As the name simplifies this chapter is all about the living creature on this earth. Each organism is different from the other to a lesser or greater extent. Here we will discuss the criteria of the living organism, their identification, classifications and nomenclature i.e. their taxonomy, why organisms are grouped into some category or other.
A large variety of plants, animals, and microbes found on earth and they differ in size, shape, colour, habitat, and many other characteristics. So, studying each of them is impossible. Therefore, scientists have devised mechanisms to classify all living organisms on certain rules and principles that allow identification, nomenclature, and finally classification of an organism.
For example, based on presence of three pairs of legs and 2 pairs of wings, an organism is identified as an insect, it will be given a scientific name and then grouped with other similar organisms. Thus, various groups or taxon include organisms based on their similarity and differences.
Therefore, the biological classification helps in revealing the relationship between various organisms. It also helps in making study of organisms easy and organized.
Millions of plants, animals, and microorganisms found on earth, have been identified by the scientists while many new species are still being discovered around the world. Therefore, to classify these newly discovered species, with new characters, new systems of classification have to be devised every now and then. This creates the requirement to change the existing systems of classification.
To classify a class of forty students, let us start the classification on the basis of sexes of the students. This classification will result in the formation of two major groups- boys and girls. Each of these two groups can be further classified on the basis of the names of the students falling in these groups.
Since it is possible that more than one student can have a particular name, these names can be further suffixed with the surnames.
Since there is still some chance that more than one student can have the same surname, the final level of classification will be based on the roll numbers of each student.
The knowledge of characteristics of an individual or its entire population helps in the identification of similarities and dissimilarities among the individuals of same kind or between different types of organisms. It helps the scientists to classify organisms in various categories depending upon these similarities and dissimilarities.
In binomial system of nomenclature, the generic name of a species always starts with a capital letter whereas the specific name starts with a small letter. Therefore, the correct scientific name of Mango is Mangifera indica.
Each unit or category of classification is termed as a taxon. It represents a rank. For example, the basic level of classification is species, followed by genus, family, order, class, phylum or division, in ascending order. The highest level of classification is known as kingdom. So each of these categories can be called as a taxon.
| Phylum/Division | Class | Order | Family | Genus | Species |
| 1. Seed Plant | Flowering Plant | Yeast | Orchid | Citrus | Banyan |
| 2. Green Alga | Fern | Mushroom | Grass | Eucalyptus | Neem |
| 3. Flatworm | Insect | Spider | Bear | Crow | Tiger |
| 4. Roundworm | Bird | Lizard | Elephant | Wolf | Dog |
Both (a) and (c) represent correct sequences of taxonomic categories as the correct hierarchical arrangement of taxonomic categories in ascending order is
Species → Genus → Family → Order → Class → Phylum → Kingdom
In sequence (b), species should have been followed by genus. Therefore, it does not represent the correct sequence.
In biological terms, species is the basic unit of taxonomy. It can be defined as a group of similar organisms capable of interbreeding freely among themselves under natural conditions to produce fertile off springs.
Therefore, a group of similar individuals that are reproductively isolated from other group of individuals form a species.
Species can also be defined as group of individuals that share the same gene pool.
(i) Phylum
Phylum is second highest unit of classification after Kingdom. It includes one or more related classes of animals. In plants, instead of phylum, the term ‘division’ is used.
(ii) Class
Class is a taxonomic group consisting of one or more related orders. For example, the class, Mammalia, includes many orders like Primata (Man), Carnivora(Lim, tiger) etc.
(iii) Family
Family is a taxonomic group containing one or more related genera eg. Family hominidae contains Apes, Monkeys and Man. In plants, families are categorized on the basis of vegetative and reproductive features.
(iv) Order
Order is a taxonomic group containing one or more families. For example, the order, carnivora, includes many families.
(v) Genus
Genus is a taxonomic group including closely related species. For example, the genus, Solanum, includes many species such as nigrum, melongen a, tuberosum, etc.
Key is a taxonomical aid that helps in identification of plant and animal species. These keys are based on similarities and dissimilarities in characters, generally in a pair called couplet.
Each statement in a taxonomic key is referred to as a lead. For categorizing each taxonomic rank, such as family, genus, species, etc., different keys are used. It is also useful in identification of unknown organisms.
Keys are of two types- indented and bracketed keys. Indented key provides a sequence of choices between two or more statements while in bracketed key, a pair of contrasting characters are used.
(i) Indented key to identify different species of Rhododendron.
1. Leaves evergreen
2. leaves densely hairy below, orange or white hair; flower
appears to have separate petals
………………………. Rhododendron groenlandicum
2. hair absent on leaves, flower has five petals fused in a shallow
tube
………………………. Rhododendron maximus
1. Leaves deciduous
2. pink flowers with two free petals and three fused petals
………………………. Rhododendron canadense
3. white to pink flowers with all petals fused together
(ii) Bracketed key to identify different species of Rhododendron.
1. Leaves evergreen-----------------------------------------------2
1. Leaves deciduous-----------------------------------------------3
2. Leaves densely hairy below, orange or white hair; flower
appears to have separate petals
……….………………………. Rhododendron groenlandicum
2. Hair absent on leaves, flower has five petals fused in shallow
tube
………………………………..Rhododendron maximus
3. Pink flowers with two free petals and three fused petals
……………………………… Rhododendron canadense
3. White to pink flowers with all petals fused together-----4
The arrangement of various taxa in a hierarchical order is called taxonomic hierarchy.
In this hierarchy, species is present at the lowest level whereas kingdom is present at the highest level.
|
Kingdom ↑ Phylum or division ↑ Class ↑ Order ↑ Family ↑ Genus ↑ Species A Taxonomic hierarchy |
Classification of a plant
As an example, let us classify Solanum melongen a (Brinjal).
Kingdom – Plantae
Division – Angiospermae
Class – Dicotyledonae
Order – Solanales
Family – Solanaceae
Genus – Solanum
Species – melongen a
Classification of an animal
As an example, let us classify MAN
Kingdom – Animalia
Phylum – Chordata
Class – Mammalia
Order – Primate
Family – Hominidae
Genus – Homo
Species – Sapiens