1. THE CLASSIFICATION OF LIVING ORGANISMS
Taxonomy or Systematics: The study of classification
ODWS Paul Billiet 2011

2. Why classify organisms?
Over a million species so far identified
Estimates of up to 30 million species on Earth
Need to organise this biodiversity
Systematics tells us about the patterns in nature, the way organisms are related and how they evolve
Systematics can be used to identify organisms that are important to us
ODWS Paul Billiet 2011

3. The unit of taxonomy: The SPECIES
The term species has biological significance
Species form populations of individuals which may interbreed to form fully fertile offspring
Problem: Some species only use asexual reproduction or rarely use sexual reproduction
ODWS Paul Billiet 2011

4. How taxonomy works
The aim is to group organisms according to their evolutionary relationship (phylogeny)
Established by studying the phenotypes of living organisms or fossils
More recently DNA sequencing permits the comparisons of the genotypes
Uses characteristic features to group organisms together (e.g. all animals with feathers = Birds)
Taxonomists decide which are the most significant or "important" characteristic by the way it occurs in different groups of organisms.
ODWS Paul Billiet 2011

5. Comparing phenotypes & genotypes
Taxonomists compare a new specimen with given characteristics:
morphology
anatomy
behaviour
embryology
protein structure
karyotypes
DNA sequence (DNA fingerprints)
ODWS Paul Billiet 2011

6. Hierarchy of characters
Taxonomy uses many different characteristics to define a taxon
One character is not enough
The characteristics are grouped in a hierarchy.
ODWS Paul Billiet 2011

7. Example
Acanthostega
So having four legs with five toes is common to all land vertebrates and their fish ancestor
This would be used to group the animals we call tetrapods
Having a nerve cord running down the back is a feature common to all the tetrapods but also all the rest of the vertebrates
So it can be used to group all the vertebrates but not the tetrapods alone.
Lamprey
ODWS Paul Billiet 2011

8. The pentadactyl limb
Classification led to comparisons of shape and form that gave rise to comparative anatomy
Comparative anatomists noticed that different species have similar structures used for different functions (e.g. the pentadactyle limb of terrestrial vertebrates)
These are called homologous structures.
ODWS Paul Billiet 2011

9. The pendadactyle limb
Lizard
Frog
Bat
Human

10. Analogous or homologous characters
Even though the front legs of different mammals may look different they still use the same bones in their structure
The simplest explanation for this is that they all originated from a common ancestor, the ancestor of all mammals
This is called homology
As organisms evolved they split up and specialised in different ways of living
Their bodies changed in shape but they still retain some of their ancestors features
ODWS Paul Billiet 2011

11. Homology in mammalian fore limbs
ODWS Paul Billiet 2011

12. Analogous structures
Wolf Canis lupus
Some structures may look very similar but have evolved independently
They are the product of natural selection on an organ adapting an organism to a particular niche
Thylacine Thylacinus cynocephalus
ODWS Paul Billiet 2011