The Living World — Notes

Millions of kinds of organisms live on Earth and new ones are still being described. Classification is needed because it:

• makes the study of such a large variety manageable, by placing organisms in groups;
• gives each organism a universal name, ending the confusion caused by local names;
• allows a newly found organism to be placed quickly among those already known;
• shows relationships among organisms, so that what is known about a group helps to predict its members;
• makes information easy to store and retrieve.

Taxonomy is the science of identification, nomenclature and classification. Systematics includes taxonomy and also studies the evolutionary relationships among organisms. The word comes from the Latin systema, used by Carolus Linnaeus in the title of his book Systema Naturae (1735).

Modern branches of taxonomy supplement the study of form and structure:

• Numerical taxonomy: a large number of characters (hundreds) are coded, every character is given equal weight, and a computer calculates overall similarity. Example: on the ten characters of the table, mango and neem match in 9, a similarity of 0.9.
• Cytotaxonomy: uses cytological information: the number, size, shape and behaviour of chromosomes. Bread wheat has 42 chromosomes in a body cell on a basic number of 7, so it is 6-ploid (hexaploid).
• Chemotaxonomy: uses the chemical constituents of organisms, such as alkaloids, flavonoids, terpenes and the amino-acid sequence of proteins.

Species. The biological species concept (Ernst Mayr) defines a species as a group of interbreeding natural populations that are reproductively isolated from other such groups. Members mate and produce fertile offspring. The concept cannot be applied to organisms that reproduce only asexually or to fossils.

Taxonomic categories (the hierarchy) in descending order: Kingdom, Phylum (Division in plants), Class, Order, Family, Genus, Species. Each category is a rank; an actual group at a rank (for example the family Poaceae) is a taxon. Going down the hierarchy the number of organisms in a group decreases and the characters they share increase; going up the number of organisms increases and shared characters decrease.

The scientific names are Homo sapiens, Musca domestica, Mangifera indica and Triticum aestivum.

Binomial nomenclature gives every species a two-word scientific name. It was introduced by Carolus Linnaeus (Species Plantarum, 1753). Names are governed by international codes: the International Code of Nomenclature for algae, fungi and plants (ICN, formerly the Botanical Code) and the International Code of Zoological Nomenclature (ICZN).

Rules

1. The name is in Latin or latinised form, often drawn from Latin or Greek words.
2. The first word is the genus and begins with a capital letter; the second is the species epithet and begins with a small letter (Homo sapiens, Triticum aestivum).
3. In print both words are in italics; in handwriting each word is underlined separately.
4. The author's name (abbreviated, in roman type) follows the epithet: Mangifera indica Linn.
5. The oldest valid name has priority and no two species may share a name.

Advantages over common names: a scientific name is the same in every country, unique to one species, shows the genus (hence close relatives), and follows fixed rules, so scientists can communicate without ambiguity.

Biologists have grouped organisms on three different bases.

• Artificial system: uses one or a few characters, usually easily seen ones such as habit (herb, shrub, tree) or the number and arrangement of stamens (the sexual system of Linnaeus). It is simple, but it separates closely related organisms and brings unrelated ones together.
• Natural system: uses all the natural affinities among organisms, with external and internal characters from morphology, anatomy, embryology and chemistry. Example: the system of Bentham and Hooker (Genera Plantarum). It does not show descent.
• Phylogenetic system: groups organisms by their evolutionary relationships and common ancestry, using evidence from fossils, comparative anatomy, embryology and molecular data. Examples: Engler and Prantl, Hutchinson. Modern classification is mostly phylogenetic.

Using only the character 'compound leaves' separates neem from mango, though they match in 9 of 10 characters: this shows the weakness of an artificial key.