There are estimated to be between 8 and 10 million species of living organisms on Earth. Scientists have formally described and named approximately 2 million of them so far. Without a reliable system for organizing this extraordinary diversity, studying life would be virtually impossible.
Classification is the science of organizing living organisms into groups based on shared characteristics. It brings order to biological diversity and allows scientists anywhere in the world to communicate precisely about specific organisms.
Classification serves several important purposes in biology.
Classification uses a hierarchical system in which organisms are placed into increasingly specific groups. This system was developed by Carl Linnaeus in the 18th century and refined over centuries as understanding of evolutionary relationships improved.
The levels of classification from broadest to most specific are:
| Level | Description |
|---|---|
| Domain | Broadest category. Three domains: Bacteria, Archaea, Eukarya |
| Kingdom | Major divisions within domains |
| Phylum | Groups with a fundamental body plan in common |
| Class | Groups within a phylum sharing further characteristics |
| Order | Groups within a class |
| Family | Groups of closely related genera |
| Genus | Groups of very closely related species |
| Species | Most specific level. Organisms that can interbreed and produce fertile offspring |
Memory aid: Determined King Phillip Came Over For Good Soup
(Domain, Kingdom, Phylum, Class, Order, Family, Genus, Species)
As you move from domain to species, the organisms within each group become more and more similar to each other.
Traditionally, living organisms have been divided into five kingdoms.
A species is a group of organisms that share common characteristics, can interbreed with each other, and produce fertile offspring.
This is the biological species concept. It is the most fundamental unit of classification.
Two organisms belong to the same species if:
Horses and donkeys, for example, can mate but produce mules, which are infertile. Horses and donkeys are therefore different species.
Every species on Earth is given a unique two-part scientific name. This system is called binomial nomenclature, from the Latin words for two names.
The scientific name consists of:
Examples:
| Common Name | Scientific Name |
|---|---|
| Human | Homo sapiens |
| Common lion | Panthera leo |
| Domestic dog | Canis lupus familiaris |
| House cat | Felis catus |
| Rice | Oryza sativa |
The binomial system ensures that every species has one universally agreed name, regardless of what language scientists speak or what country they work in.
Within Kingdom Animalia, the most fundamental division is between vertebrates and invertebrates.
Vertebrates are animals with a backbone (vertebral column).
| Class | Key Features | Examples |
|---|---|---|
| Fish | Aquatic, gills, scales, ectothermic | Salmon, shark, cod |
| Amphibians | Moist skin, begin life in water, ectothermic | Frogs, toads, salamanders |
| Reptiles | Dry scaly skin, lay eggs on land, ectothermic | Snakes, lizards, crocodiles |
| Birds | Feathers, wings, warm-blooded, lay eggs | Eagles, penguins, sparrows |
| Mammals | Hair or fur, warm-blooded, nurse young with milk | Humans, whales, bats |
Invertebrates are animals without a backbone. They make up approximately 97 percent of all animal species.
Major invertebrate groups include:
Traditional classification was based primarily on physical characteristics. Modern classification increasingly uses molecular evidence, particularly DNA sequences, to determine evolutionary relationships.
This approach, called phylogenetics, groups organisms based on shared ancestry rather than just shared physical features. It has led to some significant reclassifications.
For example, birds are now understood to be a group of dinosaurs rather than a separate evolutionary branch. Molecular evidence has also revealed that some organisms that look very similar are actually not closely related at all, while others that look very different share common ancestry.
The result is a classification system that more accurately reflects the true evolutionary history of life on Earth.
A dichotomous key is a tool used to identify unknown organisms by working through a series of paired statements, choosing one of two options at each step until the organism is identified.
Each step presents two mutually exclusive characteristics. The choice made at each step leads to the next step, progressively narrowing down the possible identity of the organism until a final identification is reached.
Dichotomous keys are widely used by ecologists, naturalists, and field biologists to identify species in the wild.