There are few terms which come across when studying classification. These terms include systematic, taxonomy, classification and nomenclature.

To understand the principles used in classification these terminologies need to be clearly defined.


There are millions of living organisms in the world which exhibit great diversity and variations. Each one is different from the other in one way or another.

It is practically impossible to study and identify each and every organism. Biologists have designed a technique for identification, naming and grouping various organisms.

Systematic is a branch of biological science that deals with the study of the kinds of diversity of all organisms and their relationship.

The term systematic is derived from the Greek word ‘systema’ which simply means ‘that which is put together’.

Significance of systematic

  • Systematic gives the idea of organic diversity, its origin and evolution in the various kingdoms.
  • It helps in identification of living organisms.
  • It simplifies the study of biology.
  • It makes biology communication easier e.g. nomenclature.
  • It group together biology knowledge.
  • It helps us to understand the relationship between the organisms and their environment.
  • The subject is relevant to all sector of applied biology such as agriculture, forestry, medicine, fisheries and conservation of natural resources.

Systematic include taxonomy, identification, classification and nomenclature.


The word taxonomy is derived from the Greek words ‘taxis’ means arrangement and ‘nomos’ means law.

Taxonomy is a part of systematic which deals with the laws and principles on which classification is based.

Taxonomy can also be defined briefly as the study of classification.

It is a functional science concerned with identification, nomenclature and classification of different kinds of living organisms.

A taxonomist is the person who studies classification.


Identification refers to the determination that a particular organism is similar to some other known organism or individual.


Identification is the part of taxonomy that assigns an organism to its correct taxon.

It involves describing an organism using evidence from other branches of biology e.g. cytology, biochemistry, anatomy, ecology or morphology.

Example of identification

Consider three animals such as a, b and c. all belonging to different species. Another animal d having resemblance with animal b. The recognition of animals which is identical or similar to the already known animal b is considered its identification.

NB; much evidence need to be collected in order to assign a particular organism in a certain group.

Identification is done by using identification key/ biological key.


This refers to a schedule of characteristics data which can be matched or correlated with observable characteristics of an organism so as to identify it.


A set of observable characteristics that leads to the identification of an unknown organism

Importance of the biological key

  • Provide a convenient method which enable biologists to identify an organism and allocate different organisms in their correct taxa or group or more useful in identification of unknown organisms.

Types of biological key

There are two types of key;

  • Indented   key.
  • Bracketed key.

Indented key;This is the type of key which provides sequence of choices between two or more statements of characters of species.   The use has to make a correct choice for identification

Example of indented key



  • Single-ovuled, fruit achene, leaves opposite, compound petals absent.
  • Leaves with tendrils………………. Clematis.
  • Petals present third or terminal leaf let modified into tendrils……….naravelia.
  • Leaves alternate or radicle………….anemone
  • Many – ovuled; fruit follicle, carpel united at base;
  • Flowers regular……………………………..nigella
  • Carpel free at base; flower irregular………..Aconitium

Bracketed [dichotomous key]

This type of key provides a choice between two contrasting statements which lead into accepting one statement and rejecting the other.

The word dichotomous means – separating or branching into two.

There are two types of dichotomous key.

A] Branched [spider] key –this is diagrammatically represented. The key form two branches at each stage. This means that one is confronted with two possibilities at each stage. The description which fits the member of each new group is put at the end of new branch.

The problem of branched key is that they take up much space particularly if many organisms are involved

Branched key (figure below)


B) Numbered key

This consists of pairs of statements which are numbered. Each pair of statements is called lead or couplet. Each lead deals with a particular observable characteristic. The leads are numbered 1,2,3,4 etc, the paired statements of each lead is marked A and B.

The biological key can be short i.e. having one or two pairs of statements [leads] or it may   be long consisting of many leads.

By considering each lead at a time a large group of organisms may be broken down in progressively   small groups until unknown organism is identified.

NB; the two statements should be contrasting and mutually exclusive ( on the same character but opposing statements).

Rules used in constructing the dichotomous key.

  1. 1. Use morphological characteristics (observable features) as many as possible.
  2. Select characters that are in opposition to another so that the two statements of each lead comprise contradictory   propositions of which one fit the situation and other not apply.
  3. Select one character at a time and identify it by number e.g. 1,2,3,4, etc.
  4. The statements should be put positivity particularly the first one.
  5. Use identical first words for the two contrasting statement.

E.g.     I. a) scented flower………………….

b) Non scented flower ………………..

  1. Use macroscopic   morphological characteristics as far as possible in separating groups.
  2. Avoid generalization or overlapping variation i.e. be very specific in your description so that the two statement clearly represent different organisms e.g.
    1. Plant 1 meter and above tall.
    2. Plant 15cm to 60cm tall

Steps in construction of the dichotomous key

  1. Study the morphological characters of the organisms provided and prepare a table of differences and similarities in characters.
  2. Use the data you have tabulated to construct a dichotomizing tree of characteristics.


Given the following organisms, construct the dichotomous key to identify them: grasshopper, Housefly, butterfly, beetle, wasps, and cockroaches.

Morphological features

One pair of wings, two pairs of wings

Scale on wings, none scaled wings

Hard outer wings, without hard outer wings

Constricted waist, non constricted waist

Numbered key

  1. a) With two wings………………………………………. .Housefly

b) With four wings…………………………………………go to 2

  1. a) Scale on wings ………………………………….……..Butterfly

b) No scale on wings …………………………….………go to 3

3. a) Insect with hard outer wings………………………… Beetle

b) Insect without hard outer wings……………………go to 4

4. a) Insect with constriction between abdomen and chest……wasps.

b) Insect without constriction between the abdomen and chest……Grasshopper

Steps to follow on using a dichotomous key.

  1. Study the two couplets of the first lead and compare characters mentioned in the two couplets to the observable features of the organism in question.


  1. Read from one lead to another until you come to the lead with one of its couplet matching with the observable features on the organism.

Given the organisms below use key provide to identify them; spider, butterfly, snail, mosquito, snake, millipede, housefly, butterfly.

  1. a) Animal with wings…………………………….…..2

b) Animals without wings………………………….…3

  1. a) Body with scale………………………….……..…..4

b) Body without scale…………………………..……5

3. a) Body with legs……………………………………6

b) Body without legs …………………………………7

4. a) Organism with clubbed antennae ………………H

b) Organism without clubbed antennae………..…G

5. a) Organism with thin abdomen ………………..…C

b) Organism with broad abdomen ……………….F

6. a) Body elongated………………………………….E

b)Body not elongated………………………………A

7.  a) Animals withshell…………………………….B

b) Animal without a shell………………………….D

1a, 2a, 3a, 4a, – H (Butterfly)

1a, 2a, 3a, 4b, 5a – C (mosquito)

1b, 2b, 3b, 7a – B (snail)

Characteristics used contraction dichotomous keys in plants.

Below are some characteristics used to identify flowering plants and to construct dichotomous keys.

  1. Plant habitat i.e. being terrestrial or aquatic.
  2. Plant morphology i.e. the plant herb, shrub or tree
  3. Plant efflorescence e.g. are the flowers terminal or axial
  4. Structure of the flower

a. Simple or compound (have one carpel or many fused)

b. Number of floral parts in each whole

c. Fusion of floral parts

5.  The leaf

    1. Type of leaf (simple or compound)
    2. Types of venation (parallel or network)
    3. Margin (smooth or serrate)
    4. Arrangement on the stem i.e. alternate or opposite or whorled.
    5. Texture e.g. hairy or smooth.


Animals can be divided into vertebrates and invertebrates.


Feature that can be used include.

  1. Body covering i.e. scales, hairs or feathers
  2. Structures used for locomotion i.e. fins, wings or legs

iii) Position of the eyes i.e. frontal or lateral.


  1. Body symmetry arrangement of other part of the body in relation to the axis of the organisms e.g. radial or bilateral or asymmetrical
  2. Body segmentation
  3. Body covering with shells, exoskeleton.
  4. Appendages whether present or absent, types and numbers e.g. tentacles, antennae, wings, legs etc.


Characteristics used in a key are mainly based on

  1. Observable e.g. morphological   features
  2. Quantity e.g. number of hairs, legs, wings e.t.c
  3. Qualitative e.g. shape of abdomen.


  • ICB – international code of Botanical Nomenclature

ICZN – international code of zoological Nomenclature

ICVN – for virus Nomenclature

Nomenclature is defined as the system of naming the organisms. It is a branch of taxonomy which deals with giving or assigning biological name (scientific name) to the organism once it is identified.

Significance of using scientific names

  1. A scientific name is universally used for a particular species or particular group of organisms. This help to avoid the confusion which may arise by use of local (or vernacular) names which differ in different parts of the world.
  2. Scientific names give descriptive information about the species; the taxonomist can determine from the description exactly the kind of organism to which the name has been given.
  3. The names used are uniformly binomial specifying the name of the genus and species of the organism hence organisms with similar evolutionary history are classified together.
  4. The scientific name allows information about organisms to be organized and found easily
  5. Help biologist to avoid errors in communication.
  6. Rules followed in the procedure of scientific naming favors stability.


The system adopted international in assigning scientific name is the Binomial system of nomenclature. This is the standard system of nomenclature.

It is a system of naming where the organism is given two words name the first word standing for the genus (generic name) followed by the second word denoting species (the specific name)

The system was introduced by carolus Linnaeus (1707- 1778) a Swedish naturalist.

The term binomial come from two Latin words (bi=two; nomen = name)

Rules of Binomial nomenclature.

Scientific names are assigned based on rules framed and standardized by the international bodies of Biological nomenclature. For example there are international codes of Botanical nomenclature (ICBN), the international code of Botanical nomenclature (ICZN) e.t.c

These codes are useful in avoiding errors, duplication, confusion and ambiguity in scientific names.

A few commonly followed rules are given bellow;

    1. Every organism can have only one scientific name.
    2. A scientific name of each species is binomial name. This is made up of two parts, a specific (species) name and a generic (genus) name. For example scientific name of man is Homo sapiens.

Homo – generic name, sapiens – specific names

    1. The scientific name should be in italics if printed and underlined if hand written
    2. The generic name should always begin with a capital letter but the specific name should not. For example the name of the mango plant is Mangifera indica.
    3. All scientific names should be in Latin or Latinized
    4. The name of the author should be in most cases included at the end of the biological name. If the Author is well known e.g. Linnaeus only the first letter is written at the end e.g. Canis lupus (L), Homo sapiens .L or (Linn) where L or Linn is abbreviated form of Linnaeus
    5. When several different names have been given to an organisms, the earlier name that published after Linnaeus system of classification is to be considered.
    6. Within a kingdom, no two genera can have the same name and within a genus, no two species can the same name.


Example of scientific names

Scientific names                        common name

Homo sapiens                            Human

Panthera tigris                           Tiger

Canis familiars                           Dog

Musca domestica                        Housefly

Periplaneta americana                cockroach

Columba livia                             pigeon

Mangifera indica                         mango tree

Triticum aestivum                       wheat

Ananas comosus                         pineapple

Carica papaya                            paw paw

Hibiscus rose                              China rose

Citrus sinensis                            orange

Lycopersion esculanta                  Tomato

Persea Americana                        Avocado

Zea mays                                    Maize

Pisum sativum                              pea

Classification refers to the method of identifying distinctions (distinguishing characters) among different organisms and placing them into groups that exhibit their most significant features and relationship.   Or classification refers to the branch of taxonomy that deals with the study of grouping living organism into series according to their resemblance and differences.


  1. To identify the different kinds of the living organisms on earth
  2. To arrange the different kinds of the living organisms into scheme that would show the true relationship among the organisms.
  3. To recognize the difference and show similarities between organisms.

The significance of classification

  1. It organizes the huge number of organisms into categories that could be named, remembered and studied. It is difficult to know everything about all living organisms. classifying them into groups according to their similarities make it easier to study their characteristics
  2. Classification helps us to understand the kinds of diversity that occur in living organisms and their relationship.


In classification organisms are put into groups, categories or ranks based on their similarities and differences. The main objective of taxonomy is to put organisms in different groups which show their evolutionary relationship

The ranks or categories used in classification are known as taxa(in plural) (singular taxon)

The taxa or taxonomic groups are arranged in hierarchical order i.e. from the largest group to the smallest group (descending sequence)

Taxonomic hierarchy refers to the sequential arrangement of the taxonomic group in definite order from higher to lower categories.

The taxonomic hierarchy consists of seven main categories or taxa. These are the kingdom, phylum, class, order, family, Genus and species. In plants however the division is used in categories in place of phylum while remaining categories are the same.

Linnaeus was the first taxonomist to establish a definite hierarchy of taxonomic ranks.

Sub – categories have been introduction to make a precise taxonomic position of the species. The prefix super is put for the category above the taxonomic category e.g. super class, super order etc. the prefix sub is added to make a lower category than the one existing e.g. sub order, sub family etc.

The figure below is an example of taxa or hierarchy of taxonomic ranks


Characteristics of the taxonomic hierarchy

  • Members of a taxon show similar characteristics which are different from other taxa.
  • As you go up the hierarchy organisms show much differences and few similarities. With that they evolved earlier from their ancestors.
  • Down the hierarchy organisms show many similarities which these they show to have evolved recently from their ancestors
  • As one descend along the hierarchy of ranks the size of the group decreases and vice versa

Taxonomic categories

  • The kingdom: This is the highest taxonomic category. it includes all organism that share a set of distinguishing common characters.
  • The phylum / Division: The term phylum is used for animals and division for plants.The phylum consists of closely related classes. A phylum can have one or more classes.
  • Class: Class is a group of closely related orders. For example class dicotyledonae of flowering plants include all dicots which are grouped into several orders such as Rosales, passiflorales, pelmoniales e.t.c.
  • Order: Consists of a group of closely related family.
  • Family: Family is a group of closely related genus
  • Genus: Genus is a group of closely related species
  • Species: Specie is the lowest rank of classification or A species refers to a group of organisms which are similar and can freely interbreed to produce fertile offspring this means that the species is reproductively isolated from other species.

Fertile– means capable of having offspring i.e. members of the same species can, if left alone in their natural environment, mate with one another to produce offspring which in turn, are also able to produce offspring.

Exception of the species definition.

If a male tiger mate with female lion production an offspring which is fertile (can reproduce)

  • male lion and female tiger produce offspring (the liger )which is infertile
  • male donkey and female horse produce infertile offspring (the mule)
  • male horse and female donkey produce offspring the (hinny) infertile

Example of taxonomic hierarchy

1. Domestic cat

Kingdom         –         Animalia

Phylum           –         chordata

Class              –         mammalia

Order              –         carnvera

Family            –          Felidae

Genus             –         Felis

Species           –         Felis gatus


Kingdom         –          Animalia

Phylum           –         chordata

Class              –         mammalia

Order             –         primate

Family            –         Homonidae

Genus            –         Homo

Species          –         Homo sapiens


Kingdom        –           plantae

Division         –           Angiospermaphyta

Class             –           Dicotyledoneae

Order            –           Sapindales

Family          –            Anacardiaciae

Genus            –           mangifera

Species          –          Mangifera indica


Illustrate the concept of taxonomic hierarchy for the following organisms.

Dog, maize, earth worm and Honey bee.


There are two different types of classifications which have been proposed so far by different taxonomic

These are as follows:

  • Artificial system of classification
  • Natural system of classification

1.Artificial system of classification

  • This system of classification / method of classifying organisms was based on one or two (few) superficial similarities or based on mere easy features of identification


The system which was based on simple arbitrary chosen criterion instead of an evolution of totality of character.

For example Aristotle (384 – 322; BC) a Greek Biologist produced the first classified animals by looking the way they move e.g. bats, insects and birds were grouped together based on their ability to fly.

He classified plants based on the appearance and size e.g. he grouped plants into herbs, shrubs and trees.

Other criteria could be used for example utility where we have edible and non edible plants, medicinal and non medicinal plants etc.

An artificial system of classification is based on mere easy features of identification.

Advantages / Merits

  1. It is useful for quick reference and identification of organisms
  2. Identification requires few characteristics which are not difficult to study and apply.
  3. Organisms which are poorly known can be filled in the system very easily
  4. It is a time saving since it is quick and easy to prepare.
  5. It is very stable, does not subjected to changes.

Disadvantages / Demerits

  1. The system did not reflect any natural relationship existing among the organisms
  2. The system limit amount of information about members i.e. it rigid
  3. It did not consider many important characteristics and could not explain the evolution significance
  4. The system put unrelated organisms under one heading
  5. It cannot not add any information about an organism.

2. Natural system of classification

  • This is the system of classification based on characters of similarities which indicate natural relationship. The basis of a natural classification is similarities
  • The system of classification based on the real nature of organism (s)
  • In this system more numbers of characters rather than single character are used for determining similarities.For example phylogenetic and phenetic.

Phylogenetic system of classification.

This system of classification is based on evolution and generic relationship among organisms. This system enables us to find the ancestors or derivatives of any taxonomy. It reflects the true relationship among organisms. The system was proposed by two Germany botanists Adolf Eagler (1844- 1930) and Karl A.E prantl (1887- 1893)

  • Relationship based on the evolutionary aspect of organisms. Darwin’s concept; tells on regional relationship among organisms
  • If relies on fossils records and is dynamic.
  • They vary between zoologists and botanists as zoologists rely on structural aspect while Botanists don’t.

Phenetic classification

  • Type of phylogenetic classification which relies on similar and dissimilar features present in today’s organism without including evolutionary and other reflected aspects.
  • Cladistics.

The system is based on similarities of form known as phonetic. Phonetic system of classification emphasizes overall similarity. Phenetic reflect evolutionary relationship.

The character taken for identifying similarities are homologous not analogous structures.

  • Homologous structure – these are structural or biochemical features which are shared between organisms by virtue of common ancestry e.g. backbone, pentadyctyl limb plan of mammals, birds and reptiles.
  • Analogous structure- structural features which have the same function but basically different in structure e.g. wings of bats, birds and insects.

Features studied in natural classification

  1. Various features of the organisms like external form (morphology ), internal structure (anatomy), cell structure (cytology) life process (physiology), biochemistry e.t.c
  2. Homologous organs.
  3. Molecular similarities ( DNA, RNA and comparative proteins).

Advantages of natural system of classification

  1. Closed related species are placed in the same group and hence reflect the evolutionary relationship
  2. Many characteristics are considered in classifying organisms by so doing it is more accurate than artificial classification
  • Like artificial system of classification, it is useful for reference identification of organisms.
  • It is flexible hence can easily accommodate newly discovered organisms.
  • It is based on biological important similarities of organisms.
  • it offer information (knowledge ) about organisms
  • The system brings out natural relationships among organisms.

Disadvantages of natural system

  • It is time consuming since it takes more time and much effort also many attribute are taken into consideration.
  • It is difficult as too much information is needed to group organisms.
  • the system is not stable since it is liable to change with time increase of knowledge of living organisms
  • Poorly known organisms are not easy to classify.
  • It always need a skilled personnel hence it is expensive since identification use scientification and use of scientific instruments.

The phylogenetic system of classification.

This system of classification is based on evolution and genetic relationship among organisms.

This system enables us to find out the ancestors or derivatives of any taxon. It reflects the true relationship among the organism.

The system was first proposed by two German botanists Adolf Eagler (1844 – 1930) and Karl A. E prantl (1887 – 1893)

Problems encountered in classifying living organisms

    1. The process is time consuming hence tiresome work
    2. Some organisms may show living and non living properties hence difficult to place them in any group e.g. viruses
    3. The process always need skilled personnel and instrument to place living organisms to their correct group therefore it is very expensive.
    4. Sometimes evolutional information about an organism can be missing hence difficult to classify.
    5. Classification is subject to change.


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