THE CLASSIFICATION OF LIVING ORGANISMS Taxonomy or Systematics: The study of classification © 2016 Paul Billiet ODWS

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, how they evolve Systematics can be used to identify organisms that are important to us. © 2016 Paul Billiet ODWS

Systematics Collections of animals and plants in museums from 17th century Need for systematic classification established Carl Linneus (1735) The binomial classification To “put order into God’s creation”. Jardins des Plantes Paris © 2016 Paul Billiet ODWS

Binomial system Homo sapiens Genus Species Capital case Small case © 2016 Paul Billiet ODWS

Levels of hierarchy Taxon Domain Kingdom Phylum Class Based on relatedness (phylogeny) but artificial Order Family Genus Species Some biological basis © 2016 Paul Billiet ODWS

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. © 2016 Paul Billiet ODWS

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 DNA sequencing compares the genotypes Use characteristic features to group organisms (e.g. all animals with feathers = Birds) Taxonomists decide which are the most significant or "important" characteristics by the way they occur in different groups of organisms. © 2016 Paul Billiet ODWS

Comparing phenotypes & genotypes Taxonomists compare a new specimen with given characteristics to other specimens: morphology anatomy behaviour embryology protein structure karyotypes DNA sequence (DNA fingerprints). Phenotype Genotype © 2016 Paul Billiet ODWS

Hierarchy of characters Taxonomy uses many different characteristics to define a taxon One character is not enough The characteristics are grouped in a hierarchy. © 2016 Paul Billiet ODWS

Example Acanthostega 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 © 2016 Paul Billiet ODWS

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. © 2016 Paul Billiet ODWS

The pendadactyle limb Lizard Frog Bat Human © 2016 Paul Billiet ODWS

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. © 2016 Paul Billiet ODWS

Homology in mammalian fore limbs © 2016 Paul Billiet ODWS

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 © 2016 Paul Billiet ODWS