Classifying living things

Earth holds an estimated 5 million to 50 million species, and biologists sort them the same way evolution produced them: by ancestry. The field of biology that groups and classifies species is taxonomy.

Grouping by evolutionary history

Classification follows vertical descent, the passing of genes from ancestors to their descendants. The more recently two species shared an ancestor, the more closely they are grouped. Species that split apart in the very distant past end up in different groups.

That makes taxonomy useful in two ways. It helps you appreciate both the unity and the diversity of life on Earth. And because it is based on evolution, it shows how living species are related to one another and to extinct species.

Domains, supergroups, and kingdoms

At the broadest level, all life is divided into three groups called domains: Bacteria, Archaea, and Eukarya. What separates them shows up inside their cells.

The cells of organisms in Eukarya contain internal compartments that carry out different functions. Their defining feature is a nucleus, a compartment wrapped in a membrane that holds the cell’s genetic material. Organisms with this kind of cell are eukaryotes.

Bacteria and archaea have simpler cells with no nucleus, so they are prokaryotes. Although bacteria and archaea look alike, their cells differ significantly at the molecular level.

All three domains trace back to one single-celled ancestor, the last universal common ancestor (LUCA). Bacteria and archaea split from each other at least 2.5 billion years ago. Eukaryotes appeared later: they first arose within the archaea and then acquired genetic material from certain bacteria.

Biologists once divided Eukarya into four kingdoms: Protista (protists, mostly single-celled eukaryotes), Plantae (plants), Fungi, and Animalia (animals). New evidence about evolutionary relationships showed that protists don’t form a single kingdom. Most eukaryotes are now placed in seven broad groups called supergroups, and kingdoms such as Animalia fall within them.

This work turned up some surprises. Fungi, for instance, are more closely related to animals than to plants.

The taxonomic hierarchy

Within a domain, each species is placed into smaller and smaller groups. Each step down contains fewer species, and those species are more closely related. From broadest to narrowest, the ranks are domain, supergroup, kingdom, phylum, class, order, family, genus, and species.

Here is the full classification of the ocellaris clownfish, the orange-and-white fish popular in saltwater aquariums:

To judge how closely two species are related, find the narrowest rank they share. Two fish in the same genus are much closer relatives than two animals that share only a kingdom.

Scientific names

Each species gets a unique two-part scientific name. This naming system is binomial nomenclature.

The first part of the name is the genus. The second is the specific epithet, a descriptor that picks out the species within its genus. The ocellaris clownfish is Amphiprion ocellaris, and humans are Homo sapiens: genus Homo, specific epithet sapiens.

Scientific names follow a few conventions:

• The genus is capitalized, and the specific epithet is not.
• Both words are italicized.
• Names are usually Latinized (made to look like Latin words), though they can come from Latin, Greek, or other sources, even a person’s name.

Check yourselfA salmon is in phylum Chordata and class Actinopterygii. A jellyfish is in kingdom Animalia but phylum Cnidaria. Which is more closely related to the clownfish, and why?

The salmon. It shares the clownfish’s domain, supergroup, kingdom, phylum, and class, while the jellyfish shares only the domain, supergroup, and kingdom. The more ranks two species share, the more recently they had a common ancestor.

Check yourselfThe gray wolf’s scientific name is Canis lupus. Which word is the genus, and how can you tell?

Canis. In binomial nomenclature the genus comes first and is capitalized, while the specific epithet (lupus) comes second and is lowercase.