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CLADISTICS. Phylogenetic systematics. ODWS Paul Billiet 2011. The basic assumption. All life on Earth shares a common origin Therefore, two different organisms will share a common ancestor. ODWS Paul Billiet 2011. Distant cousins. Merlin is clearly a cat and I am a human

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cladistics

CLADISTICS

Phylogenetic systematics

ODWS Paul Billiet 2011

the basic assumption
The basic assumption
  • All life on Earth shares a common origin
  • Therefore, two different organisms will share a common ancestor

ODWS Paul Billiet 2011

distant cousins
Distant cousins
  • 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

Vertebrates
  • 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

Tetrapods
  • 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

ODWS Paul Billiet 2011

amniotes

Crocodile hatchling

Amniotes
  • 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

Kangaroo suckling from mother

Mammals
  • 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

ODWS Paul Billiet 2011

eutherians

Human embryo

Eutherians
  • Merlin and I spent the early parts of our life in a womb supported by a placenta
  • We are eutherian mammals

ODWS Paul Billiet 2011

merlin s relatedness to me
Merlin’s relatedness to me

ODWS Paul Billiet 2011

what we know and what we don t know
What we know and what we don’t know
  • We know that Merlin and I shared a common ancestor
  • We do not know:whenwhere
  • 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
Adding in another cousin
  • Soup is another cat-like animal
  • Soup shares more features with Merlin than I do

ODWS Paul Billiet 2011

an alternative view
An alternative view
  • There is more than one way we three could be related

ODWS Paul Billiet 2011

cladograms and clades
Cladograms and clades
  • 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
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

cladistics16
Cladistics
  • 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
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
  • All hypotheses are provisional

ODWS Paul Billiet 2011

the most parsimonious route
The most parsimonious route
  • 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
An alternative hypothesis
  • 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
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
Albert is not a eutherian mammal

ODWS Paul Billiet 2011

two cladograms are possible
Two cladograms are possible
  • 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 possible23
Two cladograms are possible
  • 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
Applying Occum’s razor
  • Hence the cladogram on the left offers the simplest (most parsimonious) route

ODWS Paul Billiet 2011

the power of cladistics
The power of cladistics
  • Cladistics tests all possible hypotheses objectively
  • It can lead to some surprising conclusions

ODWS Paul Billiet 2011

cladogram of birds and dinosaurs
Cladogram of birds and dinosaurs

Node

ODWS Paul Billiet 2011

what is a bird
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

Torvosauroid theropod of the midJurassic

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

ODWS Paul Billiet 2011

what really is a bird
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
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

fossils in cladograms
Fossils in cladograms
  • 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

Archaeopteryx

Archaeopteryx
  • 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

ODWS Paul Billiet 2011

bird cladogram
Bird cladogram
  • A more modern view

ODWS Paul Billiet 2011