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Classification systems and taxonomic rank is taught in the note for Classification and Taxonomy. Read it below and look for the section on this topic.

Classification and Taxonomy

12 min read

Why we classify

There are millions of species of living organisms. Taxonomy is the science of naming and grouping organisms. Classification makes it easy to identify them and to show how they are related.

Levels of classificationA funnel of seven levels from the biggest group, kingdom, down to the smallest, species. The lion is used as an example at every level: Animalia, Chordata, Mammalia, Carnivora, Felidae, Panthera, Panthera leo.KingdomAnimaliaPhylumChordataClassMammaliaOrderCarnivoraFamilyFelidaeGenusPantheraSpeciesPanthera leo (lion)Example: the lion

Taxonomic ranks

Organisms are placed in a hierarchy of groups. Each rank contains the one below it.

RankExample: lionExample: maize
KingdomAnimaliaPlantae
PhylumChordataAngiospermophyta
ClassMammaliaMonocotyledoneae
OrderCarnivoraPoales
FamilyFelidaePoaceae
GenusPantheraZea
SpeciesPanthera leoZea mays

Binomial nomenclature

Each species has a two-part scientific name, given by Carolus Linnaeus: the genus (starting with a capital letter) and the species (all lower case), for example Homo sapiens. It is written in italics or underlined. The system is useful because the name is universal, avoids confusion from local names, and shows relationship, since species in one genus are close relatives.

The five kingdomsFive coloured boxes naming the five kingdoms of living things with an example of each: Monera such as bacteria, Protoctista such as amoeba and algae, Fungi such as mushrooms and yeast, Plantae such as maize, and Animalia such as fish, birds and humans.MonerabacteriaProtoctistaamoeba, algaeFungimushroom, yeastPlantaemaize, mango treeAnimaliafish, bird, humanLiving things are sorted into five kingdoms
A dichotomous keyA branching key that sorts four animals by yes or no questions: does it have legs, does it have six legs, does it have wings. The answers lead to snake, spider, butterfly or ant.Does it have legs?noyesSnakeDoes it have 6 legs?noyesSpiderDoes it have wings?noyes

The five kingdoms

KingdomCell typeFeedingExamples
Monera (Prokaryotae)Prokaryote, no nucleusVariousBacteria, cyanobacteria
ProtoctistaEukaryote, mostly single-celledVariousAmoeba, Paramecium, algae
FungiEukaryote, cell wall of chitinAbsorb food from dead matterMushrooms, yeast, mould
PlantaeEukaryote, cellulose wallPhotosynthesisMosses, ferns, flowering plants
AnimaliaEukaryote, no cell wallEat other organismsInsects, fish, mammals
Try itThe funnel of classification

Each rank is a smaller group. Choose a rank to see how many kinds of cats may be grouped there (illustrative numbers).

There are many more organisms in a kingdom than in a species.
Check yourself How should the scientific name of a lion be written?
Check yourself Which rank is the smallest?
Check yourself Which kingdom has chitin in its cell walls?

Key terms

Taxonomy
The science of naming and classifying organisms.
Binomial nomenclature
The two-name system of naming species.
Species
A group of organisms that can breed together to make fertile offspring.
Genus
A group of closely related species.

Summary

  • Taxonomic ranks: kingdom, phylum, class, order, family, genus, species.
  • Binomial names have a genus (capital) and a species (lower case).
  • The five kingdoms are Monera, Protoctista, Fungi, Plantae and Animalia.
  • A universal name avoids the confusion of local names.
Binomial nomenclature