# Topic 29 Biodiversity

F.5 HKDSE Biology · Topic XXIX · Lesson 3 (29.1–29.4)

How living things are classified: six kingdoms, three domains, and dichotomous keys.

## Figure

Three domains, six kingdoms: from rRNA evidence, Archaea are closer to Eukarya than to Bacteria, and fungi are closer to animals than to plants.

## Key points

- **Biodiversity ／ 生物多樣性** = the variety of living organisms; each species has **adaptations** to its **habitat ／ 生境** (woodpecker: chisel-like beak, gripping toes; black kite: hooked beak, talons).
- **Seven ranks**: **kingdom, phylum, class, order, family, genus, species**; a group at any rank is a **taxon**. Downwards, the number of organisms **decreases** and their similarities **increase**.
- **Species**: can **interbreed** to produce **fertile offspring**. **Binomial nomenclature ／ 雙名法**: *Genus species* (capital + small letter), italics in print, underlined by hand, then *H. sapiens*.
- **Phylogenetics ／ 系統發生學** = classification by evolutionary history. Woese's **rRNA** comparison → **three domains** (Bacteria, Archaea, Eukarya), **six kingdoms**; fewer rRNA differences = more closely related.
- Sharing a **lower rank** (same genus) means a **more recent common ancestor** than sharing only a higher rank (same family, different genera).
- **Prokaryotes** (Eubacteria, Archaebacteria): no true nucleus or membrane-bound organelles; **binary fission**. Eubacteria walls of **peptidoglycan ／ 肽聚糖**; archaebacteria in extreme habitats, no peptidoglycan.
- **Eukaryotes**: Protista (mostly unicellular); Fungi (**hyphae** forming a **mycelium**, **chitin** walls, saprophytic or parasitic; yeast is unicellular); Plantae (cellulose walls, photosynthesis); Animalia (no cell wall, heterotrophic).
- **Dichotomous key ／ 二叉式檢索表**: paired statements of **alternative** external features leading to a name; use features that are always visible and need no subjective judgement. It **identifies**; it does not show relatedness.

## Must know

| Kingdom (domain) | Cells | Cell wall | Nutrition | Examples |
|---|---|---|---|---|
| **Eubacteria** (Bacteria) | prokaryotic, unicellular | peptidoglycan | most heterotrophic; some autotrophic (cyanobacteria) | cholera and tetanus bacteria |
| **Archaebacteria** (Archaea) | prokaryotic, unicellular | in most, without peptidoglycan | some autotrophic, some heterotrophic | archaebacteria of hot springs, very salty water and swamps |
| **Protista** (Eukarya) | eukaryotic, mostly unicellular | in some (algae) | autotrophic (algae) or heterotrophic (protozoans) | *Amoeba*, *Paramecium*, kelps, slime moulds |
| **Fungi** (Eukarya) | eukaryotic, mostly multicellular (hyphae) | chitin | heterotrophic: saprophytic or parasitic | mushrooms, bread mould, yeast |
| **Plantae** (Eukarya) | eukaryotic, multicellular | cellulose | autotrophic (photosynthesis) | moss, ferns, pine, maize, rose |
| **Animalia** (Eukarya) | eukaryotic, multicellular | none | heterotrophic | jellyfish, insects, fish, humans |

Plants: mosses (no vascular tissue, rhizoids, spores) · ferns (vascular, spores) · conifers (naked seeds on cones, no flowers) · flowering plants (seeds in fruits; monocots, dicots). Vertebrates: fish (gills, slimy scales, external fertilization) · amphibians (moist skin, lungs and skin, tadpoles) · reptiles (dry scales, internal fertilization, shelled eggs) · birds (feathers, hard-shelled eggs) · mammals (hair, milk, mostly live young); only birds and mammals are homoiotherms.

## Common mistakes

- ✗ "The key shows that fish A and B are closely related." ✓ A dichotomous key only **identifies** organisms by observable features; relatedness comes from **rRNA** comparison or the **lowest shared rank**.
- ✗ "Archaebacteria are bacteria, so they are closest to eubacteria." ✓ Both are **prokaryotes**, but rRNA shows archaebacteria are **more closely related to eukaryotes** than to bacteria.
- ✗ "Fungi are plants." ✓ Fungi have **no chlorophyll**, walls of **chitin** (not cellulose) and feed **heterotrophically**; they are closer to animals than to plants.
- ✗ "Conifers bear fruits; mosses and ferns have seeds." ✓ Conifers have **naked seeds on cones** and no flowers or fruits; mosses and ferns reproduce by **spores**.
- ✗ "*Thymus vulgaris* and *Phaseolus vulgaris* are most closely related: same species name." ✓ The **genus name** shows relatedness: *Phaseolus vulgaris* and *Phaseolus lunatus* are in the same genus. A species name ("vulgaris" = common) is only a label within its genus.

## Quick check

1. Arrange class, genus, phylum and species in order of increasing similarity among their members.
2. Four shrimps: *Leptochela japonicus*, *Metapenaeus japonicus*, *Metapenaeus joyneri*, *Metapenaeopsis dura*. Which two have the closest phylogenetic relationship? Why?
3. A bat flies, but it is a mammal, not a bird. Give two features that show this.
4. A multicellular organism has cell walls of chitin and does not photosynthesize. Name its kingdom, and give one feature it shares with plants and one that differs.
5. Five new species of eubacteria are found in Antarctic ice. Which of nucleus, cell wall, chloroplast and mitochondrion do their cells have? Explain.

## Answers

1. Phylum, class, genus, species.
2. *Metapenaeus japonicus* and *Metapenaeus joyneri*: they share the genus name, so they belong to the same genus, the lowest shared rank.
3. A bat has hair, not feathers; it gives birth to live young and feeds them on milk, instead of laying hard-shelled eggs.
4. Fungi. Shared: cell walls, and they do not move about. Different: no photosynthesis, and the wall is chitin rather than cellulose.
5. Cell wall only. Eubacteria are prokaryotes: no nucleus and no membrane-bound organelles such as chloroplasts or mitochondria.

## Did you know?

The Hong Kong newt is called *Paramesotriton hongkongensis*: its species name, *hongkongensis*, simply means "from Hong Kong".