What makes something living?
Living things grow, reproduce, respond to their surroundings and carry out metabolism (all the chemical reactions in a cell). Growth and reproduction alone are not enough: a crystal grows, and a mule cannot reproduce. Metabolism and consciousness (sensing the surroundings) are the features every living cell shows. The cell is the basic unit of life.
Biodiversity: how many kinds of life?
Biodiversity means the number and variety of living things on Earth. About 1.7 to 1.8 million species have been named so far, and many more are still unknown. Each kind of plant, animal or microbe that is new to science must be studied, named and placed in a group.
India is a mega-diverse country: it has a small share of the world's land but a large share of its species.
Why do we classify living things?
We cannot study millions of organisms one by one. Classification means putting organisms into groups based on features we can see or test. It helps us to:
- identify a new organism quickly,
- learn about a whole group by studying a few members,
- see how groups are related through evolution,
- use one clear name everywhere in the world.
Each group at any level is called a taxon (plural taxa).
The three domains of life
Carl Woese compared the RNA inside ribosomes and found that life has three great branches, called domains:
- Bacteria: prokaryotes (no nucleus), common everywhere.
- Archaea: also prokaryotes, but with different cell membranes and chemistry; many live in very hot, salty or oxygen-free places.
- Eukarya: cells with a nucleus. Protists, fungi, plants and animals all belong here.
The domain sits above kingdom. Five-kingdom classification (next lesson) sorts living things inside these domains.
Taxonomy and systematics
Taxonomy is the science of characterisation (describing features), identification, nomenclature (naming) and classification. Systematics is wider: it also studies how organisms are related by evolution. The word comes from the Latin systema, meaning an ordered arrangement.
Taxonomical aids
Tools that help us identify and study organisms:
- Herbarium: dried, pressed plant samples on sheets with labels.
- Botanical garden: living plants grown and labelled for study.
- Museum: preserved animals and skeletons in jars or cases.
- Zoological park: wild animals kept close to their natural home.
- Key: pairs of opposite choices (couplets) that lead you to a name, like "leaves needle-like / leaves flat".
- Flora, manual, monograph, catalogue: written records of the plants or animals of an area or group.
Species and the taxonomic hierarchy
A species is a group of organisms that are very alike and can breed with each other to give fertile young. Lion and tiger are two species of the same genus, Panthera.
Ranks are arranged in steps from wide to narrow. This order is the taxonomic hierarchy:
Kingdom → Phylum (for animals) / Division (for plants) → Class → Order → Family → Genus → Species
Going up the ladder, members share fewer features. Going down, they share more.
Example: the full address of three organisms
- Human: Animalia → Chordata → Mammalia → Primata → Hominidae → Homo → sapiens
- Housefly: Animalia → Arthropoda → Insecta → Diptera → Muscidae → Musca → domestica
- Mango: Plantae → Angiospermae → Dicotyledonae → Sapindales → Anacardiaceae → Mangifera → indica
Binomial naming: rules for scientific names
Local names change from place to place. Carolus Linnaeus gave us binomial nomenclature: every species gets a two-word name. The codes ICBN (plants) and ICZN (animals) set the rules:
- Names are in Latin or Latinised.
- First word = genus, starts with a capital letter. Second word = specific epithet, all small letters.
- Printed in italics; when handwritten, each word is underlined separately.
- The name of the scientist who first described it may follow, in short form: Mangifera indica Linn.
Try it: make your own key
Collect 6 leaves from around your home. Ask one yes/no question that splits them into two piles (for example, "Is the edge smooth?"). Split each pile again with another question until each leaf is alone. You have just built a dichotomous key. In the 3D free-play step, pick an organism and check its address on the ladder.
Key formulas and definitions
- Hierarchy: Kingdom → Phylum/Division → Class → Order → Family → Genus → Species
- Binomial name = Genus (capital) + specific epithet (small), in italics
- Taxonomy = characterise + identify + name + classify
- Systematics = taxonomy + evolutionary relationships
- Three domains: Bacteria, Archaea, Eukarya
Worked examples
1. Write the scientific name of mango correctly and say what each word means.
Mangifera indica. 'Mangifera' is the genus (capital M). 'indica' is the specific epithet (small i). Both are in italics, or underlined separately when handwritten.
2. Lion is Panthera leo and tiger is Panthera tigris. What can you say about them?
They share the same genus Panthera, so they are closely related. They are different species because the second word differs and they do not normally breed to give fertile young.
3. Arrange in order from widest to narrowest: Family, Kingdom, Species, Class, Genus, Order, Phylum.
Kingdom → Phylum → Class → Order → Family → Genus → Species.
4. A student finds a new leaf. Which taxonomical aid will help her name it in the field, and which one stores dried samples for later?
A key (pairs of yes/no choices) helps identify it in the field. A herbarium stores dried, pressed samples on labelled sheets.
5. Why is Archaea kept apart from Bacteria even though both lack a nucleus?
Their ribosomal RNA, cell membrane lipids and cell wall chemistry are very different. On the tree of life, Archaea are actually closer to Eukarya than to Bacteria.
Common mistakes
- Writing 'Homo Sapiens' with a capital S. Only the genus starts with a capital: Homo sapiens.
- Thinking taxonomy and systematics mean exactly the same. Systematics also includes evolutionary relationships.
- Using 'phylum' for plants. For plants the rank is usually called 'division'.
- Believing growth alone proves something is alive. Crystals grow too; metabolism is the key feature.