Course 3.1Lesson 2 of 3

The hierarchy of groups and scientific names

The groups go from kingdom, phylum, class, order, family and genus down to species, from the largest to the smallest. Living things that share more levels are more closely related. A scientific name has two parts, the genus and the species, and it is the same in every country.

about 30 min1 simulation2 practice questions

Lesson 2 of 3Show all lessons

Try the code and the simulations beside the lesson.

Groups within groups

Think of how you are sorted. You are a person in a family, in a street, in a town, in a state, in a country. Each is inside a bigger one. Living things are sorted in this way too. The levels, from the largest group to the smallest, are

\displaystyle \text{Kingdom} \to \text{Phylum} \to \text{Class} \to \text{Order}

\displaystyle \to \text{Family} \to \text{Genus} \to \text{Species}

(In plants, the word division is used for the level of the phylum.)

A funnel of seven levels from the biggest group at the top to the smallest at the bottom: Kingdom, Phylum, Class, Order, Family, Genus and Species. Two examples are written beside it. The human is Animalia, Chordata, Mammalia, Primates, Hominidae, Homo, sapiens. The mango is Plantae, Angiosperms, Dicotyledons, Sapindales, Anacardiaceae, Mangifera, indica.

A kingdom is the biggest group. Animals are one kingdom.
A species is the smallest group: living things that can mate with each other and have young that can also have young.
Each level has the next level inside it. Members of the same group have more in common as we go down.

Closely related or not?

Two living things that share many levels are closely related.

Level	Human	Cat	Lion	Tiger
Kingdom	Animalia	Animalia	Animalia	Animalia
Phylum	Chordata	Chordata	Chordata	Chordata
Class	Mammalia	Mammalia	Mammalia	Mammalia
Order	Primates	Carnivora	Carnivora	Carnivora
Family	Hominidae	Felidae	Felidae	Felidae
Genus	Homo	Felis	Panthera	Panthera
Species	sapiens	catus	leo	tigris

The lion and the tiger share the genus, so they are very close. The lion and the cat share the family. The lion and the human share only the class.

You should find that the dog shares $4$ levels with the lion, down to the order Carnivora. The frog shares only $2$ : the kingdom and the phylum Chordata.

The scientific name

Common names change from place to place. One plant has different names in different languages, and sometimes one name stands for two different animals. So scientists use a scientific name, the same all over the world. The Swedish scientist Carolus Linnaeus gave this system. It is called binomial nomenclature (a two-part name).

The scientific name has two parts: the genus and the species.

The name of the genus starts with a capital letter.
The name of the species starts with a small letter.
In print, the name is written in italics. By hand, we underline each part separately.

Examples: Homo sapiens (human), Panthera leo (lion), Panthera tigris (tiger), Mangifera indica (mango), Felis catus (cat).

Try it in Python

This program counts how many levels two animals share.

The next program checks the form of a scientific name.

Practise with new numbers

Common mistakes

Mixing up the order of the levels. From the largest: kingdom, phylum, class, order, family, genus, species.
Writing both parts with a capital letter. Only the genus starts with a capital.
Thinking animals of one class share a genus. A cat and a human are both mammals, but their genera differ.
Trusting a common name. It changes from place to place. The scientific name does not.

Remember these

The levels from largest to smallest: kingdom, phylum, class, order, family, genus, species.
A scientific name has two parts: Genus species. The genus starts with a capital letter, and the species with a small letter.
Living things that share more levels are more closely related.
Groups multiply as we go down: a family holds many genera, and a genus holds many species.