Diversity in the Living World

1The taxonomic hierarchy

Taxonomy sorts organisms into nested ranks. From broadest to narrowest the sequence is Kingdom, Phylum (or Division in plants), Class, Order, Family, Genus and Species. Species is the basic unit: a group of organisms that can interbreed in nature and produce fertile offspring. As you move down the ladder, the number of shared characters rises and the number of organisms in the taxon falls.

A useful mnemonic keeps the seven ranks in order: King Philip Came Over For Good Soup. Phylum is the animal word; Division is the plant equivalent at the same rank. Questions that scramble Family and Order are testing whether you treat the mnemonic as a list you can reverse, not just recite.

Figure. Each band is narrower than the one above it: shared characters rise, membership falls. Phylum and Division share one band because they are the same rank under two names.

Narrowing a specimen

1. KingdomBroadest cut — animals against plants against fungi, and so on.
2. Phylum / DivisionSame rank, different word: Phylum for animals, Division for plants.
3. Class → Order → FamilyEach step adds shared characters and drops organisms that lack them.
4. Genus → SpeciesGenus groups close species; species is the basic unit that can interbreed.

2Binomial nomenclature

Carl Linnaeus gave each organism a two-word Latin name: the genus name first, then the specific epithet. Together they form the species name — Homo sapiens for humans, Mangifera indica for the mango. Both words are italicised in print; handwritten, they are underlined. The genus alone may be abbreviated to its initial once it has been written in full, so H. sapiens is legal after Homo sapiens has appeared.

Two codes keep names from colliding: the International Code of Botanical Nomenclature (ICBN) for plants, and the International Code of Zoological Nomenclature (ICZN) for animals. A name is not a description; it is a unique label that lets every biologist point at the same organism.

Figure. Two boxes make one species name. The left box is always capitalised; the right box never is. Italics apply to both together.

3Whittaker's five kingdoms

R.H. Whittaker (1969) replaced the older two-kingdom plant–animal split with five kingdoms: Monera, Protista, Fungi, Plantae and Animalia. The cut used five criteria together — cell structure (prokaryotic against eukaryotic), thallus or body organisation, mode of nutrition, reproduction, and phylogenetic relationships — not any one of them alone.

Monera holds every prokaryote. Protista holds unicellular eukaryotes. Fungi are multicellular (mostly) heterotrophs with chitinous walls. Plantae are multicellular autotrophs with cellulose walls. Animalia are multicellular heterotrophs without cell walls. Viruses, viroids and prions sit outside all five.

Figure. Five equal boxes, one kingdom each. The tag under each name is the single character that most often decides the placement in an exam stem.

4Monera — the prokaryotes

Monera is the kingdom of prokaryotes: bacteria and the cyanobacteria (blue-green algae). There is no nuclear membrane and no membrane-bound organelle. The genetic material is a single circular DNA molecule in a nucleoid, and the ribosomes are 70S. Many bacteria also carry small circular plasmids.

Shape is the first character a stem usually gives. Coccus is spherical, bacillus is rod-shaped, vibrio is comma-shaped and spirillum is spiral. Nutrition spans the whole range — photosynthetic cyanobacteria, chemosynthetic nitrifiers, and heterotrophs that are saprophytic, parasitic or symbiotic — which is why mode of nutrition alone never places a bacterium.

Figure. Four named shape classes as equal boxes. The figure teaches the vocabulary, not the anatomy — outlines of cocci and spirilla need closed curves the renderer does not have.

5Protista — unicellular eukaryotes

Protista is Whittaker's kingdom for unicellular eukaryotes. They have a true nucleus and membrane-bound organelles, which is the whole difference from Monera. Chrysophytes, dinoflagellates, euglenoids, slime moulds and protozoans (Amoeba, Paramoecium, Trypanosoma) all sit here. Many are aquatic; some photosynthesise and some engulf food.

The exam trap is to treat 'eukaryotic' as enough to leave Protista. Multicellular eukaryotes leave for Fungi, Plantae or Animalia. Unicellular plus eukaryotic is the pair that almost always means Protista in the five-kingdom scheme.

Figure. Two kingdoms that are both unicellular. The nuclear membrane is the character that splits them; ribosome size follows the same cut.

6Fungi — chitin and heterotrophy

Fungi are eukaryotic heterotrophs with cell walls of chitin, not cellulose. Most are multicellular filaments (hyphae) forming a mycelium; yeasts are the unicellular exception. They never photosynthesise. Nutrition is saprophytic (dead organic matter), parasitic (living hosts) or symbiotic — lichen with an alga or cyanobacterium, and mycorrhiza with plant roots.

Lichen is the partnership that exams use as a pollution indicator: the fungal partner provides shelter and absorbs minerals; the algal or cyanobacterial partner photosynthesises. Mycorrhiza trades soil minerals to the plant for carbohydrates. Neither partnership moves the fungus into Plantae — the wall is still chitin and the fungus itself still cannot fix carbon.

Figure. Lichen as two boxes with exchange arrows. The fungal box keeps the chitin tag so the partnership is not misread as a plant.

How a lichen is built

1. Fungal partnerProvides the body, absorbs water and minerals, and shelters the photobiont.
2. Algal / cyanobacterial partnerPhotosynthesises and shares the carbon compounds.
3. Still a fungusThe wall remains chitin; the dual organism is classified with fungi for the fungal partner, and used as a pollution indicator because lichens are sensitive to SO₂.

7Plantae — from algae to angiosperms

Plantae in the five-kingdom scheme holds the algae, bryophytes, pteridophytes, gymnosperms and angiosperms. The sequence is a ladder of land adaptation: algae are mostly aquatic with no true embryos; bryophytes are the amphibians of the plant kingdom — they have a land body but still need water for fertilisation; pteridophytes add a vascular system; gymnosperms add naked seeds; angiosperms enclose seeds in fruits and have flowers.

The characters that move an organism up the ladder are the ones exams ask for. Bryophytes lack true vascular tissue. Pteridophytes have xylem and phloem but no seeds. Gymnosperm seeds sit exposed on megasporophylls. Angiosperm seeds sit inside a fruit wall.

Figure. Left to right, each group keeps what the previous one gained and adds one more land character. Bryophyta still carries the 'need water' tag that earns the amphibian label.

8Viruses, viroids and prions

Viruses, viroids and prions are acellular and sit outside Whittaker's five kingdoms. A virus is a nucleoprotein particle — genetic material (DNA or RNA, never both) wrapped in a protein coat (capsid) — and is an obligate parasite: inert outside a host, replicating only inside one. Plant viruses are usually RNA viruses; bacteriophages are usually DNA viruses.

Viroids are even smaller: free RNA with no protein coat, causing diseases such as potato spindle tuber. Prions are infectious proteins with no nucleic acid at all — the agent of mad-cow disease and related encephalopathies. The three are distinguished by what they are made of, not by the symptoms they cause.

Figure. Acellular infectious agents classified by composition: virus has nucleic acid plus coat, viroid is naked RNA, prion is protein only.