THE CLASSIFICATION OF LIVING ORGANISMS Taxonomy or Systematics: The study of classification ODWSODWS...

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


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


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.


Comparing phenotypes & genotypesTaxonomists compare a new specimen with given

characteristics: morphology anatomy behaviour embryology protein structure karyotypes DNA sequence (DNA fingerprints)


Hierarchy of characters Taxonomy uses many different characteristics to

define a taxon One character is not enough The characteristics are grouped in a hierarchy.


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


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.


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


Homology in mammalian fore limbs


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


THE CLASSIFICATION OF LIVING ORGANISMS Taxonomy or Systematics: The study of classification ODWSODWS...

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