A3.2 Classification and cladistics

Question 1

Need for classification of organisms

Answer 1

1. Classification is needed because of the immense diversity of species
2. Classification allows for organisms to be arranged into groups –> can be used to make generalisations regarding an organism based on their classification

After classification is complete, a broad range of further study is facilitated

Question 2

Taxonomy

Answer 2

Taxonomy is the field of biology that focuses on the classification of life on earth

• to classify life in a systematic manner, a hierarchy must be developed, with smaller and smaller groups under each larger group
• each group at each level of taxonomy is known as a taxon (plural: taxa)
• THEREFORE, the taxa near the top contain a larger number of organisms, and are more generic, while the taxa near the bottom are more and more specific, but contain a smaller number of organisms

Question 3

Mnemonic for traditional hierarchy of taxa

Answer 3

Did King Phillip Come Over For Grandma’s Soup?

Domain-Kingdom-Phylum-Class-Order-Family-Genus-Species [species is at the top, domain is the bottom]

Question 4

Difficulties classifying organisms into the traditional hierarchy of taxa

Answer 4

1. Limitations to traditional method of classification
   The traditional hierarchy (eg kingdom, phylum..) does not always correspond to patterns of divergence generated by evolution
   - because the traditional classification efforts were based on observable traits instead of genetic information
   Evolutionary history is often given a secondary importance after the primary importance of group organisms together with common traits [viruses are classified more based on evolutionary history than traits]
   - hence the divergence patterns is based more on when features appear instead of changes within the genes
2. Limitations due to fixed levels within hierarchy
   The traditional method has 8 fixed levels
   - some organism groups do not need 8 levels to group them, leading to certain levels being unnecessary
   - other groups need more than 8 levels to be classified properly, hence the traditional method is too restrictive
   - eg there is a need to create a new level between Phylum and Class, which is called sub-phylum for eg insects: Subphylum Hexapoda

Question 5

Modern taxonomic levels

Answer 5

Sub-phylum exists as a level below the phylum, while supra-class exists as a level above the Class
- the distinction between a sub-phylum and supra-class is not clear
- hence biologists decided that there is no need for a fixed levels in classification, and there is no need to give a fixed name to the levels
- scientists now use unranked clades to classify organisms, using levels based on evolutionary relationships