Before you can count biodiversity you need to know what counts as a species and where it belongs. This lesson covers three ways of defining a species, the three domains of life, the taxonomic hierarchy, the four eukaryote kingdoms and how viruses are classified.
A species can be defined in more than one way: by the ability to interbreed to produce fertile offspring (biological), by shared physical features (morphological) or by a shared ecological niche (ecological). All cellular life is placed in three domains: Bacteria and Archaea, which are both prokaryotes but differ in their membrane lipids, ribosomal RNA and cell wall composition, and Eukarya. Eukaryotes are classified into a hierarchy of kingdom, phylum, class, order, family, genus and species, with four kingdoms: Protoctista, Fungi, Plantae and Animalia. Viruses are not in any domain; they are classified by whether their nucleic acid is DNA or RNA and whether it is single-stranded or double-stranded.
Switch between the biological, morphological and ecological concepts. For each one, note what it uses to decide whether two groups are the same species, and the problem the simulator points out.
| Concept | A species is... | Main limitations |
|---|---|---|
| biological | a group of organisms that can interbreed to produce fertile offspring and is reproductively isolated from other such groups | cannot be used for asexual organisms, fossils, or populations that never meet; some separate species hybridise |
| morphological | a group of organisms that share the same observable physical (and anatomical) features | similar-looking species can be different (chiffchaff and willow warbler); one species can look very varied (males and females, larvae and adults) |
| ecological | a group of organisms that share the same ecological niche: the same role, food, habitat and interactions | niches overlap and change, and the niches of many organisms are poorly known |
Producing offspring is not enough for the biological species concept: the offspring must be fertile. A horse and a donkey produce a mule, but the mule is infertile, so they are two species.
Click Bacteria, Archaea and Eukarya in turn and read how they differ. Then choose the hierarchy chip to see how the lion is classified from kingdom down to species.
| Feature | Bacteria | Archaea | Eukarya |
|---|---|---|---|
| cell type | prokaryote | prokaryote | eukaryote, with a nucleus and membrane-bound organelles |
| membrane lipids | unbranched fatty acids joined to glycerol by ester bonds | branched chains joined to glycerol by ether bonds | unbranched fatty acids, ester bonds |
| ribosomal RNA | bacterial rRNA sequences | rRNA sequences differ from Bacteria and are closer to eukaryotes | eukaryotic rRNA, 80S ribosomes |
| cell wall | contains peptidoglycan | no peptidoglycan (other polysaccharides or proteins) | cellulose (plants), chitin (fungi) or none (animals) |
Archaea are not a kind of bacteria, even though both are prokaryotes. They are a separate domain with different membrane lipids, different rRNA and no peptidoglycan in their cell walls.
Four quick checks. Each correct answer earns XP and records this skill on your profile.
In the species concepts tab, why are horses and donkeys treated as separate species?
The simulator shows the chiffchaff and the willow warbler. Which concept do they cause problems for?
Which feature, shown in the Domains tab, is found in the cell walls of Bacteria but not of Archaea?
In the viruses view, HIV and influenza virus are both placed in which group?
On the kingdoms chip, click each organism to read its kingdom's features. Then switch to viruses and click each capsid: the red strand inside shows whether the nucleic acid is single- or double-stranded.
| Kingdom | Key features |
|---|---|
| Protoctista | eukaryotes that do not fit the other kingdoms; mostly unicellular (Amoeba, Plasmodium), some multicellular algae; some photosynthetic, some heterotrophic; no differentiated tissues |
| Fungi | usually multicellular hyphae forming a mycelium (yeasts are unicellular); cell walls of chitin; heterotrophic, secreting enzymes and absorbing the products (saprotrophic or parasitic); store glycogen; reproduce by spores |
| Plantae | multicellular; chloroplasts and photosynthesis; cellulose cell walls; large permanent vacuole; store starch; tissues and organs |
| Animalia | multicellular; heterotrophic; no cell walls; usually nervous coordination and movement; store glycogen; tissues and organs |
Unlocks once the four checks above are done. Worth more XP, written to A Level Paper 4 standard.
Which list shows the taxonomic ranks in order from the largest group to the smallest?
Which pair of features would place an organism in the kingdom Fungi?
A newly discovered prokaryote lives in a hot spring. Its membranes contain branched lipids joined to glycerol by ether bonds. In which domain should it be placed?
Why can the biological species concept not be applied to an extinct species of trilobite known only from fossils?
This skill is now recorded on your profile. Keep the chain going.