CLADISTICS Phylogenetic systematics ODWSODWS Paul Billiet 2011

The basic assumption All life on Earth shares a common origin Therefore, two different organisms will share a common ancestor ODWSODWS 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 ODWSODWS 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 Silky shark Carcharhinus falciformis ODWSODWS Paul Billiet 2011

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 Common frog Rana temporaria ODWSODWS Paul Billiet 2011

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 Crocodile hatchling ODWSODWS Paul Billiet 2011

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 Kangaroo suckling from mother ODWSODWS Paul Billiet 2011

Eutherians Merlin and I spent the early parts of our life in a womb supported by a placenta We are eutherian mammals Human embryo ODWSODWS Paul Billiet 2011

Merlin’s relatedness to me ODWSODWS 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 ODWSODWS Paul Billiet 2011

Adding in another cousin Soup is another cat-like animal Soup shares more features with Merlin than I do ODWSODWS Paul Billiet 2011

An extended family: Merlin, Soup and I ODWSODWS Paul Billiet 2011

An alternative view There is more than one way we three could be related ODWSODWS 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 ODWSODWS 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 ODWSODWS 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 ODWSODWS 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 ODWSODWS 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 ODWSODWS 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 ODWSODWS 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… ODWSODWS Paul Billiet 2011

Albert is not a eutherian mammal ODWSODWS 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 ODWSODWS 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 ODWSODWS Paul Billiet 2011

Applying Occum’s razor Hence the cladogram on the left offers the simplest (most parsimonious) route ODWSODWS Paul Billiet 2011

The power of cladistics Cladistics tests all possible hypotheses objectively It can lead to some surprising conclusions ODWSODWS Paul Billiet 2011

Cladogram of birds and dinosaurs Node ODWSODWS 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 ODWSODWS 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 Torvosauroid theropod of the midJurassic ODWSODWS 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 ODWSODWS 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 ODWSODWS 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 ODWSODWS 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 Archaeopteryx ODWSODWS Paul Billiet 2011

Bird cladogram A more modern view ODWSODWS Paul Billiet 2011