5.4: Cladistics
Teaching time: 3 hours Practical time: 1 hour
key vocabulary
Prior learning and retrieval practice
Review topic 5.3 classification
Review topic 3.1 genes
Cladograms
U1: A clade is a group of organisms that have evolved from a common ancestor.
S1: Analysis of cladograms to deduce evolutionary relationships.
A1: Cladograms including humans and other primates.
Exercise 1: Identify the clades in the diagrams below and mark their common ancestors. Deduce the homologous features that suggest decent from a common ancestor
Deducing relationships in clades
Essential idea: The ancestry of groups of species can be deduced by comparing their base or amino acid sequences.
U2: Evidence for which species are part of a clade can be obtained from the base sequences of a gene or the corresponding amino acid sequence of a protein.
U3: Sequence differences accumulate gradually so there is a positive correlation between the number of differences between two species and the time since they diverged from a common ancestor.
U4: Traits can be analogous or homologous.
U5: Cladograms are tree diagrams that show the most probable sequence of divergence in clades.
Exercise 2: work through the exercises in the google doc
Constructing Cladograms
Practical
Linking to Topic 3.1: Once DNA sequences have been aligned in clustal X you can create a phylogenetic tree that can be viewed in NJplot. I think that this would make an excellent area for a student to conduct an IA in. They could look at how different genes compare across the same species in determining evolutionary relationships.
Reclassification of organisms based on genetic data
NoS: Falsification of theories with one theory being superseded by another—plant families have been reclassified as a result of evidence from cladistics. (1.9)
A2: Reclassification of the figwort family using evidence from cladistics.
U6: Evidence from cladistics has shown that classifications of some groups based on structure did not correspond with the evolutionary origins of a group or species.
