Classification of Organisms

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  • Classification of Organisms
    • Classification is the arrangement of organisms into groups based on shared features
    • Taxonomy is the branch of biology concerned with naming and classifying the diverse forms of life
      • Linnaean Taxonomy
        • Carl Linnaeus was the first scientist to introduce a formal system of taxonomy
          • It is a system with two main characteristics;
            • Uses a hierarchy approach
              • A large group of organisms sub-divided into smaller groups
                • Members of a group sharing features unique to that group
                  • There is no overlap between groups
              • There are 7 catagories within this system
                • Each successive group from kingdom to species contains less organisms
                  • Organisms are more closely related with progression from kingdom to species
                    • A taxon is a level in this classification hierarchy and is a collection of organisms sharing basic features
              • Kingdom
                • Phylum
                  • Class
                    • Order
                      • Family
                        • Genus
                          • Species
            • It uses a binomial system (two part name for each species)
              • The last two taxa, genus and species are used to give the organism it's name
                • The biological name is always written in italics, when printed, underlines or handwritten
                  • Binomial system used to avoid confusion of local, common names and languages
              • The genus is written with a capital letter at the beginning and the species is written in lower case
    • Phylogenetic Classification
      • Biological classification of organisms is based on;
        • The presence of shared characteristics
        • Evolutionary relationships between species
          • This is known as the phylogenetic system
            • The presence of shared characteristics
      • Shared characteristics include;
        • Morphology
          • Physical characteristics and the presence of homologous structures e.g. the pentadactyl linm
          • Dangers of using morphology to classify organisms;
            • Similarities in morphology can show relatedness
              • Example: The pentadactyl limb found in mammals, birds, reptiles and amphibians is a structure that indicates these organisms are truly related, therefore it is called a homologous structure
                • However, some physical characteristics e.g. the wings on insects and birds may appear the same on the surface and therefore suggest that the organisms are related but on furthur examination show that the structures are different
                  • These are called analogous structures and do not indicate relatedness. They have the same function but a different structure e.g. birds wing and insects wing
          • Homologous structures
            • Same fundamental structure
            • May not have the same function
            • Evolved from a common ancestor
          • Analogous structures
            • Differ in fundamental structure
            • Have same function
            • Superficial resemblance - no relatedness
            • e.g. Dorsal fin
            • Due to environmental pressures
        • Biological make up
          • e.g. DNA fingerprinting and amino acid sequences of common proteins are used to estimate relatedness between species
    • Convergent Evolution
      • Sometimes analogous features evolve in different organisms due to convergent evolution
        • Convergent evolution happens when different species adapt to a common environment by evolving similar physical characteristics, even though they may be biologically different
      • Convergent evolution is 'The tendency of unrelated organisms to aquire similar physical structures in response to their environment'
    • Biochemical techniques
      • To overcome the problems that can be caused by classification by morphology alone, biochemical analysis is also used
      • The development of certain biochemical techniques has improved the accuracy of classification, as it is now possible to;
        • Compare the sequence of amino acids in proteins - the more similar a sequence, the more closely related a species
        • Compare the sequence of bases in the DNA of genes (DNA fingerprinting) - the more similar they are, the more closely related a species
          • Proteins are displayed in bands on electrophoresis gel
    • Evolutionary relationships
      • Fossil records and biochemical analysis are used to work out evolutionary relationships between organisms
        • It is believed that all organisms come from a common ancestor
          • Species with many common characteristics are closely related and share a recent common ancestor
            • Species with fewer common characteristics are not closely related and share a less recent common ancestor
      • A phylogenetic tree shows the ancestry of groups or points of divergence
        • The key to phylogenetic classification is looking for the sorts of common features that must be due to common ancestors and not to evolutionary pressure
          • The phylogenetic relationships of different species can be represented by a diagram called a phylogenetic tree
            • The closer the branches the more recent the points of divergence of species and therefore the closer the evolutionary relationships
              • Shows ancestry of groups, points of divergence and who is closely related to who

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