DP Topic 5.5

1. DP Topic 5.5 Cladistics

2. Classification • Why do we classify organisms? • Essential to Biology because there are so many species; they need to be organised into manageable categories for ease of study • Classification: • Assists in identifying organisms • Suggests evolutionary links • Allows prediction of characteristics shared by a group

3. Biochemical evidence & classification • What does Biochemistry tell us about evolution and the phylogeny of organisms? • DNA structure • Transcription & translation • Respiration • ATP • Photosynthesis (in autotrophs) • Proteins use the same 20 amino acids • Biochemical commonality suggests common origins for life • Some of the earliest events must have been biochemical and the results inherited widely

4. Cladistics • U1: A clade is a group of organisms that have evolved from a common ancestor 
 • U5: Cladograms are tree diagrams that show the most probable sequence of divergence in clades. • A1: Cladograms including humans and other primates. • S1: Analysis of cladograms to deduce evolutionary relationships.

5. Source: Clegg (2007) Biology for the IB Diploma

6. Source: Clegg (2007) Biology for the IB Diploma

7. Using biochemistry to indicate evolutionary relationships • DNA changes through mutation resulting in changes in amino acid sequences in polypeptides • Measure the relatedness of different groups of organisms using biochemical changes in DNA and proteins • Differences in Haemoglobin β-chain amino acid sequences Source: Clegg (2007) Biology for the IB Diploma

8. More closely related species will have fewer differences • DNA sequencing techniques have allowed the establishment of biochemical phylogenies based on sequences of mitochondrial DNA • Similarities in polypeptide sequences of certain proteins can be compared • Proteins that have been used include haemoglobin, cytochrome C and chlorophyll

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10. Constructing Cladograms • Cladograms can be constructed using morphological or biochemical evidence • If using morphology taxonomists tend to use “primitive” characteristics, ones likely to have been shared by a common ancestor to work out the phylogenetic relationship • Taxonomists must be careful to build cladograms based on homologous structures • Biochemical evidence (immunological studies) can also be used to create cladograms. Combined with morphological data these studies tend to give a more reliable phylogeny

11. Source: Clegg (2007) Biology for the IB Diploma

12. Source: Clegg (2007) Biology for the IB Diploma

13. Source: Clegg (2007) Biology for the IB Diploma

14. The molecular clock • Biochemical changes can be used as a molecular or evolutionary clock • The number of differences between molecules gives us an idea of how long ago speciation may have occurred • We need to be careful about which molecules we use and how we interpret data resolved in their use • Molecular clocks may not “tick” at a steady rate because evolutionary changes may not occur at a constant rate • Data can be verified by the work of palaeontology and radiometric dating

15. Source: Clegg (2007) Biology for the IB Diploma

16. Homologous & Analogous structures • Homologous structures are those based on the same plan but may be adapted for different purposes • E.g. bat’s wing, human arm and seal flipper are all modifications of the same basic plan (divergent evolution) • This similarity in form indicates a common ancestor, and can be used to create a natural or phylogenetic classification • Analogous structures are those that resemble each other in function but differ in their fundamental structure (convergent evolution) • E.g. bat’s wing and insect wing, squid and vertebrate eyes • Classification based on analogous structures results in an artificial classification

17. Source: Clegg (2007) Biology for the IB Diploma