Scientists have named well over a million kinds of living things. To keep track, they sort them into groups inside groups, by the traits they share, much like a library sorts books into sections, shelves and rows. Every group is built on the characteristics of life and on traits of the body and cells.
Key idea
The eight main ranks, from broadest to narrowest: domain, kingdom, phylum, class, order, family, genus, species. Each rank holds fewer kinds of organisms that share more traits. A memory trick: Dear King Philip Came Over For Good Soup.
| Rank | Gray wolf | House cat |
|---|---|---|
| Domain | Eukarya | Eukarya |
| Kingdom | Animalia | Animalia |
| Phylum | Chordata | Chordata |
| Class | Mammalia | Mammalia |
| Order | Carnivora | Carnivora |
| Family | Canidae | Felidae |
| Genus | Canis | Felis |
| Species | Canis lupus | Felis catus |
Why it works
The kingdom Animalia holds every animal: sponges, insects, fish and you. The order Carnivora holds only meat-eating mammals like wolves, cats and bears. The genus Canis holds just wolves, coyotes and their close kin. As you go down, each group gets smaller and its members get more alike.
Worked examples
Worked example: Which holds more?
Which holds more kinds of organisms: a class or a genus?
- Class comes higher in the list than genus.
- Higher ranks are broader and hold more kinds.
Answer: A class.
Worked example: Closest shared rank
The lion is Panthera leo and the tiger is Panthera tigris. What is the narrowest rank they share?
- Both are in the genus Panthera.
- Their species differ.
Answer: Genus.
Common mistake
Thinking a "higher" rank like kingdom holds fewer organisms because it sounds more important. The broadest ranks hold the most kinds; the species holds just one. Check: is every cat an animal? Yes, so the animal kingdom must be bigger than the cat family.