- 1. CLADISTICS Phylogenetic systematics ODWSPaul Billiet
2011
2. The basic assumption
- All life on Earth shares a common origin
- Therefore, two different organisms will sharea common
ancestor
ODWSPaul Billiet 2011 3. Distant cousins
- Merlin is clearly a cat and I am a human
- We share a common ancestry that can be seen in our anatomy
ODWSPaul Billiet 2011 4. 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
ODWSPaul Billiet 2011 Silky shark Carcharhinus falciformis 5.
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
calledtetrapods
ODWSPaul Billiet 2011 Common frogRana temporaria 6. 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
ODWSPaul Billiet 2011 Crocodile hatchling 7. 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
ODWSPaul Billiet 2011 Kangaroo suckling from mother 8.
Eutherians
- Merlin and I spent the early parts of our life ina womb
supported by a placenta
ODWSPaul Billiet 2011 Human embryo 9. Merlins relatedness to me
ODWSPaul Billiet 2011 10. What we know and what we dont know
- We know that Merlin and I shared a common ancestor
- We have some ideas on what it might have looked like
- We donotknow how we came to be the way we are
ODWSPaul Billiet 2011 11. Adding in another cousin
- Soup is another cat-like animal
- Soup shares more features with Merlin than I do
ODWSPaul Billiet 2011 12. An extended family:Merlin, Soup and I
ODWSPaul Billiet 2011 13. An alternative view
- There is more than one way we three could be related
ODWSPaul Billiet 2011 14. Cladograms and clades
- These diagrams are calledcladograms
- Comes from the Greek word meaning a branch
- Each branch point ornoderepresents a common ancestor
- The branches above a node represent aclade
- All the organisms in a clade share a number of features
ODWSPaul Billiet 2011 15. 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 notscientific
ODWSPaul Billiet 2011 16. Cladistics
- Cladograms belong to a method of taxonomy calledcladistics(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 ofOccums razorto decide which is the
most plausible hypothesis
ODWSPaul Billiet 2011 17. Occums razor
- Occums razor states that if there are two or more conflicting
hypotheses to explaina phenomenon thesimplestis chosen as the
working hypothesis
- This is calledThe Principle of Parsimony
- This does not mean that it is the right hypothesis
- It still needs to be tested
- All hypotheses are provisional
ODWSPaul Billiet 2011 18. The most parsimonious route
- The cladogram on the left implies that cat-like features
evolved onlyoncein the clade containing Soup and Merlin
- The one on the right implies that they
evolvedtwiceindependently
- So it seems from first analysis that the first cladogram is the
one to retain
ODWSPaul Billiet 2011 19. An alternative hypothesis
- Evolution is not just about gaining new characters it is also
involveslosingcharacters
- Suppose that the ancestors of humans and cats were all
cat-like
- and these characters werelost just onceduring the evolution
towards me as shown on the right
- This hypothesis is just as parsimonious as the first
ODWSPaul Billiet 2011 20. How do we resolve the problem?
- The two hypotheses can betestedusing 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 anoutgroupand the test is called anoutgroup
comparison
ODWSPaul Billiet 2011 21. Albert is not a eutherian mammal
ODWSPaul Billiet 2011 22. Two cladograms are possible
- The cladogram on the left requires cat-like features to have
evolvedjust onceon the branch to Merlin and Soup
ODWSPaul Billiet 2011 23. Two cladograms are possible
- The one on the right requires either: that cat-like features
evolvedtwice independentlyto Merlin and Soup
- Or: Cat-like features evolvedoncein the common ancestor of
Merlin, Soup and myself
- AND was thenlostin the evolution of myself
ODWSPaul Billiet 2011 24. Applying Occums razor
- Hence the cladogram on the left offers the simplest (most
parsimonious) route
ODWSPaul Billiet 2011 25. The power of cladistics
- Cladistics tests all possible hypotheses objectively
- It can lead to some surprising conclusions
ODWSPaul Billiet 2011 26. Cladogram of birds and dinosaurs Node
ODWSPaul Billiet 2011 27. What is a bird?
- Birds are birds not just because they have feathers but because
they have:
- they are endothermic (warm-blooded),
- they have fused clavicles (the "wishbone"),
- a characteristic egg shell,
- three toes pointing forwards and one toe pointing back
ODWSPaul Billiet 2011 28. Unexpected links
- All of the characteristics of birds listed above have been
found in fossils of a group of dinosaurs called the theropods
(includesTyrannosaurus rex )
- This led the taxonomists to the conclusion that birds are
really dinosaurs
ODWSPaul Billiet 2011 Torvosauroid theropod of the midJurassic
29. 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 ODWSPaul Billiet 2011 30. 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
ODWSPaul Billiet 2011 31. 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
ODWSPaul Billiet 2011 32. 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
ODWSPaul Billiet 2011 Archaeopteryx 33. Bird cladogram
ODWSPaul Billiet 2011