Diversity in the living world and the need for naming
A vegetable that you call brinjal at home is called baingan in another state and eggplant in another country. Shopping gets confusing when one thing has many names, and science has the same problem on a much bigger scale.
Biodiversity is the variety of organisms on earth; about 1.7–1.8 million species are known and described, and new ones are still being found. Identification means finding what an organism is, and nomenclature means giving it one standard name used everywhere.
In the figure : Naming is possible only after the organism is described and identified.
Rule 1: Local names are not enough — A local name changes from place to place, so scientists use one worldwide scientific name for each organism.
- Easy : A flower called by three different names in three villages needs a single scientific name.
- Medium : Two scientists in different countries read the same scientific name and picture the same organism.
- Exam-type : Q: Why is a scientific name needed? A: To avoid confusion and refer to a species the same way all over the world.
Rule 2: Codes of nomenclature — Plants are named by the International Code of Botanical Nomenclature (ICBN) and animals by the International Code of Zoological Nomenclature (ICZN). A name must be unique and must not repeat for another organism.
- Easy : Plant names follow ICBN.
- Medium : A newly found beetle is named under ICZN, and the name must not already belong to another animal.
- Exam-type : Q: Which code governs naming of animals? A: ICZN.
Description → identification → naming. One organism has only one correct scientific name, and no two organisms share it.
Thinking the known 1.7–1.8 million species are all that exist. This is only the number described so far; exploration keeps adding to it.
Memory clue: DIN — Describe, Identify, Name (nomenclature).
Spot the difference
| Point | Identification | Nomenclature |
|---|---|---|
| Meaning | Finding out what the organism is | Giving it a standard name |
| Comes | First | After identification |
| Governed by | Description of characters | ICBN (plants), ICZN (animals) |
Practice questions
- 1What is biodiversity?
- 2Expand ICBN.
- 3About how many species are known and described?
- 4Why can local names not be used in science?
- 5Which code is used for naming animals?
- 6Why must identification come before nomenclature?
- 7Two students use different local names for the same bird. How do the codes of nomenclature solve this?
- 8Why must a scientific name belong to only one organism?
Show answers (try them yourself first)
- 1The variety of living organisms found on earth.
- 2International Code of Botanical Nomenclature.
- 3About 1.7–1.8 million.
- 4They differ from place to place, so they cause confusion.
- 5ICZN, the International Code of Zoological Nomenclature.
- 6A name can be attached only to an organism that is correctly described and known.
- 7Each species gets one scientific name that is unique and accepted worldwide, so both can refer to it by that name.
- 8So that anyone describing an organism anywhere arrives at the same name, and no two organisms are confused with each other.
Binomial nomenclature and its rules
Your name has two parts: the family name that you share and your own given name. The scientific name works in the same way: the first word is shared by close relatives and the second word picks out one kind.
Binomial nomenclature is a system of giving every organism a two-word name: the genus (generic name) and the specific epithet. It was introduced by Carolus Linnaeus.
In the figure : The name Mangifera indica Linn. has a genus, a specific epithet and the author's abbreviated name.
Rule 1: Writing the name — Names are in Latin. The genus starts with a capital letter, the specific epithet with a small letter. In print the name is in italics; when handwritten, both words are underlined separately.
- Easy : Correct: Mangifera indica. Wrong: Mangifera Indica.
- Medium : Handwritten: Panthera leo, with each word underlined separately.
- Exam-type : Q: Pick the correct form: Homo Sapiens / Homo sapiens. A: Homo sapiens.
Rule 2: Author citation — The author's name comes after the specific epithet in short form. It shows who first described the species.
- Easy : Mangifera indica Linn. means Linnaeus first described the mango.
- Medium : The author's name is written in short form after the epithet, for example Linn.
- Exam-type : Q: What does Linn. at the end of a name show? A: The species was first described by Linnaeus.
Two words, Latin, genus first with a capital, species second with a small letter, italic or separately underlined.
Writing the specific epithet with a capital letter or underlining the two words together as a single line.
Memory clue: G-Capital, s-small — the Genus has a Capital, the species is small.
Spot the difference
| Point | Genus | Specific epithet |
|---|---|---|
| Position | First word | Second word |
| First letter | Capital | Small |
| Example in Panthera leo | Panthera | leo |
Practice questions
- 1Who introduced binomial nomenclature?
- 2What are the two words of a scientific name called?
- 3Write the scientific name of mango correctly.
- 4In the name Panthera leo, which word is the genus?
- 5What is the language of scientific names?
- 6How do you show that a handwritten name is a scientific name?
- 7Find the error: potato is written as solanum Tuberosum.
- 8Why is the author's name placed at the end?
Show answers (try them yourself first)
- 1Carolus Linnaeus.
- 2The genus (generic name) and the specific epithet.
- 3Mangifera indica.
- 4Panthera.
- 5Latin (or Latinised words).
- 6Underline the genus and the specific epithet separately.
- 7The genus must begin with a capital and the epithet with a small letter: Solanum tuberosum.
- 8It records who first described the species, for example Linn. for Linnaeus; it is written in abbreviated form after the specific epithet.
Classification, taxonomy, taxa and systematics
In a library, books are grouped into subjects, then shelves, then rows. You find any book quickly because the grouping follows easy rules. Classifying organisms works in the same way.
Classification is grouping organisms into convenient categories on the basis of observable characters. Each group is a taxon (plural: taxa), and the science of identification, naming and classification together is taxonomy. Systematics also takes evolutionary relationships into account.
In the figure : The four basic processes of taxonomy.
Rule 1: Taxa are at different levels — 'Plants', 'mammals' and 'dogs' are all taxa, but at different levels: dog is a mammal and mammals are animals.
- Easy : Wheat is a taxon; plants are also a taxon.
- Medium : Cat, mammals and animals are three taxa of different rank.
- Exam-type : Q: Is 'mammals' a taxon? A: Yes; any group of organisms at any rank is a taxon.
Rule 2: Basis of modern taxonomy — External and internal structure, cell structure, development and ecological information are all used now. Early classifications were based on the uses of organisms for food, clothing and shelter.
- Easy : Early people grouped plants as edible and non-edible.
- Medium : A modern taxonomist studies cell structure and development, not only the look of a plant.
- Exam-type : Q: What is systematics? A: The study of kinds of organisms and their relationships; the word comes from 'systema' (systematic arrangement), and Linnaeus's book was titled Systema Naturae.
Characterisation, identification, classification and nomenclature are the four basic processes of taxonomy.
Treating a taxon as only one fixed rank. A taxon can be at any level, from species to kingdom.
Memory clue: CICN — Characterise, Identify, Classify, Name.
Spot the difference
| Point | Taxonomy | Systematics |
|---|---|---|
| Deals with | Identification, naming, classification | All of taxonomy plus evolutionary relationships |
| Word origin | Taxon = category | Latin 'systema' |
| Famous book | — | Systema Naturae (Linnaeus) |
Practice questions
- 1Define classification.
- 2What is a taxon?
- 3Name the four basic processes of taxonomy.
- 4What did the earliest classifications use as a basis?
- 5What does the Latin word 'systema' mean?
- 6Why is classification needed?
- 7How does systematics differ from taxonomy?
- 8Give three taxa from the same example, and arrange them from the smallest group to the largest.
Show answers (try them yourself first)
- 1Grouping organisms into convenient categories on the basis of easily observable characters.
- 2A group of organisms at any level of classification, such as dogs, mammals or animals.
- 3Characterisation, identification, classification and nomenclature.
- 4The uses of organisms to humans for food, clothing and shelter.
- 5Systematic arrangement of organisms.
- 6It is nearly impossible to study every organism one by one, and grouping makes the study easy.
- 7Systematics takes evolutionary relationships among organisms into account in addition to identification, naming and classification.
- 8Dogs, mammals, animals: a dog is a mammal and mammals are animals.
Taxonomic categories: species, genus and family
Think of a school: each class has students, several classes make a standard and several standards make a section. Every step up includes more students, but they share fewer things in common.
Classification is done in steps, and each step is a taxonomic category (rank). All the ranks together form the taxonomic hierarchy. A species is a group of individuals with fundamental similarities that can be told apart from other close species; a genus is a group of closely related species; a family is a group of related genera.
In the figure : Lion, as the species Panthera leo, belongs to the genus Panthera and the family Felidae.
Rule 1: Species and genus — Mangifera indica, Solanum tuberosum and Panthera leo have the epithets indica, tuberosum and leo. One genus may hold many species: Panthera has leo, tigris and pardus, and Solanum has tuberosum, nigrum and melongena.
- Easy : Potato and brinjal are different species but belong to the same genus Solanum.
- Medium : Lion, tiger and leopard are in Panthera, but cats belong to another genus, Felis.
- Exam-type : Q: Man belongs to which genus and species? A: Genus Homo, species sapiens.
Rule 2: Family — A family groups related genera and uses both vegetative and reproductive features in plants. Solanum, Petunia and Datura are in family Solanaceae; Panthera and Felis are in family Felidae; cat and dog are in different families, Felidae and Canidae.
- Easy : Cats belong to Felidae and dogs to Canidae.
- Medium : Solanum, Petunia and Datura share many floral features, so they are placed in Solanaceae.
- Exam-type : Q: Name the family of lion and tiger. A: Felidae.
Species have the most characters in common; every higher rank shares fewer common characters.
Thinking that potato and brinjal are the same species because they share a genus. They are different species of the same genus.
Memory clue: S-G-F — Species → Genus → Family, climbing from small to big.
Spot the difference
| Rank | Group of | Example |
|---|---|---|
| Species | Individuals with fundamental similarities | Panthera leo |
| Genus | Closely related species | Panthera |
| Family | Related genera | Felidae |
Practice questions
- 1Name the lowest taxonomic category.
- 2Which genus do lion, tiger and leopard belong to?
- 3What is a genus?
- 4Give the family of cat and dog.
- 5Name two plant genera that are in the family Solanaceae besides Solanum.
- 6Are potato and brinjal in the same species or the same genus?
- 7Why are cat and dog placed in different families?
- 8Pick the odd one in terms of rank: Panthera, Felis, Felidae.
Show answers (try them yourself first)
- 1Species.
- 2Panthera.
- 3A group of closely related species that share more characters than species of other genera.
- 4Cat: Felidae; dog: Canidae.
- 5Petunia and Datura.
- 6Same genus (Solanum), different species.
- 7They have some similar and some different features; the differences keep them in Felidae and Canidae.
- 8Felidae, because it is a family while Panthera and Felis are genera.
Higher categories and the taxonomic hierarchy
A postal address goes from your house, to the street, the town, the district and the state. Each step covers a wider area. Classification of organisms moves in the same way from species up to kingdom.
The higher categories are order (related families), class (related orders), phylum (related classes; called division in plants) and kingdom (the highest). The order from the bottom is species, genus, family, order, class, phylum/division, kingdom.
In the figure : The taxonomic categories in ascending order from species to kingdom.
Rule 1: Moving up the hierarchy — As you go from species to kingdom, the number of common characters decreases. The lower the taxon, the more characters its members share. It also becomes harder to find the relationship between taxa at higher levels.
- Easy : Individuals of one species look very much alike.
- Medium : Members of one order share only a few characters, mostly floral in plants.
- Exam-type : Q: In which direction do common characters decrease? A: From species to kingdom.
Rule 2: Examples of placement — Man: Homo sapiens, Hominidae, Primata, Mammalia, Chordata. Housefly: Musca domestica, Muscidae, Diptera, Insecta, Arthropoda. Mango: Mangifera indica, Anacardiaceae, Sapindales, Dicotyledonae, Angiospermae. Wheat: Triticum aestivum, Poaceae, Poales, Monocotyledonae, Angiospermae.
- Easy : Housefly is an insect (class Insecta).
- Medium : Order Carnivora (cat, dog) and order Primata (monkey) both belong to class Mammalia.
- Exam-type : Q: Which phylum includes fishes, amphibians, reptiles, birds and mammals? A: Chordata (notochord and a dorsal hollow neural system).
In plants the rank below kingdom is division; in animals it is phylum. Species is the lowest rank in both.
Writing the sequence as species, order, genus, family. Genus comes before family.
Memory clue: Kings Play Chess On Fine Green Squares — Kingdom, Phylum, Class, Order, Family, Genus, Species (top to bottom).
Spot the difference
| Point | Plants | Animals |
|---|---|---|
| Rank below kingdom | Division | Phylum |
| Example of a class | Dicotyledonae | Mammalia |
| Lowest rank | Species | Species |
Practice questions
- 1Name the highest taxonomic category.
- 2What is the plant equivalent of phylum?
- 3Write the correct sequence from species to class.
- 4Name the phylum of man and the phylum of housefly.
- 5What are the class and order of mango?
- 6Which class contains orders Primata and Carnivora?
- 7Why does the difficulty of finding relationships increase at higher categories?
- 8Mango and wheat are both Angiospermae but in different classes. Name the classes and the order of each.
Show answers (try them yourself first)
- 1Kingdom.
- 2Division.
- 3Species, genus, family, order, class.
- 4Man: Chordata; housefly: Arthropoda.
- 5Class Dicotyledonae and order Sapindales.
- 6Class Mammalia.
- 7Fewer characters are common among members, so it is harder to be sure about how the taxa at the same level are related.
- 8Mango: Dicotyledonae, order Sapindales; wheat: Monocotyledonae, order Poales.