L3 Cladistics Assumption
All life on Earth shares a common origin
¨  Therefore, two different organisms will share a
common ancestor
¨ 
ODWS Paul Billiet 2011
L3 The Cladogram
L3 Distant cousins
¨ 
¨ 
Syber 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
L3 Vertebrates
¨ 
Both Syber and I have
a skull
¤  a spine
¤  paired sense organs,
¤  a tail that continues on
beyond the anus (as a
fetus in humans)
¤ 
¨ 
All vertebrates have
these, they must have a
shared ancestor
Silky shark
Carcharhinus
falciformis
ODWS Paul Billiet 2011
L3 Syber’s relatedness to me
ODWS Paul Billiet 2011
L3 What we know and what we don’t
know
We know that Syber and I shared a common
ancestor.
¨  We do not know:
when
where
or how
¨ 
ODWS Paul Billiet 2011
L3 Adding in another cousin
Bob is another cat-like animal
¨  Bob shares more features with Syber than I do
¨ 
ODWS Paul Billiet 2011
L3 An extended family: Syber, Bob and I
ODWS Paul Billiet 2011
L3 An alternative view
¨ 
There is more than one way we three could be
related
ODWS Paul Billiet 2011
L3 Cladograms and clades
These diagrams are called cladograms
¨  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.
¨ 
¤  Eg.
Syber, Bob and I are all mammals which feed milk
to young.
ODWS Paul Billiet 2011
L3 Common sense v Science
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
L3 Occum’s razor
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
¨ 
ODWS Paul Billiet 2011
The most parsimonious route
¨ 
¨ 
The cladogram on the left implies that cat-like features
evolved only once in the clade containing Bob and Syber
The one on the right implies that they evolved twice
independently
ODWS Paul Billiet 2011
How do we resolve the problem?
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…
¨ 
ODWS Paul Billiet 2011
Albert is not a eutherian mammal
ODWS Paul Billiet 2011
Two cladograms are possible
¨ 
The cladogram on the left requires cat-like features
to have evolved just once on the branch to Syber
and Bob
ODWS Paul Billiet 2011
Two cladograms are possible
¨ 
¨ 
The one on the right requires either:
that cat-like features evolved twice independently to
Syber and Bob
Or:
Cat-like features evolved once in the common ancestor of
Syber, Bob and myself …
ODWS Paul Billiet 2011
Applying Occum’s razor
¨ 
Hence the cladogram on the left offers the simplest
(most parsimonious) route
ODWS Paul Billiet 2011
The power of cladistics
Cladistics tests all possible hypotheses objectively
¨  It can lead to some surprising conclusions
¨ 
ODWS Paul Billiet 2011
L3 Cladogram of birds and dinosaurs
Node
ODWS Paul Billiet 2011
L3 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
L3 Unexpected links
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
ODWS Paul Billiet 2011
L3 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
L3 The absence of a characteristic is not
relevant
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
L3 Archaeopterx
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
L3 Bird cladogram
¨ 
A more modern
view
ODWS Paul Billiet 2011