Taxonomy - Classifying the Living World
Taxonomy is the branch of biology that deals with classification of organisms into a system of categories, based on their characteristics. Modern taxonomy rests on four fundamental processes:
- Characterisation - listing the characters (features) of an organism.
- Identification - correctly recognising what the organism is.
- Classification - placing it into groups and sub-groups (taxa).
- Nomenclature - giving it a standardised scientific name.
On what basis do we classify?
Early classifications used only gross morphology. Modern classification uses a wide range of characters:
- External and internal structure (morphology and anatomy),
- Structure of the cell (cytology),
- Development process (embryology/ontogeny), and
- Ecological information (habitat, interactions).
Classification is therefore never based on a single trait but on the totality of characters.
Systematics, Taxa and the Rules of Naming
Systematics
The word systematics comes from the Latin 'systema', meaning the systematic arrangement of organisms. Linnaeus titled his famous work 'Systema Naturae'.
Systematics is broader than taxonomy: it is the study of the diversity of organisms and of all their comparative and evolutionary relationships. In other words, systematics = taxonomy plus the study of phylogeny (evolutionary history). Taxonomy tells us what an organism is and where it fits; systematics additionally tells us how organisms are related.
Taxon
A taxon (plural taxa) is a taxonomic group or unit of any rank that is real enough to be assigned a category. For example, 'insects' represent a taxon (at the level of a class/order), 'mammals' represent a taxon, and so does a single genus like Solanum. Each taxonomic category in the hierarchy - species, genus, family, and so on - is a rank, and the actual group placed at that rank is a taxon.
Distinguish carefully: a category is a rank/level (e.g., 'family'); a taxon is the actual group occupying it (e.g., the family 'Felidae').
Tautonyms - a code difference
A tautonym is a scientific name in which the genus and species names are identical - e.g., Naja naja (cobra), Rattus rattus (rat), Bufo bufo (toad).
- Zoology (ICZN): tautonyms are ALLOWED.
- Botany (ICBN): tautonyms are NOT allowed.
This is a favourite discriminating point in exams.
Naming codes at a glance
- ICZN — International Code of Zoological Nomenclature → governs the naming of animals.
- ICBN, now renamed ICN (International Code of Nomenclature for algae, fungi and plants) → governs plants (and algae and fungi).
- Suffix clues: animal family ends in -idae (Hominidae, Muscidae, Felidae, Canidae); plant family in -aceae (Anacardiaceae, Solanaceae, Poaceae); plant order in -ales (Sapindales, Polymoniales, Poales); plant class in -ae (Dicotyledonae, Monocotyledonae).
Two species concepts — don't mix them up
- Morphological species concept (the NCERT wording): a species is a group of individuals with fundamental similarities that can be distinguished from closely related species by distinct morphological differences.
- Biological species concept (Ernst Mayr): a species is a group of interbreeding natural populations that are reproductively isolated from other such groups.