Key Points

  • Taxonomy (biological nomenclature) is a way of having a universal set of names for groups of living things.
  • A formal set of rules exists for naming and organizing taxa (named groups of organisms). Among these are that taxa are named in Latin (or at least in Latinate style) and that they are organized as a nested hierarchy.
  • Species represent a fundamental unit in taxonomy. Species are grouped into genera.
  • There is no single universally recognized method of identifying when two individuals are in the same species This is even more problematic for fossils, where some relevant information (such as interbreeding, DNA, and so on) is not available.
  • Taxonomic differences are not the only reason that two individuals might be different: ontogenetic, sexual, geographic & individual variations have to be considered as well.

Taxonomy

Taxon (pl. taxa): a named group of organisms. From this term, we get taxonomy: the study and principles for applying names to groups of organisms.

Traditionally, each culture had its own name for the animals, plants, and other organisms in their region. But EACH culture had its own set of names, so the same type of animal might have many different names. During the 1600s and 1700s, methods were proposed for a formal scientific set of names.

Carlos Linnaeus developed a universal set of rules in the Systema Naturae ("System of Nature") in 1758; later workers added and modified the system (primarily with the addition of new "ranks").

Some of the Linnaean rules:

Linnaean taxonomy has its own special set of grammatical rules:

  • Genera have one word names (e.g., Panthera, Homo, Ginkgo, Tyrannosaurus)
  • Species have two word names, the first part of which is the same as the genus name (e.g., Panthera leo, Homo sapiens, Ginkgo biloba, Tyrannosaurus rex)
  • All taxon names other than species have one word names, which are capitalized; all taxon names other than genera and species are in roman letters (i.e., they are never italicized/underlined): Dinosauria, Tyrannosauridae, Animalia; not Dinosauria, tyrannosauridae, or animalia.
  • Taxon names of whatever "rank" have some etymology (derivation) (that's true of all words, really). Sometimes the name might be descriptive (e.g., "Triceratops horridus", the "roughened three-horned face") or it might honor a place of discovery (e.g., "Albertosaurus", found in the Canadian Province of Alberta) or some individual (e.g., "Diplodocus carnegii", after billionaire Andrew Carnegie who's funding supported the expedition and museum which found this species). But the name can be inaccurate (e.g., "Basilosaurus"--King Reptile--is a whale, not a reptile!) but if the name was formed obeying the rules of taxonomy, that inaccurate descriptor is fine.

    • Because there is disagreement about the features used to define a particular species or genus, different biologists and paleontologists will sometimes disagree about which specimens belong in a particular species, and which species belong in a particular genus (and so forth).

    For those interested in a website concerning some unusual Linnaean species names, click here.

    What Are Species?

    What is a species? Above we see the rules for these names, but it doesn't tell us about what it is being named.

    Linnaeus' "species" were taxa like lions, tigers, black bears, etc. These were assemblages of individuals that share certain attributes:

    Darwin did not regard species as a distinct "kind" of biological entity. Instead, he considered them as essentially the same thing as geographic or stratigraphic variations (see these below), but ones in which extinction has removed the intermediate forms that otherwise would blend into the closest living relative group.

    20th Century biologist Ernst Mayr (and most contemporary biologists) formalized their definition of a species as a "naturally occurring populations that interbreed and produce viable fertile offspring".

    But there are some problems with this. For one: hybrids (crosses between two separate species) do occur naturally, and many of these are actually fertile! And for paleontologists: we can't test interfertility between populations because they are dead!

    So we are stuck looking only at shapes (and in fact, only the shapes of those hard parts that survive fossilization).

    The question then becomes: how different do two individuals, or two populations, have to be for us to consider them different species? This is actually a terribly difficult question even with living organisms!! There are several sources of variation:

    In fact, the recognition that species were NOT absolute kinds, but instead have "fuzzy" boundaries that blend into each other, is one of the main clues to the discovery of evolution.

    Supplementary Video