4.5.3Genetics, Variation and Interdependence
Hierarchy of the Taxa — AQA A Level Biology 7402 Revision Notes & Topical Questions
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4.5.3 Hierarchy of the Taxa — AQA A Level Biology 7402 notes: List the taxonomic hierarchy from domain to species and use the binomial system to name organisms correctly.Classification is hierarchical: a large group is divided into smaller groups, and each of these is divided aga…
- nested hierarchy
- fewer organisms
- more features
- Protoctista
- first word
- second word
Section 1 of 9A hierarchy of groups
Classification is hierarchical: a large group is divided into smaller groups, and each of these is divided again, with no overlap between groups at the same level. This is a nested hierarchy, in which each group is contained within the one above. The groups are called taxa (singular: taxon).
The taxa, from the largest to the smallest, are:
A common memory aid is
- Dear King Philip Came Over For Good Soup.
- As you move down the list, there are fewer organisms in each group, but the members share more features and are more closely related.
Section 2 of 9The three domains
The highest level is the domain. Three domains are recognised, based mainly on differences in ribosomal RNA, cell membrane lipids and the type of cell.
| Domain | Cell type | Examples |
|---|---|---|
| Archaea | Prokaryotic; unusual membrane lipids; often in extreme environments | Methane producers, salt-lovers |
| Bacteria | Prokaryotic; peptidoglycan cell walls | E. coli, Staphylococcus |
| Eukarya | Eukaryotic; membrane-bound organelles and a nucleus | Animals, plants, fungi, protoctists |
Domain Eukarya contains the kingdoms Animalia, Plantae, Fungi and Protoctista. Prokaryotes (Bacteria and Archaea) were once all placed in a single kingdom, Prokaryotae.
Section 3 of 9The binomial system
Common names differ between countries and can refer to several species. In the binomial system, devised by Linnaeus, each species is given a two-part Latin name that is the same all over the world.
For example, humans are Homo sapiens, and the closely related extinct species Homo neanderthalensis belongs to the same genus. Members of the same genus are closely related, but they are different species and are not expected to produce fertile offspring together. After first use, the genus may be abbreviated: H. sapiens.
The advantages are that the name is universal (it allows scientists of all languages to communicate), it is unambiguous and it shows the genus relationship between species.
Section 4 of 9A worked classification
| Taxon | Human | Domestic cat |
|---|---|---|
| Domain | Eukarya | Eukarya |
| Kingdom | Animalia | Animalia |
| Phylum | Chordata | Chordata |
| Class | Mammalia | Mammalia |
| Order | Primates | Carnivora |
| Family | Hominidae | Felidae |
| Genus | Homo | Felis |
| Species | sapiens | catus |
The two animals share the same taxa down to class and differ from the order downwards, which shows that they are related but not very closely. Both lines split from a common mammalian ancestor.
Section 5 of 9Limits of the system
The species level uses the definition of interbreeding, which cannot be applied to asexual organisms such as bacteria, to fossils, or to species that rarely meet.
Section 6 of 9Features used at each level
At the top, domains are separated by cell structure and molecular features.
Notice that the lower the taxon, the more specific the shared features. All members of a family have many characteristics in common, while members of one kingdom share only broad features. This is why the system is called a hierarchy: each level fits completely inside the one above it.
Exam tip. If asked to place two organisms, find the lowest taxon they share. Two animals that are in the same class but different orders are less closely related than two that share an order. The more taxa shared, the closer the relationship.
Viruses are not placed in the three domains, as they are not cells and are not considered living by many biologists.
Section 7 of 9Exam-style questions
Name the taxonomic groups in order, from kingdom to species. [2]2 marks
Kingdom, phylum, class, order, family, genus, species. [2 marks for the correct sequence; 1 mark if one adjacent pair is swapped or one is missing.]
The wolf is called Canis lupus and the coyote Canis latrans. State two conclusions you can draw from these names. [2]2 marks
1. They belong to the same genus (Canis), so they are closely related / share a recent common ancestor. [1]
2. They are different species (lupus and latrans). [1]
Explain the advantages of using the binomial system to name organisms. [3]3 marks
Exam-style questions1. It gives each species a single name that is accepted worldwide, so scientists in different countries can communicate. [1]2. It avoids confusion caused by local common names, which may refer to different species. [1]3. The first word shows the genus, which indicates close relationship. [1]
Section 8 of 9Key terms
| Term | Meaning |
|---|---|
| Hierarchy | An arrangement in which groups are nested within larger groups without overlap. |
| Domain | The highest taxon: Archaea, Bacteria or Eukarya. |
| Genus | A group of closely related species. |
| Species | The lowest taxon: organisms that interbreed to give fertile offspring. |
| Binomial system | Naming each species with two Latin words, genus then species. |
| Taxon | A group of organisms at any level in the hierarchy. |
| Kingdom | A major group within a domain, for example Animalia. |
Section 9 of 9Exam technique
Marked practiceQuizzes for this lesson
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