Cladistics

1. CLADISTICS Phylogenetic systematics ODWSPaul Billiet 2011

All life on Earth shares a common origin

Therefore, two different organisms will sharea common ancestor

Merlin is clearly a cat and I am a human

We share a common ancestry that can be seen in our anatomy

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

Merlin and I both have jaws with teeth and two pairs of limbs

We share these features with a more select group of vertebrates calledtetrapods

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

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 aremammals

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

We are eutherian mammals

We know that Merlin and I shared a common ancestor

We donotknow: when where

We have some ideas on what it might have looked like

We donotknow how we came to be the way we are

Soup is another cat-like animal

Soup shares more features with Merlin than I do

There is more than one way we three could be related

These diagrams are calledcladograms

Comes from the Greek word meaning a branch

Each branch point ornoderepresents a common ancestor

The branches above a node represent aclade

All the organisms in a clade share a number of features

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 notscientific

Cladograms belong to a method of taxonomy calledcladistics(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 ofOccums razorto decide which is the most plausible hypothesis

Occums razor states that if there are two or more conflicting hypotheses to explaina phenomenon thesimplestis chosen as the working hypothesis

This is calledThe Principle of Parsimony

This does not mean that it is the right hypothesis

It still needs to be tested

All hypotheses are provisional

The cladogram on the left implies that cat-like features evolved onlyoncein the clade containing Soup and Merlin

The one on the right implies that they evolvedtwiceindependently

So it seems from first analysis that the first cladogram is the one to retain

for the moment

Evolution is not just about gaining new characters it is also involveslosingcharacters

Suppose that the ancestors of humans and cats were all cat-like

and these characters werelost just onceduring the evolution towards me as shown on the right

This hypothesis is just as parsimonious as the first

The two hypotheses can betestedusing 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 anoutgroupand the test is called anoutgroup comparison

Enter Albert

The cladogram on the left requires cat-like features to have evolvedjust onceon the branch to Merlin and Soup

The one on the right requires either: that cat-like features evolvedtwice independentlyto Merlin and Soup

Or: Cat-like features evolvedoncein the common ancestor of Merlin, Soup and myself

AND was thenlostin the evolution of myself

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

Cladistics tests all possible hypotheses objectively

It can lead to some surprising conclusions

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

All of the characteristics of birds listed above have been found in fossils of a group of dinosaurs called the theropods (includesTyrannosaurus rex )

This led the taxonomists to the conclusion that birds are really dinosaurs

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

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

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

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

A more modern view

Date post:	21-Jan-2015
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Cladistics

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