A3.2:  Classification and Cladistics

Guiding Questions:  
Guiding questions help students view the content of the syllabus through the conceptual lenses of both the themes and the levels of biological organization.

• What tools are used to classify organisms into taxonomic groups? 
• How do cladistic methods differ from traditional taxonomic methods? ​ 

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​Linking Questions:  
Linking questions strengthen students’ understanding by making connections between topics.  The ideal outcome of the linking questions is networked knowledge.

• How can similarities between distantly related organisms be explained?
• What are some examples of ideas over which biologists disagree? 

Resources:

• At SHS, Topic A3.2 is taught in the Nature of Biology and Cladistics units. 
• Quizlet study set for this topic.  Coming soon!
• View the general sequence of the SHS course
• View the SHS units mapped to the IB Biology curriculum roadmap

AHL A3.2.1— Need for classification of organisms. 

• ​Define “taxonomy”
• List the levels of classification in the traditional hierarchy of taxa.
• Outline the benefits of having a system of classification of organisms. ​

AHL A3.2.2— Difficulties classifying organisms into the traditional hierarchy of taxa.  

• List the levels of classification in the traditional hierarchy of taxa.
• Discuss limitations of the traditional classification system.

AHL A3.2.3—  Advantages of classification corresponding to evolutionary relationships.

• Define “synapomorphies.”
• Discuss advantages of a classification system that corresponds to evolutionary relationships.

AHL A3.2.4— Clades as groups of organisms with common ancestry and shared characteristics.

• Define clade.
• Identify a clade as a branch in a cladogram.
• List evidence used for placing organisms in a clade.​

AHL A3.2.5— Gradual accumulation of sequence differences as the basis for estimates of when clades diverged from a common ancestor.

• Outline the relationship between time, evolutionary relationships and biological sequences (nitrogenous base or amino acid). 
• State the source of differences between biological sequences (nitrogenous base or amino acid).
• Outline the use of a “molecular clock” to determine time since divergence between two clades.​

AHL A3.2.6—  Base sequences of genes or amino acid sequences of proteins as the basis for constructing cladograms.

• State that DNA and amino acids sequences can be the basis for constructing cladograms. .
• Outline the relationship between time, evolutionary relationships and biological sequence (nitrogenous base or amino acid) similarities between species. 
• Define “parsimony” as used in a biological context.
• Outline how the principle of parsimony relates to evolutionary divergence between the members of a clade.​

AHL A3.2.7—  Analyzing cladograms. 

• Define “cladogram”.
• Outline what is represented by a “root”, “node” and “terminal branch” on a cladogram. 
• Analyze a cladogram to deduce evolutionary relationships, common ancestry and clades.

AHL A3.2.8— Using cladistics to investigate whether the classification of groups corresponds to evolutionary relationships.

• Explain why the development of cladistics lead to the reclassification of some species.
• Outline the reason and evidence for the reclassification of the figwort family.

AHL A3.2.9— Classification of all organisms into three domains using evidence from rRNA base sequences.

• List the three domains of life.
• Discuss evidence from rRNA base sequences that led to the reclassification of life from two cell types (prokaryotic and eukaryotic) to three domains. 
• Interpret the tree diagram that illustrates the evolutionary relationship between organisms of the three domains.