AHL Content Statements

• A3.2.1

  Need for classification of organisms

• 
  

  Classification is needed because of the immense diversity of species. After classification is completed, a broad range of further study is facilitated.

• A3.2.2

  Difficulties classifying organisms into the traditional hierarchy of taxa

• 
  

  The traditional hierarchy of kingdom, phylum, class, order, family, genus and species does not always correspond to patterns of divergence generated by evolution.
  NOS: A fixed ranking of taxa (kingdom, phylum and so on) is arbitrary because it does not reflect the gradation of variation. Cladistics offers an alternative approach to classification using unranked clades. This is an example of the paradigm shift that sometimes occurs in scientific theories.
  

• A3.2.3

  Advantages of classification corresponding to evolutionary relationships

• 
  

  The ideal classification follows evolutionary relationships, so all the members of a taxonomic group have evolved from a common ancestor. Characteristics of organisms within such a group can be predicted because they are shared within a clade.
  

• A3.2.4

  Clades as groups of organisms with common ancestry and shared characteristics

• 
  

  The most objective evidence for placing organisms in the same clade comes from base sequences of genes or amino acid sequences of proteins. Morphological traits can be used to assign organisms to clades.

• A3.2.5

  Gradual accumulation of sequence differences as the basis for estimates of when clades diverged from a common ancestor

• 
  

  This method of estimating times is known as the “molecular clock”. The molecular clock can only give estimates because mutation rates are affected by the length of the generation time, the size of a population, the intensity of selective pressure and other factors.
  

• A3.2.6

  Base sequences of genes or amino acid sequences of proteins as the basis for constructing cladograms

• 
  

  Examples can be simple and based on sample data to illustrate the tool.
  NOS: Students should recognize that different criteria for judgement can lead to different hypotheses. Here, parsimony analysis is used to select the most probable cladogram, in which observed sequence variation between clades is accounted for with the smallest number of sequence changes.
  

• A3.2.7

  Analysing cladograms

• 
  

  Students should be able to deduce evolutionary relationships, common ancestors and clades from a cladogram. They should understand the terms “root”, “node” and “terminal branch” and also that a node represents a hypothetical common ancestor.
  

• A3.2.8

  Using cladistics to investigate whether the classification of groups corresponds to evolutionary relationships

• 
  

  A case study of transfer of plant species between families could be used to develop understanding, for example the reclassification of the figwort family (Scrophulariaceae). However, students are not required to memorize the details of the case study.
  NOS: Students should appreciate that theories and other scientific knowledge claims may eventually be falsified. In this example, similarities in morphology due to convergent evolution rather than common ancestry suggested a classification that by cladistics has been shown to be false.
  

• A3.2.9

  Classification of all organisms into three domains using evidence from rRNA base sequences

• 
  

  This is the revolutionary reclassification with an extra taxonomic level above kingdoms that was proposed in 1977.