CLADISTICS Phylogenetic systematics ODWSODWS Paul Billiet 2011.

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


Distant cousins

Merlin is clearly a cat and I am a human We share a common ancestry that can be seen in

our anatomy


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


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


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


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


Eutherians

Merlin and I spent the early parts of our life in a womb supported by a placenta

We are eutherian mammals

Human embryo


Merlin’s relatedness to me


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


Adding in another cousin

Soup is another cat-like animal Soup shares more features with Merlin than I do


An extended family: Merlin, Soup and I


An alternative view

There is more than one way we three could be related


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


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


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


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


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


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


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…


Albert is not a eutherian mammal


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


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 myselfODWS Paul Billiet 2011

Applying Occum’s razor

Hence the cladogram on the left offers the simplest (most parsimonious) route


The power of cladistics

Cladistics tests all possible hypotheses objectively

It can lead to some surprising conclusions


Cladogram of birds and dinosaurs

Node


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


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 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


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


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


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


Bird cladogram

A more modern view


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CLADISTICS Phylogenetic systematics ODWSODWS Paul Billiet 2011.

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