Classification

Classification = the act of arranging organisms into groups based on their similarities and differences.
1. This makes it easier for scientists to identify them.
2. Taxonomy = the study of classification.
3. Similar organisms are placed into 1 of 3 domains i.e. animals, plants or fungus
4. As you move down the hierarchy, there are more groups at each level but fewer organisms in each group.
5. The hierarchy ends with species which only contains one type of organism e.g. human, dog or E. coli
6. Animals can be placed into 1 of 5 kingdoms i.e. prokaryotae, protoctista, fungi, plantae, animalia.
  1. Prokaryotae are prokaryotic, unicellular, with no nucleus and less than 5um.
  2. Protoctista are eukaryotic usually living in water, single celled or simple multicellular organisms.
  3. Fungi are eukaryotic with a chitin cell wall and saprotrophic (absorb substances from dead or decaying organisms)
  4. Plantae are eukaryotic, multicellular with walls made of cellulose, can perform photosynthesis, contain chlorophyll and are autotrophic (produce their own food)
  5. Animalia are eukaryotic multicellular cells with no cell wall and are heterotropic (consume plants and animals)
Binomial Naming System
1. This is used for classification as it involves giving all organisms one internationally accepted Latin name.
2. The first part is the genus which is written with a capital letter. The second part is the species name and it is written with a lower case. Both are written with italics or underlined if handwritten.
3. This helps to avoid the confusion of using common names. For example, there are over 100 different plant species called raspberries.
Phylogeny
1. This is the study of the evolutionary history of groups of organisms.
2. Since all organisms have evolved from one common ancestor, phylogeny shows which organisms is related to which ancestor and how closely.
3. Closely related species diverged away from each other most recently.
Evolution of Classification Systems
1. Early Classification
  1. Classification systems only used observable features such as laying eggs and flying.
  2. However scientists don't always agree on the relative importance of these features as they may not show how closely related the organisms are.
2. Modern Classification
  1. Molecular Evidence
    1. Similarities in proteins in DNA. Closely related organisms will have more similar molecules.
    2. The way DNA is stored, the sequence of bases and the sequence of amino acids in proteins can all be compared.
  2. Embryological Evidence
    1. The similarities in the early stages of the embryo's development.
  3. Anatomical Evidence
    1. The similarities in structure and function of different body parts.
  4. Behavioural Evidence
    1. The similarities in behaviour and social organisation of an organism.
3. The 3 Domain System
  1. This is relatively new (1990) and was suggested because of new evidence.
    1. Molecular Evidence showed the enzyme RNA polymerase is different in Bacteria and Archaea because Archaea have similar histones to Eukarya
    2. Cell Membrane Evidence showed that the bonds of lipids in the cell membranes of Bacteria and Archaea are different
  2. In the older system, the largest groups were the kingdoms.
  3. Most scientists now agree that Bacteria and Archaea evolved separately and that Archaea are more closely related to Eukarya than Bacteria.
Dichotomous Keys
1. These provide a way to identify organisms based on observable features such as colour of leaves etc.

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