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DP Topic 5.5
Cladistics
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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
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Biochemical evidence & classification
• What does Biochemistry tell us about evolution and the
phylogeny of organisms?
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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
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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.
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Source: Clegg (2007) Biology for the IB Diploma
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Source: Clegg (2007) Biology for the IB Diploma
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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
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• 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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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
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Source: Clegg (2007) Biology for the IB Diploma
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Source: Clegg (2007) Biology for the IB Diploma
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Source: Clegg (2007) Biology for the IB Diploma
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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
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Source: Clegg (2007) Biology for the IB Diploma
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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
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Source: Clegg (2007) Biology for the IB Diploma
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