Binomial nomenclature
You need to know what binomial nomenclature is, who gave it, the two parts of a scientific name, and every rule for writing one: Latin or Latinised, printed in italics or each word underlined separately when handwritten, the genus with a capital letter and the specific epithet with a small letter, and the author's name abbreviated at the end, as in Mangifera indica Linn.
Two words for every organism
Every scientific name has exactly two parts.
1. The generic name, the name of the genus: a group of closely related kinds.
2. The specific epithet: the word that picks out one particular kind (one species) inside that genus.
Giving each organism a two-part name in this way is binomial nomenclature ("bi" two, "nomen" name).
It was given by Carolus Linnaeus, the eighteenth-century Swedish naturalist, and biologists all over the world still use it, because a two-word format proved convenient: short enough to remember, yet enough to name one kind exactly.
Take the mango.
Its scientific name is Mangifera indica.
Mangifera is the genus; indica is the specific epithet, the particular species.
You can read meaning into both: Mangifera means "mango-bearing" and indica means "of India".
That is typical.
Names are built from Latin (or from words of any language turned into Latin form), and they often describe the organism or where it came from.
Why two words work
The two parts do different jobs, and that is the whole idea of the system.
- The genus name groups. All the species in one genus share many features. Solanum tuberosum (potato), Solanum melongena (brinjal) and Solanum nigrum (black nightshade, the makoi weed of field edges) are three species in one genus. The shared first word tells you at once that they are close relatives.
- The specific epithet separates. Inside Solanum, the epithets tuberosum, melongena and nigrum keep the three apart.
Look at the flowers of the three plants in the drawing below: the same star-shaped plan with a central cone of yellow anthers is the kind of shared feature that puts them in one genus, while the leaves, fruits and tubers keep them apart as separate species.
- An epithet means nothing on its own. The same epithet can be used in many different genera. Mangifera indica (mango), Azadirachta indica (neem) and Tamarindus indica (tamarind) all end in indica, but they are three unrelated trees. So you must never write the epithet alone as if it were a name: "indica" names nothing. The genus and epithet together name one species.
Why it works (not examined). Before Linnaeus, a plant was often given a long Latin phrase describing it, sometimes a dozen words, and different botanists wrote different phrases for the same plant.
Linnaeus kept a short two-word label for each species and moved the description elsewhere.
His plant book Species Plantarum (1753) is the starting point for plant names today, and the tenth edition of his Systema Naturae (1758) is the starting point for animal names.
The rules of writing a scientific name
The international codes lay down universal rules.
You need to know all of them, and you need to apply them, because questions often show a badly written name and ask you to correct it.
1. Latin or Latinised. Names are Latin, or turned into Latin form, and printed in italics. A word from any language (Sanskrit, Hindi, Greek, a person's surname, a place) is turned into Latin form. Azadirachta, the neem genus, comes from the Persian name for the tree; benghalensis in Ficus benghalensis (banyan) comes from Bengal.
2. Order of the parts. The first word is the genus; the second is the specific epithet. Never reverse them.
3. Italics or separate underlining. In print both words are in italics. When you handwrite a name (as in your examination), you cannot write italics, so you underline each word separately: one short line under Mangifera and another under indica, with a gap between them, never one continuous line under both words. The underlining or italics shows the Latin origin.
4. Capital and small letters. The genus starts with a capital letter; the specific epithet starts with a small letter, even when it comes from a place or person. So it is Mangifera indica, never Mangifera Indica, and Ficus benghalensis, never Ficus Benghalensis.
5. Author's name at the end. The name of the scientist who first described the species is written after the specific epithet, at the end of the name, in a shortened form. In Mangifera indica Linn., "Linn." shows that Linnaeus first described the mango. The author's name is not italicised or underlined, because it is not part of the Latin name. Neem is written Azadirachta indica A. Juss., after the French botanist Adrien de Jussieu, who first described it.
Two writing habits follow from these rules.
- Shortening the genus. Once a genus has been written out in full, later mentions in the same passage may shorten it to its capital initial with a full stop: tiger Panthera tigris, then lion P. leo and leopard P. pardus. Never shorten the specific epithet, and never start an answer with the shortened form before the full genus has appeared.
- Human beings are Homo sapiens: genus Homo, specific epithet sapiens ("wise").
Name as written
Correct?
Why
Panthera tigris
Yes
genus capital, epithet small, both italic
Panthera Tigris
No
the specific epithet must start with a small letter
panthera tigris
No
the genus must start with a capital letter
Panthera tigris (handwritten with one line under both words)
No
each word is underlined separately
tigris Panthera
No
the genus comes first
Azadirachta indica A. Juss.
Yes
author abbreviation at the end, not italic
Connect it. The genus and the species are the two lowest taxonomic categories, which you meet in full later in this unit, so every scientific name already tells you two levels of the hierarchy.
In the chapter on the animal kingdom you will need to write dozens of scientific names; the habit of capital genus, small epithet and separate underlining starts here.
- Binomial nomenclature: Each name has two components – the Generic name and the specific epithet. This system of providing a name with two components is called Binomial nomenclature.
- Rules of nomenclature (as printed, with the mango as the example):
- Biological names are generally in Latin and written in italics. They are Latinised or derived from Latin irrespective of their origin.
- The first word in a biological name represents the genus while the second component denotes the specific epithet.
- Both the words in a biological name, when handwritten, are separately underlined, or printed in italics to indicate their Latin origin.
- The first word denoting the genus starts with a capital letter while the specific epithet starts with a small letter.
- Author's name: Name of the author appears after the specific epithet, i.e., at the end of the biological name and is written in an abbreviated form, e.g., Mangifera indica Linn.
A label in a school herbal garden in Mysuru reads: "Ocimum Tenuiflorum (Tulsi)" and "ficus benghalensis (Banyan)", the second written by hand with one unbroken line under both words.
Rewrite both names correctly as you would in an answer book and state the rule each label broke. [2]
Answer:
- Ocimum tenuiflorum, with Ocimum and tenuiflorum each underlined: the specific epithet starts with a small letter (½ for the corrected name, ½ for the rule).
- Ficus benghalensis, with Ficus and benghalensis each underlined separately: the genus starts with a capital letter, and when handwritten each word is underlined separately (½ for the corrected name, ½ for the rule).
A weaker answer writes "Ocimum tenuiflorum" without underlining.
In an answer book the name must be underlined, word by word, because you cannot write italics; the corrected name then loses its ½.
Read the passage and answer the questions that follow.
Kavya's grandmother runs a small nursery near Thrissur.
A new seed packet from a supplier in Gujarat lists "Solanum melongena L. (brinjal)" and "Solanum nigrum L. (black nightshade)".
A customer says the two must be "the same plant, because they share a name".
(a) Identify the genus and the specific epithet in the first name. (1)
(b) Explain why the customer is wrong, using both names. (2)
Attempt either subpart (c) or (d):
(c) What does the letter "L." at the end of each name tell you? (1)
OR
(d) How should Kavya write the first name in her nursery register by hand? (1) [4]
Answer:
- (a) Genus Solanum; specific epithet melongena (½ + ½).
- (b) The shared word Solanum is the genus, so the two are related species of one genus (1); the different specific epithets melongena and nigrum show that they are two different species / different plants (1).
- (c) The species was first described by Linnaeus; it is the author's name in abbreviated form (1).
- OR (d) Solanum melongena L., with Solanum and melongena each underlined separately and L. not underlined, the genus capital and the epithet small (1).
Answer each part on its own labelled line, and attempt only one of (c) and (d).
Which one of these is written according to the rules of binomial nomenclature?
(A) Homo Sapiens
(B) Sapiens homo
(C) Homo sapiens
(D) homo sapiens
1. (A): the epithet has a capital S. Wrong.
2. (B): epithet first, genus second, and the genus is small. Wrong.
3. (C): genus first with a capital, epithet second with a small letter, both italic. Right.
4. (D): the genus has a small h. Wrong.
Answer: (C) / Homo sapiens (1)
Mango is Mangifera indica, neem is Azadirachta indica and tamarind is Tamarindus indica.
A student concludes that the three trees "belong to the same species indica".
(a) Why is the student wrong?
(b) What would two trees need to share in their names to be placed in the same genus?
(c) Write the name of neem with its author's name and say which part is not italicised. [3]
Answer:
- (a) A species is named by the genus and the epithet together; indica alone is only the specific epithet, and the same epithet can be used in different genera, so these are three different species in three genera (1).
- (b) The same first word / generic name (1).
- (c) Azadirachta indica A. Juss.; the author's abbreviation "A. Juss." is not italicised (½ + ½).
Every time you write a scientific name in your answer book, underline the genus and the specific epithet separately, with a capital first letter for the genus only.
A correct organism with a badly written name can lose the ½ for the name, and a question on the rules themselves usually gives ½ per rule, so learn all five (Latin, order, italics or separate underlining, capital genus and small epithet, author at the end).
- Capitalising the epithet because it comes from a place (Mangifera Indica, Ficus Benghalensis). The epithet always starts with a small letter, whatever its origin.
- One long underline under both words. Each word is underlined separately, because each is a separate Latin word.
- Italicising or underlining the author's name. "Linn." and "A. Juss." are written plainly after the Latin name.
- Treating the epithet as the species name on its own. indica or leo alone names nothing; the species is always the two words together.
- Writing "Binomial nomenclature was given by Darwin" or "by Whittaker". It was Carolus Linnaeus.
Check your understanding
1. In the name Panthera pardus, the word pardus is the
(A) genus
(B) family
(C) specific epithet
(D) author's name [1]
2. Assertion (A): In the name Mangifera indica Linn., "Linn." is printed in italics.
Reason (R): The name of the author who first described the species is written at the end of the biological name in an abbreviated form.
Choose the correct option.
(A) Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation for Assertion (A).
(B) Both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct explanation for Assertion (A).
(C) Assertion (A) is true, but Reason (R) is false.
(D) Assertion (A) is false, but Reason (R) is true. [1]
3. Which one of the following names breaks two rules of binomial nomenclature?
(A) Oryza sativa
(B) oryza Sativa
(C) Oryza Sativa
(D) Pavo cristatus Linn. [1]
4. State the two components of a binomial name and name the scientist who gave this system. [2]
5. Write any four rules that must be followed when writing the scientific name of an organism. (½ x 4) [2]
6. A newspaper in Guwahati prints: "The one-horned rhinoceros, Rhinoceros Unicornis, has recovered in Kaziranga."
(a) Identify the two errors in the way the name is printed and rewrite it correctly for print. (1)
(b) How would you write it correctly by hand? (1)
(c) Explain why the genus name alone, Rhinoceros, is not enough to tell a reader exactly which animal is meant. (1) [3]
7. Why is the same specific epithet, such as indica, allowed in many different scientific names, while the full two-word name is used for only one species? [2]
8. Explain why scientific names are Latinised, even for organisms first known by a name in an Indian language. [2]
Answers
1. (C) / specific epithet (1)
2. (D) / Assertion (A) is false, but Reason (R) is true. (1). The author's abbreviation is not part of the Latin name and is not italicised, so A is false; R is a correct rule.
3. (B) / oryza Sativa (1). The genus starts with a small letter and the epithet with a capital, two errors; (C) has one error only.
4. Generic name / genus (½) and specific epithet (½); Carolus Linnaeus (1).
5. Any four, ½ each: names are in Latin or Latinised; the first word is the genus and the second the specific epithet; printed in italics or, when handwritten, each word underlined separately; the genus starts with a capital letter and the epithet with a small letter; the author's name is written in abbreviated form at the end.
6. (a) The name is not in italics, and the specific epithet should start with a small letter (½); Rhinoceros unicornis (½).
(b) Rhinoceros unicornis, with Rhinoceros and unicornis each underlined separately (1).
(c) A genus can contain more than one species, so the specific epithet is needed to pick out the one-horned rhinoceros from other species of the same genus (1).
7. An epithet only distinguishes species inside one genus, so the same epithet can be reused in different genera (1); the combination of genus and epithet is unique, so each species has only one name and no name is used for two organisms (1).
8. Latin is one common language used by biologists of every country, so a Latinised name is the same everywhere (1); names derived from any language are given Latin form so that they follow the same rules and cannot be confused with local names (1).