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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 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
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
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
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
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
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 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: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 • 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
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? 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 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 • A more modern view ODWS Paul Billiet 2011
