5.4 Cladistics

IB Syllabus Statements

Understandings:

A clade is a group of organisms that have evolved from a common ancestor.
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.
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.
Traits can be analogous or homologous.
Cladograms are tree diagrams that show the most probable sequence of divergence in clades.
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.

Applications and skills:

Application: Cladograms including humans and other primates.
Application: Reclassification of the figwort family using evidence from cladistics.
Skill: Analysis of cladograms to deduce evolutionary relationships.

Explain what ‘falsification of theories’ means and why this would happen.
Define clade.
What kind of evidence is used to determine which species are included in a clade.
Outline the relationship between the number of base sequence differences between two species and the time since they diverged from a common ancestor.
Compare analogous vs. homologous traits.
What are cladograms and what are they used for?
Explain how classifying organisms based on cladistics can be more useful than classifying organisms based on morphological features.
Explain what you would expect to see on a cladogram that includes humans and other primates.
Outline the reclassification of the figwort family using evidence from cladistics.
Explain how you would use a cladogram to deduce evolutionary relationships.
How can the ancestry of groups of species be deduced?