What is living?
Defining "living" is harder than it seems. Several features are often listed, but not all of them are unique to living things:
Growth: living things grow by cell division, from inside. But non-living crystals and mountains also "grow" by adding material from outside, so growth alone is not a defining feature.
Reproduction: producing offspring. But mules, sterile worker bees and infertile couples don't reproduce, yet they are clearly alive. So reproduction is not an all-inclusive defining feature either.
Metabolism: the sum of all chemical reactions in the body. No non-living object shows metabolism, so it is a defining feature of life. Isolated metabolic reactions can be carried out in a test tube, but the test tube is not alive.
Cellular organisation: all living things are made of one or more cells, another defining feature.
Consciousness: the ability to sense and respond to the environment (light, temperature, touch, chemicals). All organisms, from bacteria to humans, respond to their surroundings. Humans are the only organisms aware of themselves (self-consciousness).
So we can describe living organisms as self-replicating, evolving and self-regulating interactive systems capable of responding to external stimuli.
Diversity in the living world
About 1.7 to 1.8 million species have been described so far, and many more remain unknown. This variety is biodiversity. India, with only about 2.4% of the world's land area, has around 8% of its recorded species.
To study so many organisms, biologists need a standard way to identify, name and classify them.
Nomenclature
Common names differ from place to place: the mango is aam in Hindi and mamidi in Telugu. Scientists need one universal name for each organism.
Binomial nomenclature, introduced by Carolus Linnaeus, gives every organism a two-part scientific name:
the generic name (genus), and
the specific epithet (species).
Example. Mango is *Mangifera indica: Mangifera is the genus and indica the species. Humans are Homo sapiens; the domestic cat is Felis catus; wheat is Triticum aestivum*.
Rules:
Names are in Latin (or Latinised), whatever their origin.
The genus starts with a capital letter; the species starts with a small letter.
When printed, names are in italics; when handwritten, the two words are underlined separately.
The name of the scientist who first described the species may follow in abbreviated form: Mangifera indica L. (Linnaeus).
Names for plants follow the International Code for Botanical Nomenclature (ICBN), and for animals the International Code of Zoological Nomenclature (ICZN).
Classification and the taxonomic hierarchy
Classification is grouping organisms into convenient categories based on easily observable characters. Each category is a taxon (plural taxa). Taxonomy is the science of identification, nomenclature and classification. Systematics also studies the evolutionary relationships between organisms.
The categories form a hierarchy, from the most specific to the most general:
Category | Example: human | Example: mango |
|---|---|---|
Species | sapiens | indica |
Genus | Homo | Mangifera |
Family | Hominidae | Anacardiaceae |
Order | Primates | Sapindales |
Class | Mammalia | Dicotyledonae |
Phylum / Division | Chordata | Angiospermae |
Kingdom | Animalia | Plantae |
As we move up the hierarchy, the number of common characters decreases and the number of organisms included increases. Members of a species share the most features; members of a kingdom share very few.
Species: a group of individuals with fundamental similarities that can interbreed to produce fertile offspring. Example: Panthera leo (lion), Panthera tigris (tiger).
Genus: related species with more features in common with each other than with species of other genera. Lion, tiger and leopard are all in Panthera.
Family: related genera. The cat family Felidae includes Panthera and Felis.
Similar families form an order, orders form a class, classes a phylum (called a division for plants), and phyla a kingdom.
Taxonomic aids
Collections and tools that help identify and classify organisms:
Aid | What it is |
|---|---|
Herbarium | a store of dried, pressed and preserved plant specimens on sheets, with labels giving name, place and date of collection and the collector's name |
Botanical garden | a collection of living plants for reference, each labelled with its scientific name and family. Examples: Royal Botanic Gardens at Kew (England), Indian Botanical Garden at Howrah, National Botanical Research Institute at Lucknow |
Museum | preserved plant and animal specimens: in jars of preservative, stuffed, or as skeletons |
Zoological park | wild animals kept in protected surroundings resembling their natural habitat, so we can study their behaviour |
Key | a set of paired, contrasting statements (couplets), each a lead, used step by step to identify an organism |
Flora, manuals, monographs and catalogues are written records that also help identification.
Summary
Metabolism and cellular organisation are defining features of life; growth and reproduction are not.
Binomial nomenclature: Genus species, in italics, genus capitalised.
The hierarchy: species → genus → family → order → class → phylum → kingdom.
Herbaria, botanical gardens, museums, zoos and keys are taxonomic aids.