Essential idea: The ancestry of groups of species can be deduced by comparing their base or amino acid sequences.
The images above are both cladograms. They show the statistical similarities between species based on their DNA/RNA. The cladogram on the left show bacteria classified using the 16 S ribosomal RNA gene. The cladogram on the right uses retroviral sequences which have been incoporated into ape DNA and then inherited to build a picture of the relationships between apes. It’s an Interesting fact that humans have around 100,000 endogenous retroviruses incorporated into their DNA.
Understandings, applications and skills
|5.4.U1||A clade is a group of organisms that have evolved from a common ancestor.|
|5.4.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.|
|5.4.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.|
|5.4.U4||Traits can be analogous or homologous.|
|5.4.U5||Cladograms are tree diagrams that show the most probable sequence of divergence in clades.|
|5.4.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.|
|5.4.A1||Cladograms including humans and other primates.|
|5.4.A2||Reclassification of the figwort family using evidence from cladistics.|
|5.4.S1||Analysis of cladograms to deduce evolutionary relationships.|
[Text in square brackets indicates guidance notes]
Presentation and notes
The presentation is designed to help your understanding. The notes outline is intended to be used as a framework for the development of student notes to aid revision.
Use the Cornell style template to collate your own notes for Topic 5.4 Cladistics.
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Falsification of theories with one theory being superseded by another - plant families have been reclassified as a result of evidence from cladistics.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) [Covered by 5.4.A2]
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?