CLASSIFICATION OF ORGANISMS
Introduction
The earth contains millions of different organisms, ranging from tiny bacteria to large animals such as elephants and whales. These organisms differ in their structure, size, method of nutrition, reproduction, habitat and other characteristics. Because of the enormous number of organisms, scientists need a systematic way of identifying, studying and naming them.
Classification is the scientific process of grouping organisms according to their similarities, differences and evolutionary relationships.
The branch of biology concerned with the identification, naming and classification of organisms is known as taxonomy.
Meaning of Classification
Biological classification is the arrangement of living organisms into groups and subgroups based on their similarities, differences and relationships.
Organisms that have many characteristics in common are placed in the same group, while organisms with significant differences are placed in different groups.
For example, humans, dogs, cats and lions are all mammals because they possess important characteristics such as having hair, possessing mammary glands and breathing with lungs. However, they are different enough to belong to different species.
Simple diagram of classification
LIVING ORGANISMS
│
┌──────────────┴──────────────┐
│ │
PLANTS ANIMALS
│ │
┌───────┴───────┐ ┌───────┴───────┐
│ │ │ │
Flowering Non-flowering Vertebrates Invertebrates
│ │ │
Maize Fern Mammals
│
┌───────┼───────┐
│ │ │
Human Dog Lion
Why Classification Is Necessary
Classification is important for several reasons:
1. It makes the study of organisms easier
There are millions of organisms in the world. Grouping them makes it easier for scientists and students to study them systematically.
2. It helps in identification
Classification provides characteristics that can be used to identify an organism correctly.
For example, an organism can be identified as a mammal if it has characteristics such as hair and mammary glands.
3. It avoids confusion caused by common names
An organism may have different common names in different languages or regions. Scientific names provide a universally accepted name.
For example, the organism commonly called "maize" in English is scientifically known as Zea mays.
4. It shows relationships between organisms
Classification can reveal how closely organisms are related. Organisms placed in the same smaller group usually share more characteristics than organisms placed in larger groups.
5. It helps scientists communicate
Scientists from different countries can use scientific names to refer to the same organism without misunderstanding.
6. It helps in predicting characteristics
When an organism is placed in a particular group, scientists can often predict some of its characteristics based on those of related organisms.
7. It helps in understanding evolution
Modern classification takes evolutionary relationships into consideration. It therefore helps scientists understand how different organisms may have evolved and how they are related.
Examples of Classification
Organisms can be classified into different groups based on their characteristics.
One traditional system divides living organisms into five kingdoms:
- Monera
- Protista
- Fungi
- Plantae
- Animalia
Modern biological classification may use systems with three domains and several kingdoms, but the five-kingdom system remains useful for introducing the basic idea of classification.
Five-kingdom classification
LIVING ORGANISMS
│
┌───────────┬───────────┼───────────┬───────────┐
│ │ │ │ │
MONERA PROTISTA FUNGI PLANTAE ANIMALIA
│ │ │ │ │
Bacteria Amoeba Mushroom Maize Man
Euglena Yeast Fern Dog
Examples
| Kingdom | Examples | Major characteristics |
|---|---|---|
| Monera | Bacteria | Mostly unicellular; cells lack a true nucleus |
| Protista | Amoeba, Euglena | Mostly unicellular eukaryotes |
| Fungi | Mushroom, yeast | Obtain food by absorption; do not photosynthesise |
| Plantae | Maize, mango, fern | Generally multicellular and photosynthetic |
| Animalia | Human, dog, fish | Multicellular; obtain food by ingestion |
Taxonomic Hierarchy
Classification is arranged in a series of groups called taxonomic ranks.
The major ranks, from the broadest to the most specific, are:
Kingdom → Phylum → Class → Order → Family → Genus → Species
In modern classification, Domain is placed above Kingdom.
Diagram of taxonomic hierarchy
DOMAIN
│
KINGDOM
│
PHYLUM
│
CLASS
│
ORDER
│
FAMILY
│
GENUS
│
SPECIES
As we move downward through the hierarchy, the number of organisms in each group decreases, while the similarities among the organisms generally increase.
BINOMIAL SYSTEM OF NAMING ORGANISMS
Meaning of Binomial Nomenclature
The binomial system of nomenclature is a scientific system of naming organisms using two names.
The word binomial means "two names."
This system was developed and popularised by the Swedish scientist Carolus Linnaeus.
Under this system, every species is given a two-part scientific name:
- Genus name
- Specific epithet
Together, these form the organism's scientific name.
For example:
Human — Homo sapiens
- Homo = genus
- sapiens = specific epithet
Rules of Binomial Nomenclature
There are important rules for writing scientific names.
1. The genus name comes first
The first part of the name is the genus and begins with a capital letter.
Example:
Homo sapiens
2. The specific epithet comes second
The second part begins with a small letter.
Example:
Homo sapiens
Not:
Homo Sapiens
3. Scientific names are written in italics
When typed or printed, both words are written in italics.
Example:
Zea mays
4. Names are Latin or Latinised
Scientific names are generally based on Latin or Latinised words, allowing scientists throughout the world to use the same naming system.
5. When handwritten, each word is underlined separately
For example:
Homo sapiens can be handwritten as:
Homo sapiens
_____ _______
Examples of Binomial Names
| Common name | Scientific name |
|---|---|
| Human | Homo sapiens |
| Domestic dog | Canis lupus familiaris |
| Domestic cat | Felis catus |
| Maize | Zea mays |
| Rice | Oryza sativa |
| Mango | Mangifera indica |
| Onion | Allium cepa |
| Housefly | Musca domestica |
The scientific name is useful because it remains more consistent than common names, which may vary between countries and languages.
Relationship Between Classification and Binomial Nomenclature
Classification and binomial nomenclature are closely related.
Classification groups organisms according to their characteristics and relationships, while binomial nomenclature gives each species a standard scientific name.
For example, the classification of humans can be represented as:
Domain → Eukaryota
Kingdom → Animalia
Phylum → Chordata
Class → Mammalia
Order → Primates
Family → Hominidae
Genus → Homo
Species → Homo sapiens
The scientific name Homo sapiens therefore gives information about the organism's position in classification: Homo is its genus, while sapiens identifies the species within that genus.
Importance of Binomial Nomenclature
The binomial system is important because it:
- Gives every species a unique scientific name.
- Prevents confusion caused by different common names.
- Makes communication among scientists easier.
- Indicates the genus to which an organism belongs.
- Helps scientists organise and identify organisms.
- Provides a universal system that can be used internationally.
Conclusion
Classification is an essential part of biology because it allows the enormous variety of living organisms to be organised into meaningful groups. It makes organisms easier to identify, study and compare and helps scientists understand their relationships and evolutionary history.
The binomial system of nomenclature, introduced and popularised by Carolus Linnaeus, provides a universal method of naming organisms using two parts: the genus and the specific epithet. For example, humans are scientifically known as Homo sapiens.
Therefore, classification and scientific naming work together to provide an organised and internationally recognised system for studying the diversity of life on Earth.
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