Published on

1 Like
  • Be the first to comment

No Downloads
Total views
On SlideShare
From Embeds
Number of Embeds
Embeds 0
No embeds

No notes for slide


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