Unit VII. Classification of Organisms

I. Biological classification

  A. Taxonomy - study of hierarchical classification of organisms

    1. Taxonomists are scientists who study classifying

    2. Taxon ( taxa-plural) is a category into which related organisms are placed

      a. The seven taxa from broadest to most specific are --- Kingdom, Phylum, Class, Order, Family, Genus, & species

      b. Each taxa is a proper noun and should be capitalized except species

        -the different taxa can be recognized by their appropriate suffixes

      c. Each level or taxon groups together organisms that share more characteristics than the level above      

      d. Botanists use the term division instead of phylum for classifying plants

      e. Plant species are subdivided into varieties, while bacteria are subdivided into strains

Resource: Examples of classification

  B. Reasons to Classify:

    1. Shows evolutionary relationships

    2. Accurately & uniformly names organisms

    3. Prevents misnomers such as starfish & jellyfish that aren't really fish

    4. Uses same language (Latin) for all names

    5. Prevents duplicated names because all names must be approved by International Naming Congresses (International Zoological Congress)

    6. Naming rules follow the International Code for Binomial Nomenclature

  C. Early Taxonomy:

    1. Aristotle was the first taxonomist dividing organisms into land, sea, & air dwellers

    2. John Ray was the first to use Latin for naming

    3. Linnaeus developed the modern system of naming known as binomial nomenclature, a two-word name (Genus & species)

      a. Linnaeus put all organisms into one of two kingdoms --- Plantae or Animalia

      b. placed organisms into related groups called taxa (taxon-singular) based on their morphology (similar structure & function)

    4. Scientific names should be italicized in print or underlined when writing

    5. Always capitalize the genus name, but write the species in lower case

    6. The scientific name for most organisms is identified by Genus and species. Example: man is Homo sapiens

      -the two name system is called binomial nomenclature

    7. The genus name may be abbreviated, but not the species (H. sapiens)

  C. Basis for Modern taxonomy:

    1. Modern taxonomists classify organisms based on their evolutionary relationships

    2. Homologous structures have the same structure, but different functions & show common ancestry

    3. The bones in a bat's wing, human's arm, penguin's flipper are the same (homologous), but the function is different 

    4. Analogous structures have the same function, but different structures & do not show a close relationship (insect wing & bird's wing)

    5. Similarity in embryo development shows a close relationship (vertebrate embryos all have tail & gill slits)

    6. Similarity in DNA & amino acid sequences of proteins show related organisms

resource: Cell differentiation

II. Modern Taxonomic System:

  A. Modern taxonomy uses six kingdoms --- Archaebacteria, Eubacteria, Protista, Fungi, Plantae, & Animalia

    1. Archaebacteria & Eubacteria are unicellular prokaryotes lacking a nucleus, while Protista, Fungi, Plantae, & Animalia are

        all eukaryotes with a nucleus & membrane-bound organelles 

    2. All members of Plantae & Animalia are multicellular organisms

    3. Fungi & Animalia are heterotrophs, while Plantae are all autotrophs capable of making their own food 

    4. Archaebacteria live in harsh environments like very salty lakes; intestines of mammals; and hot, sulfur springs & may be autotrophs or heterotrophs

    5. Eubacteria are true bacteria some of which cause disease

    6. Protista are mainly unicellular with a few multicellular organisms and may be autotrophic (Euglena) or heterotrophic (Ameba)

    7. Fungi include multicellular mushrooms, mold, unicellular yeast, etc. & are absorptive heterotrophs (digest food & then absorb it)

    8. Animalia are ingestive heterotrophs that take in food & then digest it inside their multicellular bodies.

    9. Plantae includes all plants & are the only all multicellular, autotrophic kingdom

   10. Classification of viruses may shed light on their involvement in the evolution of life

  B. Phylogeny (Cladistics or phylogenetic systematics)- shows the evolutionary history of related organisms:

    1. Phylogenetic trees are branching diagrams showing how organisms are related 

      - Also called family trees

    2. Fossil records help establish relationships on a phylogenetic tree

    3. Organizes living things based on their evolution (systematics)

    4. Common ancestor is shown at the base of the tree

    5. Most modern organisms shown at tips of branches

    6. Each time a branch divides into a smaller branch, a new species evolves

    7. Cladograms shows how organisms are related based on shared, derived characteristics such as feathers, hair, scales, etc.

      -types of cladograms: vertical orientation, and  horizontal orientation

      -the lengths of each line represents relative time

  C. Taxonomic Keys (Dichotomous Keys)- tools for identifying organisms based on properties

    1. Dichotomous means being described as one of two parts. All keys are based on described which can be identified as either/or

    2. Example: Classification of shop fasteners

Resource: Guidelines for using taxonomic keys

  D. Three Domain System:

    1. Based on comparing sequences of ribosomal RNA in different organisms to determine ancestry

    2. All organisms placed into three broad groups called domains

    3. Domain Archaea (kingdom Archaebacteria) contains chemosynthetic bacteria living in harsh environments

    4. Domain Bacteria (kingdom Eubacteria) contains all other bacteria including those causing disease

    5. Domain Eukarya (kingdoms Protista, Fungi, Plantae, & Animalia) contains all eukaryotic organisms

Image: Cladogram of 3 Domains