Binomial nomenclature is the scientific system of naming each species using two words. The first word represents the genus, and the second is the specific epithet. Rules:

• The genus name begins with a capital letter.
• The specific epithet begins with a lowercase letter.
• Both words are italicised when printed.
• When handwritten, the two words are underlined separately.
• Scientific names are Latin or latinised.
• The author’s abbreviated name may follow the scientific name in Roman type.

Examples: Homo sapiens (human), Musca domestica (housefly) and Mangifera indica (mango).

A taxon is a group of organisms recognised as a unit at any level of classification. A taxonomic hierarchy is the ordered arrangement of taxonomic categories. The seven main categories, from lowest to highest, are:

1. Species: The basic unit of classification.
2. Genus: Groups closely related species.
3. Family: Groups related genera.
4. Order: Groups related families.
5. Class: Groups related orders.
6. Phylum or Division: Groups related classes. Phylum is commonly used for animals and division for plants.
7. Kingdom: The broadest category in this traditional hierarchy.

Sequence: Species → Genus → Family → Order → Class → Phylum/Division → Kingdom.

The taxonomic classification of humans, from lowest to highest, is:

• Species: Homo sapiens
• Genus: Homo
• Family: Hominidae
• Order: Primata
• Class: Mammalia
• Phylum: Chordata
• Kingdom: Animalia

Each higher category includes organisms that share broader characteristics.

The taxonomic classification of the housefly is:

• Species: Musca domestica
• Genus: Musca
• Family: Muscidae
• Order: Diptera
• Class: Insecta
• Phylum: Arthropoda
• Kingdom: Animalia

This classification places the housefly in the kingdom Animalia, phylum Arthropoda and class Insecta.

A taxonomic key is an identification tool that uses contrasting characteristics to help identify an unknown organism. At each step, the user chooses between two contrasting statements based on the organism’s observable features. Each choice leads to another pair of statements or to an identification. Taxonomic keys help distinguish organisms systematically and may use characteristics such as leaf arrangement, flower structure, body segments or appendages.

Taxonomy deals with the identification, nomenclature and classification of organisms. Systematics studies the diversity of organisms and their relationships, including evolutionary relationships. Both fields help scientists identify organisms, organise biodiversity, communicate using standardised names and understand relationships among living organisms.

Biological classification organises the enormous diversity of living organisms into groups based on similarities and differences.

• It makes organisms easier to identify and study.
• It helps scientists compare organisms and understand their relationships.
• It provides a systematic way to organise information about biodiversity.
• Standardised scientific names reduce confusion caused by different common or local names.
• Universal naming conventions help scientists communicate accurately across regions and languages.

Living organisms display characteristics such as growth, reproduction, metabolism, cellular organisation and the ability to sense and respond to their surroundings.

• Growth: Living organisms grow through internal processes. Growth alone is not sufficient to define life because non-living objects can increase in size through external accumulation.
• Reproduction: Many organisms reproduce, but some living organisms, such as sterile individuals, do not reproduce.
• Metabolism: Living organisms carry out chemical reactions necessary for maintaining life.
• Cellular organisation: Living organisms possess organised cellular structures.
• Response to stimuli: Living organisms respond to changes in their environment.

Therefore, life is understood through a combination of characteristics rather than a single feature alone.

Taxonomic categories arrange organisms into a hierarchy.

• Genus: Groups closely related species. Example: Homo.
• Family: Groups related genera. Example: Hominidae.
• Order: Groups related families. Example: Primata.
• Class: Groups related orders. Example: Mammalia.
• Phylum: Groups related classes in animals. Example: Chordata.
• Kingdom: A broad category that includes related phyla. Example: Animalia.

As we move from species towards kingdom, each category generally includes a wider variety of organisms, with fewer characteristics shared by all members.

Scientific name: Brassica campestris Linn.

• Genus: Brassica
• Specific epithet: campestris
• Author citation: Linn., an abbreviation referring to Linnaeus.

Together, Brassica campestris forms the binomial name. Scientific names are italicised when printed. When handwritten, the genus and specific epithet are underlined separately. The author citation is written in Roman type rather than italics.

A taxon is a recognised group of organisms at any level of classification. A taxonomic hierarchy is the ordered arrangement of taxonomic categories. Sequence from lowest to highest:
Species → Genus → Family → Order → Class → Phylum/Division → Kingdom. Human sequence:
Homo sapiens → Homo → Hominidae → Primata → Mammalia → Chordata → Animalia. Mango sequence:
Mangifera indica → Mangifera → Anacardiaceae → Sapindales → Dicotyledonae → Angiospermae → Plantae. As we move upwards from species towards kingdom, the number of organisms included generally increases, while the number of characteristics shared by all members generally decreases.

Species is the basic unit of classification because it represents a group of organisms sharing closely related characteristics. Under the biological species concept, members of a species can generally interbreed under natural conditions and produce fertile offspring, while being reproductively isolated from other such groups. Species are dynamic rather than static because populations show variation and can change over generations through evolutionary processes. Environmental conditions, natural selection and other evolutionary mechanisms can contribute to changes in populations over time.