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