom to species, i.e., in an descending order the number of similar characteristics of organisms increases, i.e., specificity increases. In taxonomic hierarchy, taxonomic groups, i.e., taxa are arranged in a definite order, from higher to lower categories and in which species serves as the basic and lowest category.

To categorise an individual or group of organisms in a definite rank, we should have all the basic knowledge of its characteristics. This would help us in identifying the similarities and dissimilarities among the individuals of the same kind of organisms as well as of other kinds of organisms. Here, we will explain all the seven broad or obligate categories of taxonomic hierarchy.

However, taxonomists have also developed some sub-categories like sub-phylum or sub-division, etc. in this hierarchy to make this arrangement more transparent and scientifically useful.

Species

It is a natural population of individuals or group of populations which resemble one another in all essential morphological (i.e., body form, size, shape, etc.) and reproductive characters so that they are able to interbreed freely in nature to produce fertile offsprings. On the basis of taxonomic studies, a group of individuals/populations with fundamental similarities are referred to as species. A species can be easily distinguished from other closely related species on the basis of their distinct morphological differences.

Let us consider, Solanum tuberosum (i.e., potato), Canis familiaris (dog) and Panthera leo (lion). In all these three names, .e., tuberosum, familiaris and leo represent the species, while the first words or names Solanum, Canis and Panthera represent the genus, which is next higher taxon than species in the taxonomic hierarchy.

Due to the similarities in some morphological features, one or more species are kept in a genus. For example, Solanum has another specific epithets than tuberosum, like nigrum and melongena. Canis has another specific epithets than familiaris like lupus and aureus. Panthera has another specific epithets than leo like tigris and pardus.

All the species represent the different organisms which are kept in the same genus due to some similarities in their morphological features but are assigned to different specific epithet due to some other distinct features like habitat or colour or size, etc.

Genus

Genus is a group of related species, which has more characters in common in comparison to species of another genera. For example, potato, makoi and brinjal are three different species but all belong to the same genus Solanum. Species like lupus, familiaris and aureus belong to genus Canis. Likewise, genus Panthera consists ot species like leo, tigris and pardus which have ability to roar as a similar character.

The different species of Canis genus resemble to each other in some characters but are different and easily distinguishable from many other species of Panthera genus. So, we can conclude that all the related species in one genus have a common ancestory, so that they have a few differences but many similarities, to be grouped in one genus. Hence, the aggregates of closely related species is a genus.

When we consider human beings, they belong to the species sapiens, which is grouped in genus Homo. Thus, our scientific name is written as Homo sapiens.

Family

The next higher category in the taxonomic hierarchy after genus is family. It includes various groups of related genera, which share less number of similarities as compared to that at genus and species level. As we have already studied that when we move from lower taxon towards higher one in hierarchy, the number of similar characters decreases. For instance, on basis of vegetative and reproductive features of some plant species, they are grouped into three different genera Solanum, Petunia, Datura and these three different genera are kept in a single family, i.e., Solanaceae due to some other common features or correlated characters. They all are distinguishable from the genera of the related family Liliaceae which includes two different but related genera, ie., Alium and Colchicum. Similarly, among animals, the genus Felis of cats and the genus Panthera including Iion, tiger and snow leopard are placed under a common family — Felidae. At same time, if we observe the features of a cat and a dog, we will find that they both are quite distinct. So, these are kept in separate families Felidae (cats) and Canidae (dogs).

Order

It is a group of related families that means related families are kept in the same order. For example, the family Felidae of cats and family Canidae of dogs are assigned to the same order Carnivora. As they both have canine teeth which is a similar feature found in animals of both the families.

Therefore, order is assemblage of families which exhibit a few similar characters inspite of many differential features. The similar characters are less in number as compared to different genera included in a single family. Different plant families like Convolvulaceae, Solanaceae are included in the order Polymoniales along with three other related families. The common feature of all different families placed in a single order, i.e, Polymoniales is similar floral characters found in them.

Class

A class is a group of one or more related orders. For example, the order Rodentia of rats, Primata of monkey, gorilla and gibbon, and Carnivora of cats and dogs are all placed in the same class Mammalia. All these orders are kept under the same class because members of these orders have milk-producing glands and hair on their skin. We can say that class is the assemblage of orders which exhibit a few similar characters.

Phylum/Division

It is a category higher than that of class. The term phylum is used for animals, while division is commonly used for plants. Phylum or division is a group of related classes, i.e., it is formed of one or more classes. Different classes, comprising animals like fishes, amphibians, reptiles, birds and mammals, together constitute the next higher category known as “Phylum“. All of these different organisms are placed in the phylum Chordata as all of the animals in these classes have some common characters like presence of notochord atleast at some stage of their lives, dorsal hollow neural system, etc. In case of plants, Classes with a few similar characters are assigned to higher category called division.

Kingdom

It is the highest category in taxonomic hierarchy. It is a group of phyla (in plural) or divisions, which have certain basic common characteristics. Animals present in all phyla lack cell wall and chlorophyll, are placed in the kingdom Animalia. All plants present in different divisions have rigid cell wall around their cells and synthesize their own food by photosynthesis, are included in the kingdom Plantae. Hence, we can call these two broad categories of plants and animals as plant and animal kingdoms.

So, now we can recall the basis of arrangement of organisms in taxonomic hierarchy. As we go higner 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. Higher the category. greateris the difficulty of determining the relationship to the other taxa at the same level. Hence, the problem of classification becomes more complex.

Table: Organisms with their taxonomic categories

Common names	Biological Name	Genus	Family	Order	Class	Phylum OR Division	Kingdom
Man	Homo sapiens	Homo	Hominidae	Primata	Mammalia	Chordata	Animalia
Housefly	Musca domestica	Musca	Muscidae	Diptera	Insecta	Arthropoda	Animalia
Mango	Mangifera indica	Mangifera	Anacardiaceae	Sapindales	Dicotyledonae	Angiospermae	Plantae
Wheat	Triticum aestivum	Triticum	Poaceae	Poales	Monocotyledonae	Angiospermae	Plantae
Brinjal	Solanum melongena	Solanum	Solanaceae	Polymoniales	Dicotyledonae	Angiospermae	Plantae