Species, genus and the taxonomic hierarchy

An address has levels

Your address has levels: a country, a state, a district, a town, a street and a house. Each level holds the next one. If we know the house, we know all the rest.

Biologists put organisms in groups in the same way. A small group sits inside a bigger group, and this sits inside a still bigger one. Each level is called a taxonomic category, or a rank. Each group at any rank is called a taxon (plural: taxa). All the ranks together are the taxonomic hierarchy.

The ranks, from the smallest

Species. A species is a group of individuals with fundamental similarities. Members of a species look alike, and they can mate and have young that can also have young. A horse and a donkey look alike and can mate, but their young, the mule, is sterile. So they are two species, both in the genus Equus.

Genus. A genus is a group of related species that have more characters in common than they have with the species of other genera. The potato (Solanum tuberosum) and the brinjal (Solanum melongena) are in one genus. The lion, the tiger and the leopard are in the genus Panthera.

Family. A family is a group of related genera, with fewer similarities than a genus. Plant families are decided from both vegetative and reproductive features. The genera Solanum, Petunia and Datura are in the family Solanaceae. The genus Panthera and the genus Felis (cat) are in the family Felidae. A cat and a dog differ more, so they are in two families: Felidae and Canidae.

Order. An order is a group of related families, with a few similar characters. The families Felidae and Canidae are in the order Carnivora. Among plants, the families Convolvulaceae and Solanaceae are in one order, because of their flowers.

Class. A class holds related orders. The order Primata (monkey, gorilla, gibbon) and the order Carnivora (tiger, cat, dog) are in the class Mammalia.

Phylum. Fishes, amphibians, reptiles, birds and mammals share features such as a notochord and a dorsal hollow nerve cord. So they are in the phylum Chordata. In plants, a group of classes with a few similar characters is called a division.

Kingdom. All the animals of all the phyla are in the kingdom Animalia. All the plants of all the divisions are in the kingdom Plantae.

A way to remember the order from the biggest to the smallest: Kind Pandas Chew Oranges For Good Sleep.

Four examples

The mango and the wheat share the division and the kingdom, and they differ from the class downward. The man and the housefly share only the kingdom.

What changes as you go up

As you go up the ladder, from species to kingdom, the group holds more organisms, and they share fewer features. As you go down, the group becomes smaller, and its members become more alike.

You should find that the chimpanzee shares five ranks with us, down to the family Hominidae. The housefly shares only the kingdom, and the mango shares nothing.

Try it in Python

This program keeps the ranks of the four organisms of the table, and it finds the lowest rank that each pair shares.

Practise with new numbers

Common mistakes

• Mixing up the order of class and order. From the top: kingdom, phylum, class, order, family, genus, species.
• Calling the plant rank a phylum. In plants it is a division.
• Thinking a horse and a donkey are one species because they can mate. Their young is sterile.
• Thinking a higher rank shares more features. It shares fewer, because it holds more kinds.

Remember these

• The ranks from the smallest: species, genus, family, order, class, phylum (division in plants), kingdom.
• A taxon is a group at any rank.
• A species has fundamental similarities, and its members can mate and have young that can also have young.
• Going up the ladder: more organisms in the group, and fewer shared features.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. In which family are the lion and the cat?

2. Which rank is called a division in plants?

3. The mango and the wheat share which ranks?

4. Why are the horse and the donkey two species, though they can mate?