The document discusses cladistics, a method for classifying organisms into clades based on their evolutionary relationships, which is visually represented by cladograms. It highlights the importance of molecular evidence, such as DNA and protein sequences, in determining relatedness and divergence among species. Additionally, it explains concepts such as homologous and analogous structures, convergent evolution, adaptive radiation, and the significance of mitochondrial DNA in tracing evolutionary paths.

1. IBDP core
Topic 5.4
Cladistics

2. Understandings:
 A clade is a group of organisms that have evolved from a common ancestor
 Cladograms are tree diagrams that show the most probable sequence of
divergence in clades
 Evidence for which species are part of a clade can be obtained from the base
sequence 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
 Evidence from cladistics has shown that classification of some groups based
on structure did not correspond with the evolutionary origins of a group or
species
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3. Clades
 Cladistics is a method of
classifying organisms into
groups of species called clades
(from Greek ‘klados' = branch)
 Cades can be organized
according to branching
diagrams (cladograms) in
order to show evolutionary
relationships
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Humans, chimpanzees, gorillas, orangutans and gibbons all belong to a
common clade – the Hominoids
The Hominoid clade forms part of a larger clade – the Anthropoids –
which includes Old World and New World monkeys.

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6. Rectangular tree (left) and diagonal ladder
(right) cladogram formats.
Source: https://my.vanderbilt.edu/lauranovick/treethinking/
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7. Source: http://ib.bioninja.com.au/standard-level/topic-5-evolution-and-biodi/54-cladistics/clades.html
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8. Cladograms
 Cladograms are tree diagrams where each branch point represents the
splitting of two new groups from a common ancestor
 Each branch point (node) represents a speciation event by which distinct
species are formed via divergent evolution
 Cladograms show the probable sequence of divergence and hence
demonstrate the likely evolutionary history (phylogeny) of a clade
 The fewer the number of nodes between two groups the more closely related
they are expected to be
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9. Species 1 and 2 are more closely related to
one another than to species 3.
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10. Phylogenetic Classification
 A cladogram shows how species may be related by descent from a common
ancestor. A classification of organisms on the basis of such relationships is
called a phylogenetic classification. A phylogenetic classification involves
placing organisms in a clade with their common ancestor.
 In the following picture birds are in the same clade as reptiles, because a
variety of evidence suggests that birds evolved from a reptile ancestor. The
cladogram places mammals in a separate clade, because evidence suggests
that mammals evolved from a different ancestor.
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11. Molecular Evidence
 All organisms use DNA and RNA as genetic material and the genetic code by
which proteins are synthesized is (almost) universal.
 This shared molecular heritage means that base and amino acid sequences
can be compared to ascertain levels of relatedness.
 Over the course of millions of years, mutations will accumulate within any
given segment of DNA.
 The number of differences between comparable base sequences
demonstrates the degree of evolutionary divergence.
 A greater number of differences between comparable base sequences
suggests more time has past since two species diverged.
 Hence, the more similar the base sequences of two species are, the more
closely related the two species are expected to be.
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12. Molecular Evidence
 Amino acid sequences are typically used to compare distantly related species
(i.e. different taxa), while DNA or RNA base sequences are often used to
compare closely related organisms (e.g. different haplogroups – such as
various human ethnic groups).
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Source: http://www.ganfyd.org/index.php?title=File:Haemoglobin.gif

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Comparison of the Haemoglobin Beta Chain in Different Species

14. Molecular clock
 Some genes or protein sequences may accumulate mutations at a relatively
constant rate (e.g. 1 change per million years)
 If this rate of change is reliable, scientists can calculate the time of
divergence according to the number of differences
 Different genes or proteins may change at different rates (e.g. haemoglobin
mutates more rapidly than cytochrome c)
 The rate of change for a particular gene may differ between different groups
of organisms
 Over long periods, earlier changes may be reversed by later changes,
potentially confounding the accuracy of predictions
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15. Visual
molecular
clock
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16. Convergent
evolution
Convergent evolution is the independent
evolution of similar features in species
with distinct lineages
It may occur when different species
occupy the same habitat and are thus
subjected to the same selection pressures
The shared conditions cause common
adaptations to be selected in different
species, resulting in structural similarity
• An example of convergent evolution is the
development of wings in birds, bats and insects
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17. Convergent evolution
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birds
bats
insects

18. Adaptive
radiation
 Closely related organisms
can exhibit very different
structural features due to
adaptive radiation (e.g.
pentadactyl limb)
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19. Divergent evolution
 Divergent evolution is the process
whereby members of a species
become more and more different,
eventually resulting in two (or
more) new species. In other words,
the Butterflies will 'diverge,' thus
creating a new species. Remember,
like humans, the Butterflies have a
lot of variation: some are large and
some are small, and some are
darker and some are lighter.
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20. Homologous structures
 Traits that are similar because they are derived from common ancestry are
termed homologous structures
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21. Analogous structures
 Traits that are superficially similar but were derived through separate
evolutionary pathways are termed analogous structures
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22.  In evolutionary biology, the term analogous structures pertain to the various
structures in different species having the same function but have evolved
separately, thus do not share common ancestor. In comparison, the
homologous structures pertain to the structures that show similar
morphology and anatomy but have different functions. Moreover, the
homologous structures are believed to have evolved from a common ancestor
since they show similar development pattern during embryonic development.
 The term analogous structures is applied in the concept of convergent
evolution (convergence), which pertains to the evolutionary process wherein
the organisms evolve bodily parts that are analogous in terms of structure and
function despite their ancestors that are very dissimilar or unrelated.
Examples of analogous structures are as follows:
 Wings of insects and birds used for flying
 Jointed legs of insects and vertebrates used for locomotion
 Fins of fish and flippers of whales (mammals)
 These structures show that they have the same function, e.g. flight or
locomotion but have a different origin or that they have evolved separately.
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23. Homologous structures vs Analogous structures
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Homologues structures Analogues structures
Similar in anatomy Dissimilar in anatomy
Doing dissimilar functions Doing similar functions
Develop in related animals Develop in unrelated animals
Inherited from a common ancestor Not inherited from a common ancestor
Similar developmental pattern Developmental pattern is not similar
Similar structure and origin Dissimilar structure and origin

24. Clade Reclassification
 Until recently, figworts were the 8th largest family of
flowering plants (angiosperms), containing 275
different genera
 This was problematic as many of the figwort plants
were too dissimilar in structure to function as a
meaningful grouping
 Taxonomists examined the chloroplast gene in figworts
and decided to split the figwort species into five
different clades
 Now less than half of the species remain in the figwort
family – which is now the 36th largest among
angiosperms.
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25. In Situ Hybridization
 DNA is double stranded and is held together by hydrogen bonds between
complementary base pairs
 DNA from different species can be separated into single strands with
sufficient heat and then mixed together
 If two strands from different species share similar sequences they will
hybridize (anneal together to form a double strand)
 The amount of heat then required to separate this hybrid molecule indicates
how similar the two sequences are
 More heat indicates more hydrogen bonds formed, meaning more
complementary base pairing due to similar sequences
 Less heat indicates fewer hydrogen bonds formed, meaning less base pairing
has occurred because sequences are dissimilar
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Source: http://ib.bioninja.com.au/standard-level/topic-5-evolution-and-biodi/54-cladistics/in-situ-hybridisation.html

27. Mitochondrial DNA
 Mitochondrial DNA (maternal DNA) is an important tool for tracing
evolutionary relationships within a species
 Mitochondrial DNA offers several benefits over nuclear DNA when determining
phylogenetic pathways, including:
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1. Unlike nuclear DNA, which comes from both parents, mitochondrial DNA
comes only from the mother.
2. As maternal DNA is passed from the mother, no recombination occurs,
maintaining sequence fidelity/ճշտություն/.
3. Mitochondria produce reactive oxygen species, which cause sequences
to mutate at a higher rate
4. As every cell has mitochondria, large amounts of maternal DNA can be
gathered for sequencing.

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30. Mitochondrial And Nuclear DNA
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