the classification of living organisms n.
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  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. 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 Example 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. Lizard Frog Human Bat The pendadactyle limb

  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. Thylacine Thylacinus cynocephalus Wolf Canis lupus 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 ODWS Paul Billiet 2011