Classification and Tree Thinking

Humans are excellent classifiers. Scientists classify organisms based upon features that are shared in common with other related organisms. The formal process of classifying organisms is referred to as taxonomy.

Traditional taxonomy relies primarily on physical traits, so that organisms that look alike are placed in the same group.

One traditional taxonomic system, called the Linnaean system of taxonomy, established the following categories:

Critter 1 Critter 2
Domain Eukarya Eukarya
Kingdom Animalia Animalia
Phylum (Division) Chordata Arthropoda
Class Mammalia Insecta
Order Carnivora Hymenoptera
Family Felidae Formicidiae
Genus Felis Solenopsis
Specific Epithet domestica invicta

Lab Question

  1. Can you tell what organisms are categorized here?

Classifying organisms based on shared evolutionary history attaches powerful information to a classification scheme. While traditional taxonomy relied primarily on classifying organisms by their shared physical characteristics (or morphology), technological advancements have allowed scientists to determine an organism’s evolutionary history more accurately. Taxonomy based on presumed evolutionary relationships is referred to as phylogeny.

Usually, traditional taxonomic systems agree with phylogenetic systems when determining where organisms should be classified. However, sometimes, striking conflicts can arise. For example, birds are traditionally placed in their own class Aves. However, a wealth of research indicates that birds are actually living dinosaurs, and are thus technically reptiles.

Check out some more information on this fascinating situation.

Part 1: Dichotomous Keys

A dichotomous key is an identification key that consists of a series of choices that ultimately lead to the identification of the object in question. In each step of a dichotomous key, the user has two choices and must pick the one that most correctly describes the object. Each choice leads to a new set of choices. To help you understand how a key like this works, consider the following four insects:

Illustrations of a housefly, a grasshopper, a dragonfly, and a ladybug.

After studying the insects, you might classify the insects by wing covering, body shape, and the direction in which the wings point.

To begin the key, you could start separating the four insects based on wing covering—”wings covered by exoskeleton” vs. “wings not covered by exoskeleton.”

The first step in the key could be organized the following way:

  1. wings covered by an exoskeleton. . . . . . . . . . . . . . go to step 2
  2. wings not covered by an exoskeleton . . . . . . . . . . go to step 3

Step 2 consists of a pair of statements that distinguishes between the ladybug and the grasshopper.

  1. body has a round shape . . . . . . . . . . . . . . . . . . . . . ladybug
  2. body has an elongated shape. . . . . . . . . . . . . . . . . grasshopper

Step 3 consists of a pair of statements that distinguishes between the dragonfly and the housefly.

  1. wings point out from the side of the body . . . . . . dragonfly
  2. wings point to the posterior of the body. . . . . . . . housefly

Notice that there were four organisms to be identified and it only took three steps to identify them. After making a key, you should end up with one less step than the total number of organisms you are trying to identify.

When constructing a key, keep the following in mind:

  • Use constant characteristics rather than variable ones.
  • Use measurements rather than terms like “large” and “small.”
  • Use characteristics that are generally available to the user of the key rather than seasonal characteristics or those seen only in the field.
  • Make the choice a positive one—something “is” instead of “is not.”
  • If possible, start both choices of a pair with the same word.
  • If possible, start different pairs of choices with different words.

When using a key, keep the following in mind:

  • Always read both choices, even if the first seems to be the logical one.
  • Be sure you understand the meaning of the terms involved. Do Not Guess.
  • When measurements are given, use a calibrated scale. Do Not Guess.
  • Since living things are always somewhat variable, do not base your conclusion on a single observation. Study several specimens to be sure your specimen is typical.
  • If the choice is not clear, for whatever reason, try both divisions. If you end up with two possible answers, read descriptions of the two choices to help you decide.
  • Having arrived at an answer in a key, do not accept this as absolutely reliable. Check a description of the organism to see if it agrees with the unknown specimen. If not, there is an error somewhere, either in the key or in its use. The ultimate check of identifications is a comparison of the unknown with an authentically named “Type Specimen.”


  1. Examine the 6 specimens at your table and use them to construct your own dichotomous key here:
  2. Use someone else’s key to identify 2 specimens. List your path to identification here (letters are fine):
    1. Specimen 1
    2. Specimen 2

Part 2: Cladistics

A new system of phylogenetic classification , called cladistics, is currently in practice today. A cladogram is a hypothesis about the evolutionary relationships between the organisms depicted on the tree. In this way, a cladogram illustrates the lines of descent for these organisms. A cladogram proposes an answer to the question “Which groups of organisms share a common ancestry?”

2 cladogramsTake a look at these two identical, generic cladograms. The capital letters indicate the terminal organisms represented in the tree. The numbers indicate characters present in organisms beyond that point. And the nodes (indicated by the lowercase letters and the dots) represent the common ancestors of the terminal organisms. Even though they look different, if you examine them closely, these two cladograms are depicting the same relationships between critters A, B and C.

In this example you see that A is more closely related to B than C based on the shared derived characteristic 1. Note that at each branch a derived characteristic is indicated that separates the left branch from the right branch of the evolutionary tree.

Now examine the cladogram at the bottom of this page illustrating the evolutionary relationships between a hagfish, shark, bony fish, frog, rat, bird, and lizard.


  1. Name each organism on the cladogram.
  2. Place a dot at every point that represents a common ancestor.
  3. Indicate one shared derived characteristic that distinguishes each branch.
  4. Who is more closely related: the shark and bony fish, or the bony fish and frog?
Cladogram of Vertebrata, showing the evolutionary relationship of a hagfish, shark, bony fish, frog, rat, bird, and lizard. The animals derive from a common ancestor separately and in the order listed.

Part 3: Making Caminalcule Cladograms

Caminalcules were created by the evolutionary biologist Joseph Caminal and were originally called Caminalcules. The 29 living species and 48 fossils were published by Robert R. Sokal (1983a) in the journal Systematic Zoology. They were used to conduct detailed research on evolutionary classification.

Lab Question

  1. As an example, list the similarities and differences among the following individuals.


  1. Draw the simplest possible evolutionary tree that contains these 4 living critters and these 3 fossil critters.
  2. Include at least one trait that distinguishes each living critter.
  3. Use each fossil as an ancestor.
  4. Remember—cladograms are hypotheses that must be tested. Your hypothesis might be different from someone else’s!

The Caminalcules

Using the provided sheet of paper with illustrated Caminalcules, cut out each individual using a pair of scissors. Next, working in groups of 2 or 3, categorize individuals into subgroupings based on shared morphological features that distinguish them from other subgroupings (known as a derived feature). Once you have categorized the species into four or five major groups, paste the animals onto a piece of paper and indicate branches delineated by a shared derived feature. The instructor will illustrate an example of a completed cladogram.

Remember! Cladograms are hypotheses that must be tested! Your hypothesis might be different from someone else’s. Be prepared to defend your decision (with evidence) to someone whose hypothesis is different.