CLADISTICS
Phylogenetic systematics
ODWS Paul Billiet 2011
The basic assumption
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All life on Earth shares a common origin
Therefore, two different organisms will share
a common ancestor
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Distant cousins
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Merlin is clearly a cat and I am a human
We share a common ancestry that can be seen in
our anatomy
ODWS Paul Billiet 2011
Vertebrates
Silky shark
Carcharhinus falciformis
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Both Merlin and I have, a skull followed by a
vertebral column, paired sense organs, a tail that
continues on beyond the anus
All vertebrates have these, they must have a shared
ancestor
ODWS Paul Billiet 2011
Tetrapods
Common frog
Rana temporaria
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Merlin and I both have jaws with teeth and two pairs
of limbs
We share these features with a more select group of
vertebrates called tetrapods
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Amniotes
Crocodile hatchling
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When we were embryos both Merlin and I were
protected by membranes
One is called the amnion that is a feature of many
terrestrial vertebrate animals
ODWS Paul Billiet 2011
Mammals
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Both Merlin and I have:
hair,
we are endothermic,
we have jaws that
connect to the skull in a
particular way,
we suckled milk when
were young,
we have a diaphragm
between our thorax and
our abdomen
We are mammals
Kangaroo suckling from mother
ODWS Paul Billiet 2011
Eutherians
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Merlin and I spent the early parts
of our life in a womb supported
by a placenta
We are eutherian mammals
Human embryo
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Merlin’s relatedness to me
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What we know and what we don’t
know
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We know that Merlin and I shared a common
ancestor
We do not know:
when
where
We have some ideas on what it might have
looked like
We do not know how we came to be the way
we are
ODWS Paul Billiet 2011
Adding in another cousin
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Soup is another cat-like animal
Soup shares more features with Merlin than I do
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An extended family:
Merlin, Soup and I
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An alternative view
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There is more than one way we three could
be related
ODWS Paul Billiet 2011
Cladograms and clades
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These diagrams are called cladograms
Comes from the Greek word meaning a
branch
Each branch point or node represents a
common ancestor
The branches above a node represent a
clade
All the organisms in a clade share a number
of features
ODWS Paul Billiet 2011
Common sense v Science
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These cladograms suggest that there may be
different ways of obtaining the same result
Common sense would suggest that the first
cladogram is the correct approach
Common sense is not objective
Common sense is not scientific
ODWS Paul Billiet 2011
Cladistics
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Cladograms belong to a method of taxonomy
called cladistics
(aka phylogenetic systematics)
Cladistics has become an accepted way of
classifying organisms
It permits hypothesis of relatedness to be
tested
It uses the the principle of Occum’s razor to
decide which is the most plausible hypothesis
ODWS Paul Billiet 2011
Occum’s razor
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Occum’s razor states that if there are two or
more conflicting hypotheses to explain a
phenomenon the simplest is chosen as the
working hypothesis
This is called The Principle of Parsimony
This does not mean that it is the right
hypothesis
It still needs to be tested
All hypotheses are provisional
ODWS Paul Billiet 2011
The most parsimonious route
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The cladogram on the left implies that cat-like
features evolved only once in the clade containing
Soup and Merlin
The one on the right implies that they evolved twice
independently
So it seems from first analysis that the first
cladogram is the one to retain…
… for the moment
ODWS Paul Billiet 2011
An alternative hypothesis
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Evolution is not just about gaining new characters it is
also involves losing characters
Suppose that the ancestors of humans and cats were
all cat-like…
…and these characters were lost just once during
the evolution towards me as shown on the right
This hypothesis is just as parsimonious as the first
ODWS Paul Billiet 2011
How do we resolve the problem?
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The two hypotheses can be tested using a
fourth organism
This organism has to be clearly unrelated to
the rest of the group
e.g. An animal that is not a eutherian
mammal
This is called an outgroup and the test is
called an outgroup comparison
Enter Albert…
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Albert is not a eutherian mammal
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Two cladograms are possible
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The cladogram on the left requires cat-like
features to have evolved just once on the
branch to Merlin and Soup
ODWS Paul Billiet 2011
Two cladograms are possible
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The one on the right requires either:
that cat-like features evolved twice independently
to Merlin and Soup
Or:
Cat-like features evolved once in the common
ancestor of Merlin, Soup and myself …
… AND was then lost in the evolution of myself
ODWS Paul Billiet 2011
Applying Occum’s razor
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Hence the cladogram on the left offers the
simplest (most parsimonious) route
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The power of cladistics
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Cladistics tests all possible hypotheses
objectively
It can lead to some surprising conclusions
ODWS Paul Billiet 2011
Cladogram of birds and dinosaurs
Node
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What is a bird?
Birds are birds not just because they have
feathers but because they have:
 hollow bones,
 flexible wrists,
 they are endothermic (warm-blooded),
 they have fused clavicles (the "wishbone"),
 a characteristic egg shell,
 three toes pointing forwards and one toe
pointing back
ODWS Paul Billiet 2011
Unexpected links
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All of the characteristics of birds listed above
have been found in fossils of a group of
dinosaurs called the theropods (includes
Tyrannosaurus rex)
This led the taxonomists to the conclusion
that birds are really dinosaurs
Torvosauroid theropod of the midJurassic
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What really is a bird?
In fact birds seem to possess only
a few characteristics which are
really their own:
 a very short tail (the parson's
nose) to manage the tail
feathers,
 fused fingers and a "thumb wing"
for slow flight,
 a deep keel to the sternum
(breast bone) to attach the flight
muscles,
 a complex breathing system to
manage at high altitudes
Bird skeleton
ODWS Paul Billiet 2011
The absence of a characteristic is not
relevant
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It is often said that a characteristic of birds is
that they lack teeth
Anteaters and tulips do not have teeth either
and you would not call them birds
ODWS Paul Billiet 2011
Fossils in cladograms
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As seen, fossils can be placed in cladograms
They occupy the same status as a living
(extant) organism
Cladograms transcend time
This means a fossil can be analysed in the
same way as a newly discovered living
species
Newly discovered fossils have rewritten the
cladogram for birds
ODWS Paul Billiet 2011
Archaeopteryx
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For many years the
fossil Archaeopteryx
was thought to be
close to the common
ancestor of modern
birds
Its age and the
discovery of other
fossils have changed
our interpretation
Archaeopteryx
ODWS Paul Billiet 2011
Bird cladogram
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A more modern
view
ODWS Paul Billiet 2011