The Living World — Notes

Biodiversity is the variety of living organisms. Life is found in habitats as different as cold mountains, deserts, deep oceans, hot springs and rainforests. The number of species that are known and described is about 1.7 to 1.8 million, and new species continue to be found.

With so many species, nobody can study each one separately. Classification is the grouping of organisms into convenient categories on the basis of observable characters. It

• makes the study of a large number of organisms manageable;
• gives each organism a standard name, so that confusion caused by local names is avoided;
• shows which organisms resemble each other and are probably close relatives;
• helps to place a newly found organism.

Three linked activities are involved. Identification is finding the correct name and position of an organism. Nomenclature is the naming of organisms. Classification is the placing of organisms in groups according to their similarities and differences.

Three domains. In 1990 Carl Woese proposed that all living organisms be placed in three domains, mainly on the basis of differences in ribosomal RNA and in cell chemistry:

• Bacteria: prokaryotes (no true nucleus, no membrane-bound organelles) whose cell walls contain peptidoglycan.
• Archaea: prokaryotes whose cell walls lack peptidoglycan and whose membrane lipids differ; many live in extreme habitats such as very salty water (halophiles), hot acidic springs (thermoacidophiles) and oxygen-free swamps (methanogens).
• Eukarya: organisms with a true nucleus and membrane-bound organelles, including the kingdoms Protista, Fungi, Plantae and Animalia.

(The three-domain scheme is a more recent addition to the five-kingdom scheme studied in the next chapter; older textbooks may not mention it.)

Taxonomy is the science of identifying, naming and classifying organisms. Systematics is the wider study of the diversity of organisms and the relationships among them, including their evolutionary history. The word is from the Latin systema, meaning a systematic arrangement of organisms; Linnaeus used it in the title of his book Systema Naturae.

Classification is carried out in steps called taxonomic categories (ranks). Every category is a unit of classification, and a group of organisms placed in a rank is called a taxon (plural taxa). The obligatory categories form a hierarchy, from the lowest to the highest:

Species → Genus → Family → Order → Class → Phylum (Division in plants) → Kingdom

• Species: a group of individuals with fundamental similarities, which can be distinguished from other species. Biologists commonly add the biological species concept: the members of a species interbreed to produce fertile offspring and are reproductively isolated from other species. (The textbook states the species idea through similarities; the interbreeding criterion is the commonly taught extension and does not apply to organisms that reproduce only asexually.)
• Genus: a group of related species that have more characters in common with each other than with species of other genera.
• Family: a group of related genera. Order: a group of related families. Class: a group of related orders.
• Phylum (animals) or Division (plants): a group of related classes. Kingdom: the highest category in the five-kingdom scheme, including all the phyla or divisions of one great group.

Going up the hierarchy, a category includes more organisms that share fewer characters; going down, a category includes fewer organisms that share more characters.

To find the lowest category two organisms share, compare their taxa from species upward and stop at the first match. The tiger and the lion share the genus Panthera; the tiger and the domestic cat (Felis catus) share the family Felidae; the tiger and the dog (Canis familiaris) share the order Carnivora.

Organisms often have different local names in different places and languages, so a standard system was needed. Carolus Linnaeus introduced binomial nomenclature, in which the scientific name of every species has two words: the genus followed by the species epithet. Naming of plants follows the International Code of Botanical Nomenclature (ICBN) and of animals the International Code of Zoological Nomenclature (ICZN). (The plant code has since been renamed to cover algae and fungi as well; the rules below are the same.)

Rules for writing a scientific name

1. Names are in Latin or Latinised form.
2. When printed, both words are in italics; when handwritten, each word is underlined separately.
3. The genus starts with a capital letter; the species epithet is written entirely in small letters.
4. The abbreviated name of the author (the scientist who first described and named the species) is written after the species epithet in ordinary type, for example Mangifera indica Linn. (Linn. stands for Linnaeus).
5. Each species has one scientific name that is accepted all over the world.

Examples: Mangifera indica (mango), Solanum tuberosum (potato), Panthera tigris (tiger), Homo sapiens (human). The wrong forms mangifera indica, Mangifera Indica and MANGIFERA INDICA each break a rule.

Why it matters. The brinjal is also called eggplant or aubergine in other places, but its scientific name, Solanum melongena, is the same everywhere. The genus also shows relationships: Solanum tuberosum and Solanum melongena belong to one genus and so share many characters.