THE CLASSIFICATION
OF LIVING ORGANISMS
Taxonomy or Systematics:
The study of classification
ODWS Paul Billiet 2011
Why classify organisms?
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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
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
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ODWS Paul Billiet 2011
How taxonomy works
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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
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
Hierarchy of characters
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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
Example
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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
Acanthostega
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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
The pentadactyl limb
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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
The pendadactyle limb
Lizard
Frog
Bat
Human
Analogous or homologous
characters
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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
Homology in mammalian fore limbs
ODWS Paul Billiet 2011
Analogous structures
Wolf Canis lupus
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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