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The Three Domains of Life

In November 2000, Dr. Carl Woese won the National Medal of Science for his work on classifying living organisms into three major Domains, based on the nucleotide sequence of different types of RNA (e.g., small subunit rRNA). (You'll recall RNA as DNA's more streamlined "cousin".)

The divergence of RNA sequences can be used as an index of how much time has passed since various taxa diverged from a common ancestor.

Ribosomal RNA is an appropriate molecule to use because

Woese proposed that all life be classified into three Domains:

We classify organisms into each of these groups based on characteristics they have in common, but place them into smaller, less inclusive groups within the three domains on the basis of characteristics that make each group unique.


Taxonomy: Describing, Naming, and Classifying Living Organisms

In the earliest studies of biodiversity...

Carl to the Rescue
Carl von Linné was born on May 23, 1707 in Stenbrohult a town in the Swedish province of Småland. His passionate love of nature led him to become a botanist, but his most enduring work is known as Systema naturae, the full title of which was Systema Naturae: Creationis telluris est gloria Dei ex opere Naturae per Hominem solum. This last part translates as "The Earth's creation is the glory of God, as seen from the works of Nature by Man alone."

Like many scientists of his day, Carl Linne was deeply religious, and saw his scientific efforts as a form of worship to a divine creator.

Systema naturae outlined a new way to classify and name living things that we still use today (sometimes referring to it fondly as the Linnaean System of classification), although--unlike Carl--we don't base our taxonomic groupings primarily on shared similarities. As we'll see shortly, each species has a unique name that is written in Greek or Latin, or at least Latinized (if the term is a name or term from another language).

In the spirit of his work, Carl von Linné started to call himself Carollus Linnaeus, and today we simply recall the Father of Modern Taxonomy as Linnaeus.

In the Linnaean system, each species is nested into successively more inclusive taxonomic groups above it:

The most inclusive group (kingdom) includes the less inclusive groups below it, all the way down to species.

More recently, and even more inclusive taxonomic designation, the Domain has been added, to group related Kingdoms.


Taxonomic Hierarchy: Organizing Life

A taxonomic hierarchy can be visualized as a series of nesting groups,
with the most inclusive (largest) groups engulfing ever less inclusive (smaller) groups.

  • Click on Kingdom 2 to look deeper inside it.


    Each Kingdom contains related phyla (singular = phylum) within it.
    Each Phylum contains related classes within it, and so on.

  • Click on Class C to look deeper inside it.





    ...and so on.

    Phylogenetic trees are constructed in a similar way. A very simplified one is shown below. Note that actual phylogenies might not be quite this clear-cut, but the general rule--with less inclusive groups branching farther up the tree--is the same.


  • Scientific Names

    Every species has its own, unique scientific name consisting of its genus and species written in Greek or Latin, or Latinized.

    Example: Strongylocentrotus purpuratus and Strongylocentrotus franciscanus

    Of all these classification groups, each generically known as a taxon (plural = taxa) the only one that is really biologically meaningful is...


    What's in a Name?
    Would a Frog by any other name be as sticky?
    Scientific names have a specific meaning, often physically describing the species. For example, Eleutherodactylus planirostris, the Greenhouse Frog:

    • eleutheros is Greek for...
    • dactyl is Greek for...
    • plani is Greek for...
    • rostris is Greek for...
    And the frog looks like this:

    Many species are at least partially named after people, but these proper names, too, must be Latinized:

      Example: Chilomeniscus savagei

    • chilo is Greek for...

    • meniscus is Greek for...

    • And "savagei" is the Latinization of the last name of Dr. Jay Savage, eminent herpetologist (and professor emeritus of the University of Miami).

    And sometimes they're just weird, such as Upupa epops

    Here are some entertaining scientific names.