About 1.5 million animal species have been scientifically described, and most estimates suggest many more remain undiscovered, especially insects and other invertebrates. To make sense of this diversity, scientists classify animals into groups based on their evolutionary relationships. Knowing the basic groups and how scientific names work is an essential first skill for anyone working with wildlife.
What you will learn
- The ranks of biological classification and how scientific names work
- What a species is, and why the answer is not always simple
- The main groups of vertebrates and invertebrates
- How DNA is changing the way animals are classified
Taxonomy and scientific names
Taxonomy is the science of naming and classifying organisms. The modern system began with the Swedish botanist Carl Linnaeus, whose 1758 edition of Systema Naturae is the official starting point for animal names.
Organisms are grouped in a hierarchy of ranks. Here is the tiger as an example:
| Rank | Tiger | Grey wolf |
|---|---|---|
| Kingdom | Animalia | Animalia |
| Phylum | Chordata | Chordata |
| Class | Mammalia | Mammalia |
| Order | Carnivora | Carnivora |
| Family | Felidae | Canidae |
| Genus | Panthera | Canis |
| Species | Panthera tigris | Canis lupus |
Every species has a two-part scientific name (binomial nomenclature): the genus, capitalised, followed by the species, in lower case, both written in italics. Subspecies add a third name, for example Panthera tigris tigris for the Bengal tiger. Scientific names are universal: “Panthera tigris” means the same animal in Kathmandu, Moscow and São Paulo, whereas common names vary between languages and regions. The name “elk”, for example, means a moose in Europe but a wapiti in North America.
What is a species?
The most widely used definition is the biological species concept: a species is a group of populations that can interbreed in nature and produce fertile offspring, and that are reproductively isolated from other groups. It works well for many animals but has limits: it cannot be applied to fossils or to organisms that reproduce asexually, and some distinct species do occasionally hybridise.
Other definitions focus on shared evolutionary history (the phylogenetic species concept) or on consistent differences in form, genetics or ecology. Different definitions can give different answers, which is why species counts change. Genetic studies have suggested, for example, that the giraffe may be four species rather than one, that Africa’s forest and savanna elephants are separate species, and that the king cobra may be four species.
These decisions matter for conservation: splitting one widespread species into several smaller ones can reveal that some of them are highly threatened.
The major animal groups
Vertebrates (animals with backbones)
| Group | Approximate described species | Key features | Examples |
|---|---|---|---|
| Fish (several classes) | Over 35,000 | Gills, fins, most lay eggs | Sharks, salmon, clownfish, coelacanth |
| Amphibians | About 8,700 | Moist skin, most have a larval stage in water | Frogs, salamanders, caecilians |
| Reptiles | About 12,000 | Scaly skin, shelled eggs (most) | Snakes, lizards, turtles, crocodiles, tuatara |
| Birds | About 11,000 | Feathers, beaks, lay eggs, warm-blooded | Eagles, penguins, hummingbirds |
| Mammals | About 6,500 | Hair, milk glands, warm-blooded | Bats, whales, primates, rodents |
Birds are, in evolutionary terms, living dinosaurs, and crocodiles are more closely related to birds than to lizards. Groupings such as “reptiles” and “fish” are traditional categories rather than single evolutionary branches.
Invertebrates (animals without backbones)
Invertebrates make up about 97 percent of animal species. The main groups include:
- Arthropods: insects, spiders, scorpions, crustaceans, centipedes and millipedes. Insects alone account for about a million described species, of which beetles make up roughly 40 percent.
- Molluscs: snails, slugs, clams, octopuses and squid.
- Annelids: segmented worms such as earthworms and leeches.
- Cnidarians: jellyfish, corals and sea anemones.
- Echinoderms: starfish, sea urchins and sea cucumbers.
- Nematodes: roundworms, extremely numerous in soils and as parasites.
How DNA is changing classification
Since the 1990s, DNA sequencing has transformed taxonomy. Genetic studies have:
- Shown that whales evolved from land mammals and are most closely related to hippos.
- Revealed that the red panda is not closely related to the giant panda.
- Uncovered cryptic species: animals that look identical but are genetically distinct, such as several species of African forest shrews and some neotropical butterflies.
- Enabled DNA barcoding, identifying species from a short standard gene sequence, used to detect illegal wildlife products and to identify species from eDNA.
Case study: the discovery of the saola
In 1992, a joint survey by Vietnam’s forestry ministry and WWF found unusual horns in a hunter’s home in the Vu Quang area of Vietnam. They belonged to a large forest mammal unknown to science: the saola (Pseudoryx nghetinhensis), a relative of wild cattle. It was the first new large mammal described in over 50 years. The saola has almost never been seen alive by scientists and is Critically Endangered, mainly because of snares set for other animals. Its discovery showed that major discoveries are still possible, even among large mammals.
Try it yourself
Pick five animals you have seen this week. Look up the scientific name, class, order and family of each. Which two are the most closely related?
Key terms
- Taxonomy: the science of naming and classifying organisms.
- Binomial nomenclature: the two-part scientific naming system.
- Species: a group of interbreeding populations reproductively isolated from others (biological species concept).
- Cryptic species: species that look alike but are genetically distinct.
- DNA barcoding: identifying species from a short standard DNA sequence.
Quick quiz
Further reading
- Catalogue of Life (catalogueoflife.org), a global index of species names.
- Mammal Diversity Database (mammaldiversity.org), American Society of Mammalogists.