A3.2:  Classification and Cladistics

This topic is HL only

Theme:  Unity and Diversity

The tools and methods used to study cells demonstrate how scientific investigation uncovers both themes:

• Light and electron microscopy allow direct observation of both the common structures that unite all cells and the unique features that distinguish different cell types
• Studying cells from different organisms reveals both shared fundamental features and remarkable adaptations to specific environments and lifestyles

Unity:

• All organisms can be placed in clades based on shared common ancestors, illustrating that all life follows the same fundamental patterns of evolutionary branching and descent with modification
• ​Organisms within the same clade share characteristics because they inherited them from the same common ancestor, allowing scientists to predict traits and understand relationships across diverse groups
• Base sequences of genes and amino acid sequences of proteins provide objective evidence for evolutionary relationships because all organisms use the same genetic code inherited from common ancestors
• The molecular clock principle works across all life forms because all organisms accumulate genetic changes over time through similar mutational processes
• The three-domain classification system is based on rRNA sequences that are conserved across all life

Diversity:

• The number and variety of species on Earth necessitates classification systems to organize and understand the diversity of life forms
• Traditional hierarchical systems and modern cladistic methods are different ways of organizing diversity
• Different organisms and genes evolve at different rates depending on generation time, population size, and selective pressures, creating complex patterns of sequence divergence 
• New molecular evidence can overturn established classifications, showing how scientific understanding of diversity continues to improve
• Different analytical criteria and analyses can lead to different hypotheses about evolutionary relationships, reflecting the complex and sometimes ambiguous nature of understanding evolutionary history

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? ​ 

​
​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? 

Key Terms to Know: all are higher level only

Amino Acid Sequence
Arbitrary
Base Sequence
Binomial Nomenclature
Clade
Cladistics
Cladogram
Class
Classification
Common Ancestor
Convergent Evolution

Divergence
Domain
Evolution
Falsify
Family
Genus
Kingdom
Molecular Clock
Morphology
Mutation
Node

Order
Paradigm Shift
Parsimony
Phylum
Root
Selection Pressure
Species
Taxa
Taxonomy
Terminal Branch

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.

• ​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.
  
• Summarize the process of creating cladograms using bioinformatic tools.​
  

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 an example of reclassification as a result analysis of molecular sequence data. 
  

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.