Cladistics
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Cladistics

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Cladistics Cladistics Presentation Transcript

  • 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
    • We share a common ancestry that can be seen in our anatomy
    ODWS Paul Billiet 2011
  • 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 Silky shark Carcharhinus falciformis
  • 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 Common frog Rana temporaria
  • 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 Crocodile hatchling
  • 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 Kangaroo suckling from mother
  • 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 Human embryo
  • Merlin’s relatedness to me ODWS Paul Billiet 2011
  • What we know and what we don’t know
    • 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
    • Soup is another cat-like animal
    • Soup shares more features with Merlin than I do
    ODWS Paul Billiet 2011
  • An extended family: Merlin, Soup and I ODWS Paul Billiet 2011
  • An alternative view
    • There is more than one way we three could be related
    ODWS Paul Billiet 2011
  • 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
    • 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
    • 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 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 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
    • 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?
    • 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 Merlin and Soup
    ODWS Paul Billiet 2011
  • 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
    • 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
  • Cladogram of birds and dinosaurs Node ODWS Paul Billiet 2011
  • 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
    • 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 Torvosauroid theropod of the midJurassic
  • 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
    • 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
    • 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
    • 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 Archaeopteryx
  • Bird cladogram
    • A more modern view
    ODWS Paul Billiet 2011