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DPTopic5.4:
Cladistics
EssentialIdea:Theancestryofgroupsofspeciescanbededucedby
comparingtheirbaseoraminoacidsequences
Clades&Cladograms
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.
Cladesincludeallthespeciesthatarestillaliveandalsothosethatareextinct.
Exercise1:Whatevolutionaryrelationshipsareindicatedbythiscladogram?
MarkthecommonancestorforthechimpanzeeandtheHuman.
Exercise2:Workthroughtheseinstructions
Constructing Cladograms
Eukaryotic
Multicellular
Vertebral Column
Amniote egg
Hair
Placenta
Opposable Thumbs
Totals
Paramecium
Flatworm
Shark
Eagle
Koala
Camel
Human
To make a cladogram (the first step has been done for you):
1. Make a list of organisms to be included
2. List as many as possible characteristics which each organism possesses, biochemical or morphological - tick
the boxes for each organism
3. Decide on the primitive characteristic (this is the one that all organisms share)
4. Construct a cladogram with the first branch being the organism with the fewest derived characteristics
Remember -
Primitive characteristics are those that have the same structure and function and evolved early on in the
organisms being studied.
Derived traits are those that have the same structure and function but evolved later as a modification to a
previous trait
The original line at the base is the common ancestors lineage
Using Biochemical Evidence
U2:
U3:
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.
Exercise 3:
Using biochemical variation to deduce evolutionary relationships, and the molecular clock
1. A A A A A T T T C C C C G G G G A A A A T T T T C C C C G G A A T
2. A T A A A T T T C C C C G G A G A A A A T T T T C C C C G G A A T
3. A A A A C T T T C C C C G G A G A A A A T T T C C C C C G G G A C
4. T A T A C T T T C C C T G G A G A A A A T A T T C C G C G G G A C
Each sequence of bases above has been taken from the same section of highly conserved DNA from four different
mammals. Taxonomists want to use them to resolve the phylogeny of these species. To help them you need to work
through the following exercise.
1.
2.
3.
4.
5.
6.
How many differences are there in the sequences between each species (Use the matrix on the next page)?
Which species do you think are most closely related based on the sequences above? Why?
Transcribe and Translate the sequences and answer the above questions using the sequence of amino acids.
From your investigation evaluate the effectiveness of using biochemical variations to deduce phylogenies
Evaluate how effective these different sequences would be as a molecular or evolutionary clock.
Using the differences within these tables create your own cladogram.
Amino acid sequence number
1
1
2
3
4
2
3
4
x
x
x
x
Amino Acid Sequence number
Amino Acid Sequence number
DNA sequence number
1
1
2
3
4
2
3
4
x
x
x
x
Analogous&HomologousTraits
U4:
•
•
Traits can be analogous or homologous.
Homologousstructuresarethosebasedonthesameplanbutmaybeadaptedfordifferent
purposes
E.g.bat’swing,humanarmandsealflipperareallmodificationsofthesamebasicplan
(divergentevolution)
•
Thissimilarityinformindicatesacommonancestor,andcanbeusedtocreateanaturalor
phylogeneticclassification
•
Analogousstructuresarethosethatresembleeachotherinfunctionbutdifferintheir
fundamentalstructure(convergentevolution)
•
E.g.bat’swingandinsectwing,squidandvertebrateeyes
•
Classificationbasedonanalogousstructuresresultsinanartificialclassification
ConstructingCladograms
•
•
•
•
Cladogramscanbeconstructedusingmorphologicalorbiochemicalevidence
Ifusingmorphologytaxonomiststendtouse“primitive”characteristics,oneslikelytohave
beensharedbyacommonancestortoworkoutthephylogeneticrelationship
Taxonomistsmustbecarefultobuildcladogramsbasedonhomologousstructures
Biochemicalevidence(immunologicalstudies)canalsobeusedtocreatecladograms.
Combinedwithmorphologicaldatathesestudiestendtogiveamorereliablephylogeny
Exercise4:
ReclassificationoftheFigworts
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.
A2:
Reclassification of the figwort family using evidence from cladistics.
Essential idea: The ancestry of groups of species can be deduced by comparing their base or amino acid sequences.
Biology guide
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)
Understandings:
Theory of knowledge:
t
A clade is a group of organisms that have evolved from a common ancestor.
t
t
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.
t
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.
t
Traits can be analogous or homologous.
t
Cladograms are tree diagrams that show the most probable sequence of
divergence in clades.
t
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.
t
Application: Cladograms including humans and other primates.
t
Application: Reclassification of the figwort family using evidence from
cladistics.
t
Skill: Analysis of cladograms to deduce evolutionary relationships.
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Topic 5: Evolution and biodiversity
Applications and skills:
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?