the classification of living organisms taxonomy or systematics: the study of classification odwsodws...
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THE CLASSIFICATION OF LIVING ORGANISMS Taxonomy or Systematics:
The study of classification
ODWS Paul Billiet 2011
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
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
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
Comparing phenotypes & genotypesTaxonomists 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 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 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.
Acanthostega
Lamprey
ODWS Paul Billiet 2011
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
The pendadactyle limb
Lizard
Human
Frog
Bat
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
Homology in mammalian fore limbs
ODWS Paul Billiet 2011
Analogous structures
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
Wolf Canis lupus
ODWS Paul Billiet 2011