The binomial system of naming – Introduction to Classification of Organisms (Senior 1 Biology) | Ganzaa

Introduction to Classification of Organisms

Lessons

Senior 1 · Biology

Introduction to Classification of Organisms

5 lessons

Lesson 2 of 5

The binomial system of naming

20 min

One plant, many names

The bean you ate yesterday is ibishyimbo in Kinyarwanda, haricot in French, maharage in Kiswahili and "bean" in English. Worse, English uses "bean" for many unrelated plants: broad beans, soya beans, and even coffee "beans", which are the seeds of a shrub that is not a bean plant at all.

A scientist in Kigali and a scientist in Brazil need one name that means exactly one kind of organism, in every language. That name for the bean is Phaseolus vulgaris.

Definition

Binomial system (binomial nomenclature): the international system of giving every species a scientific name made of two words, the genus name followed by the species name. "Bi-nomial" means "two names".

Example: in Homo sapiens, Homo is the genus and sapiens is the species name (the specific epithet).

Definition

Scientific name: the two-word Latin or Latin-style name of a species, the same in every country and language. Common name: the everyday name in one language, such as "coffee" or ikawa, which differs from place to place.

Where the system came from

The Swedish naturalist Carl Linnaeus (1707–1778) made the two-word system standard. His book Species Plantarum (1753) named about 6,000 plants this way, and the tenth edition of his Systema Naturae (1758) did the same for animals. Those two books are still the official starting points for plant and animal names.

The rules for writing a scientific name

Figure 1: How to write a scientific name. Arabica coffee is the kind grown on Rwanda's hills.

The name has two words: genus, then species.
The genus name begins with a capital letter.
The species name is written entirely in small (lower-case) letters, even if it comes from a person's name or a place.
When printed, the whole name is in italics. When handwritten, each word is underlined separately.
After the full name has been written once, the genus may be shortened to its capital initial: Phaseolus vulgaris, then P. vulgaris.
A subspecies gets a third word: the mountain gorilla is Gorilla beringei beringei.

The words often describe the organism or honour a person or place. Coffea arabica means "coffee of Arabia"; vulgaris means "common"; sapiens means "wise". Gorilla beringei honours Robert von Beringe, a German officer who shot two of these gorillas on Mount Sabyinyo in 1902, the first to be described by science.

English	Kinyarwanda	French	Scientific name
Bean	ibishyimbo	haricot	Phaseolus vulgaris
Maize	ibigori	maïs	Zea mays
Cassava	imyumbati	manioc	Manihot esculenta
Arabica coffee	ikawa	caféier arabica	Coffea arabica
Chimpanzee	impundu	chimpanzé	Pan troglodytes
Mountain gorilla	ingagi	gorille de montagne	Gorilla beringei beringei
Human	umuntu	être humain	Homo sapiens

Warning

Common mistakes. (1) A capital letter on the species name: Homo Sapiens is wrong. (2) One long underline under both words when handwriting: underline each word on its own. (3) Using the genus alone as if it were the species: Anopheles is a genus of more than 400 kinds of mosquito, not one species.

Worked example 1: correct the names

A student's notebook says: "zea Mays", "Homo Sapiens" and "the malaria mosquito, Anopheles". Correct each one.

"zea Mays": the genus needs a capital and the species a small letter, so it becomes Zea mays (underlined word by word in handwriting).
"Homo Sapiens": the species name must be all small letters: Homo sapiens.
"Anopheles" is only a genus. One important malaria-carrying species in Africa is Anopheles gambiae, so the full two-word name is needed.

Worked example 2: reading relationships from names

Which two of these are most closely related: Panthera leo (lion), Panthera pardus (leopard), Felis catus (cat)?

The first word of each name is the genus.
The lion and leopard share the genus Panthera; the cat is in a different genus, Felis.
Organisms in the same genus share a lower rank than organisms in different genera.

Answer: the lion and the leopard. The scientific name alone tells you this, even if you have never seen the animals.

Worked example 3: shortening names safely

Rewrite this sentence, shortening where the rules allow: "Plasmodium falciparum causes the most dangerous malaria. Plasmodium falciparum is carried by mosquitoes of the genus Anopheles."

The full name has been written once, so the second time the genus can be shortened: P. falciparum.
Check that no other genus in the passage starts with P. None does, so the shortening is clear.
"Anopheles" stands alone as a genus name and is never shortened.

Answer: "Plasmodium falciparum causes the most dangerous malaria. P. falciparum is carried by mosquitoes of the genus Anopheles."

Scientific names at work in Rwanda

When Rwanda exports coffee, tea or flowers, the plant health (phytosanitary) certificate names the plants and any pests by their scientific names, so inspectors in any country know exactly what was checked. Researchers at RAB report on Manihot esculenta (cassava) diseases with colleagues in Uganda and Tanzania in the same way, even though the three countries use different common names.

GCSE Biology: Classification Systems, the Linnaean System (Cognito). Watch the part on binomial names and note how the genus and species are written.

Tip

Memory hook: the Genus is the Group name, so it gets the capital G. The species is specific, so it stays small.

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Practice: scientific names

What does "binomial" mean, and what are the two parts of a scientific name?
Give two advantages of scientific names over common names.
Write the scientific name of maize correctly as it would appear in print, and describe how you would handwrite it.
Find the errors: "Coffea Arabica" and "homo sapiens".
After writing Manihot esculenta once in an essay, how may you write it the second time?
The mountain gorilla is Gorilla beringei beringei. Explain why it has three words.
Phaseolus vulgaris (common bean) and Phaseolus lunatus (lima bean): what can you conclude about these two plants from their names alone?

Show answersHide answers

Two names: the genus name followed by the species name (specific epithet).
They are the same in every language and country; each refers to exactly one species; they show relationships (same genus means close relatives).
Print: Zea mays in italics. Handwritten: Zea mays with each word underlined separately; capital Z, small m.
Coffea arabica (species name must be lower case); Homo sapiens (genus needs a capital).
M. esculenta.
The third word names the subspecies: a distinct population within the species Gorilla beringei (the other eastern gorilla subspecies is Gorilla beringei graueri).
They are in the same genus, Phaseolus, so they are close relatives; they are different species because the second words differ.

Key points

Every species has a two-word scientific name: genus (capital letter) + species (small letters).
Print it in italics; handwrite it with each word underlined separately.
Scientific names are international and show relationships; common names are not and do not.

Sources and credits

12.1: Organizing Life on Earth, from Concepts of Biology by OpenStax (Samantha Fowler, Rebecca Roush, James Wise), via LibreTexts Biology, licensed under CC BY 4.0. Adapted by Ganzaa.
Painting: Carl von Linné, Alexander Roslin (1775), public domain, via Wikimedia Commons.
Photo: RWA 4492 Rwanda Chimp Baby, Wookiemedia, CC BY-SA 4.0, via Wikimedia Commons.
Video: GCSE Biology - Classification Systems | Linnaean System, Cognito, embedded from YouTube.

Figure 1, the table of names, the worked examples and the practice questions are written by Ganzaa.

1One plant, many names

2Where the system came from

3The rules for writing a scientific name

4Scientific names at work in Rwanda

One plant, many names

Where the system came from

The rules for writing a scientific name

Scientific names at work in Rwanda