The Classification of Living Things (Taxonomy)

Nature of Classification

Classification of living things is hierarchical

There is a specific hierarchy by which all living things are classified

example of the hierarchy - classification of the house cat:

Kingdom: Animalia

Phylum: Chordata

Class: Mammalia

Order: Carnivora

Family: Felidae

Genus: Felis

Species: Felis domesticus

Why is the classification hierarchical?

Do any of these levels mean anything? That is, is there something special about a family as opposed to an order?

What is a taxon (pl. Taxa)?

Classification as Scientific Theory

A Classification represents the evolutionary (geneological) degree of relatedness between living things.

Is classification "card collecting"?

Details and methodology

What is a "type specimen"

What is a species: special problems for paleontology

  • a group of individuals that interbreeds
  • if two groups hybridize at their boundary, are they the same species?
  • but you cannot tell the breeding habits of fossils?
  • a group of individuals that shares a common set of genetic characteristics

A Classification of living things

(note: this scheme, taken in part from Lane's "Life of the Past", is just one of several classifications that you will find; There is disagreement about the relationships between organisms at several levels)

Prokaryotes (cells without true nucleus)

Kingdom Monera

      • Phylum Bacteria
      • Phylum Cyanobacteria

Eukaryotes (cells with a true nucleus)

Kingdom Protista

      • Phylum Sarcodina Amoebas
      • Phylum Flagellata: Have flagella
      • Phylum Ciliata: Have cilia

Kingdom Plantae

      • Phylum Algae: Photosynthetic aquatic plants in fresh or salt water, single-celled or multicellular.
        • Subphylum Chlorophyta: The green algae, with same photosynthetic pigments as the higher land plants.
        • Subphylum Chrysophyta: The yellow-green algae. Includes several groups, one of which, diatoms, is important as fossils. Mostly unicellular; many with a silica skeleton.
        • Subphylum Phaeophyta: The brown algae. Mostly all marine; rarely with preservable hard parts. Not important as fossils.
        • Subphylum Rhodophyta: The red algae. Exclusively marine, usually multicellular, and with a complex calcareous skeleton that is commonly preserved as a fossil.
      • Phylum Mycophyta: The fungi. Nonphotosynthetic land plants that derive food from the soil and decaying organic matter. Includes mushrooms, molds, rusts, and mildew. Not important as fossils.
      • Phylum Bryophyta: The mosses and liverworts. Photosynthetic land plants that lack vascular tissues and readily preservable hard parts. Not important as fossils.
      • Phylum Tracheophyta: The vascular land plants. Fossil record extends from the Silurian to the Recent. Includes a host of different varieties that are today dominated by the flowering plants.

Kingdom Animalia

Parazoa: Cellular grade of organization. Symmetry radial, spherical, or absent.
      • Phylum Porifera: The sponges. Skeleton of spicules.
Eumetazoa: Tissue, or organ, or organ system grade of organization.

Radiata: Primarily radial symmetry. Tissue grade of organization. Mesoderm incipient, basically two germ layers.

      • Phylum Coelenterata or Cnidaria: The corals, jellyfish, sea anemones.
Bilateria: Symmetry bilateral and some with radial symmetry (the echinoderms). Mostly with mesoderm, and a coelom is developed in most.

Acoelomata: No body cavity. Includes the flatworms and ribbon worms that are not important in the fossil record.

Pseudocoelomata: Body cavities partly lined with mesoderm. Includes several phyla of worms and rotifers not important in the fossil record.

Eucoelomata: Body cavity a true coelom.

Lophophorata: Two phlya characterized by a special circular structure around the mouth with dliated tentacles for food gathering, called a lophophore.

  • Phylum Brachiopoda: Skeleton of two valves. Very important fossils.
  • Phylum Bryozoa: The moss animals. All colonial, most with a skeleton. Important fossils.

Schizocoela: Coelom originates from a split in the mesoderm.

  • Phylum Mollusca: Unsegmented except for one small primitive group (Monoplacophora). Commonly bilaterally symmetrical. Mostly with calcareous exoskeleton secreted by a mantle that surrounds vital organs. Includes the clams (Class Pelecypoda or Bivalvia); the snails (Class Gastropoda); and the cephalopods, as well as other, less important, classes.
  • Phylum Annelida: The annelid worms. Segmented, with a round, elongate body; lack jointed appendages. Tiny chitinous jaws (scolecodonts) Found as fossils as well as rare, carbon-film impressions of body.
  • Phylum Arthropoda: The jointed-leg animals; conspicuously segmented. Includes crabs, shrimp, barnacles, spiders, and insects. The extinct trilobites are the most conspicuous fossil group.

Enterocoela: Mesoderm-lined coelom formed from outpocketings of the embryonic gut.

  • Phylum Echinodermata: The spiny-skinned animals. Conspicuous five sided radial symmetry; unique water vascular system. Includes sand dollars and sea urchins (echinoids), starfishes, sea cucumbers (holothurians), and crinoids. In addition to the five living classes, there are as many as sixteen extinct fossil classes.
  • Phylum Chordata: Elongate, rodlike cartilaginous notochord runs along the back for support. Main nerve cord dorsal (top); heart ventral (bottom). Most adults with gill slits, a bony vertebral column (backbone), or both. The most important groups are the Hemichordata, which include the extinct graptolites, and the Vertebrata, which include all fishes, amphibians, reptiles, birds, and mammals.


Using morphological traits to determine how closely something is related:

  • Ancestor-descendant lineages are untestable hypotheses
  • we use similarity instead.
    • "Primitive" (synplesiomorphies) characteristics are those shared by a "larger" group of organisms because they belonged to a distant ancestor.
      • When comparing humans to all tetrapods, having five fingers is primitive-the first tetrapod to venture on land had 5 fingers and 5 toes. Just because carnivores, rodents, anteaters, turtles and lizards have 5 fingers and 5 toes doesn't make any of these groups any more related to people than are horses.
      • The primitive covering for tetrapods is scales.
    • "Derived" characteristics (synapomorphies) are those shared by a smaller group of organisms because they were "innovated" by a recent ancestor.
      • humans and apes lack tails, monkeys and most other tetrapods still have tails.
      • horses have one toe on each foot, birds and some dinosaurs share the trait of four toes on the hind feet.
    • Without knowing the ancestors from the descendants, we determine how recently one group diverged from another by counting the number of shared characteristics, and sorting out the synapomorphies from the synplesiomorphies by comparison with an outgroup.
      • If I were trying to determine how closely different species of parrots were related to each other, I would compare them to some other bird (a crow, for example). Characteristics shared with the crow are probably primitive.
    • Convergent evolution may make it difficult to sort out synapomorphies, because if the same trait is derived independently, then it is not the "same" trait.
      • does the lack of a tail in a frog make it more closely related to a human than a salamander is?
    • the more synapomorphies are shared by two taxa, the more closely these two taxa are related, hence the more recently their lineages diverged.
    • We use the cladogram to represent these relationships

Impact of the study of evolution on taxonomy

  • if a taxon includes all of the descendants of a single species, then it is said to be "monophyletic" it may also be called a "clade"
  • if a taxon does not include all of the descendants of a single species, it is said to be "paraphyletic"