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5.4 Cladistics

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Description

Nature of science:
Falsification of theories with one theory being superseded by another—plant families have been reclassified as a result of evidence from cladistics. (1.9)
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.
 Theory of knowledge:
  • A major step forward in the study of bacteria was the recognition in 1977 by Carl Woese that Archaea have a separate line of evolutionary descent from bacteria. Famous scientists, including Luria and Mayr, objected to his division of the prokaryotes. To what extent is conservatism in science desirable?

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