Taxonomy: The Science of Classification

 

The earth formed about 4.5 billion years ago and within a half billion years the first life originated. Everything alive today is descended from those first simple life forms. In the tree of life, some groups of living things branched out along different paths long ago and now have little in common, while others have branched only recently and are still very similar. In other words, some living things are close relatives, while others are distant cousins or near total strangers. It isn't always easy to see that family history because we can only look at species that are alive today (the living buds at the tips of the branches) while the rest of the tree is hidden in the past. Sometimes a fossil of an extinct species provides a clue, but fossils are rare and finding them requires hard work and a lot of luck! The science of Taxonomy uses the similarities and differences between organisms as clues to try to figure out the puzzle of relationships.

 

A Nuts and Bolts Example:

The following are all made of steel, and are all found in the toolbox. We will assume they are all living things derived from a common ancestor. Let's decide who is most closely related and who is more distantly related.

 

Things to consider:

Homologous structures - structures which are similar because of common ancestry. Example - the front appendages of vertebrates are homologous. The flipper of a whale, the wing of a bat, the paw of a cat, the arm of a human, and the wing of a bird all have the same internal bone structure with only slight modifications in shape and size - they each contain a humerus, a radius, an ulna, carpals, metacarpals and phalanges.

Analogous structures - structures which perform similar functions but are based on completely independent designs. Example - the wing of an insect is analagous to the wing of a bird or the wing of an airplane.

Convergence - the process by which distantly related organisms come to resemble each other closely because they are adapting to the same environment and are under similar selection pressures. Example - the streamlined body and fins of dolphins and seals (both mammals) resemble those of fishes because swimming well requires these modifications.

Synapomorphy - a "shared-derived" trait. Synapomorphys are traits not found in the ancestors but which are shared by a group of descendants. Species which have shared-derived characteristics are probably closely related because the same structure rarely evolves more than once in separate lineages.

 

Dichotomous Tree of the Nuts and Bolts:

 

The major groupings:

There appear to be four major groups - the bolts, the nuts, the screws and the nails. The group that is most different from the others is the nuts, because all the others are long and have a head. Within the three remaining groups, the nails are the most different because both the screws and bolts are threaded. Finally, the screws and bolts can be divided based on the pointed tips of the screws. Someone might argue that the nails and screws have more in common because they both have a pointed tip, and that "pointy tip" is a more important trait than "threading." This is a good argument. See if a third trait lends support for grouping them based on either the threading or the pointy tips.

 

The details:

Within each of the major groups, the branching continues. Bolts 3 and 5 could be grouped as closest relatives because 3 and 5 have identical heads and bolt diameters. Or you might argue that Bolts 2 and 3 are more closely related because they are the same length while Bolt 5 is longer. In the dichotomous (splitting into twos) tree of relationships shown below, it was decided that 3 and 5 were closer because head shape and bolt diameter are two traits that are shared between 3 and 5, while length is the only trait shared between 2 and 3. Note that the position of (3 and 5) could be switched with that of (2) without changing the meaning of the tree. 2, 3 and 5 are all equally distant from the pair of screws (4 and 7).


A Dichotomous Key to the Nuts and Bolts:

A key is useful when you find an organism and you want to identify it based on its characteristics. Here is a key based on the tree for the nuts and bolts. Start at Question 1a and read the description. If the answer to 1a is "Yes", you would be sent to Question 2a. If the answer to Question 1a is "No", you go to Question 1b, which then sends you to Question 3. Keep following the directions until you know the species by its identifying number.

 

1a. With a hole

Go to Question 2

1b. Without a hole

Go to Question 3

2a. Six sided

It is Species #1

2b. Four sided

It is Species #6

3a. With threading

Go to Question 4

3b. Without threading

It is Species #8

4a. Pointy tip

Go to Question 5

4b. No pointy tip

Go to Question 6

5a. Rounded head

It is Species #4

5b. Not rounded head

It is Species #7

6a. Flat head

Go to Question 7

6b. Not flat head

It is Species #2

7a. Body length twice the width of head

It is Species #5

7b. Body length not twice the width of head

It is Species #3

 

Practice using a key:

The identifying numbers have been removed from the nuts and bolts in the following picture. Use the key to identify them and write the correct species number in the blank next to each one.

 


Advanced Lesson - Classifying A Group of Imaginary Organisms

1) Carefully examine the imaginary animals (Caminalcules) or plants (Dendrogrammaceae). In both cases, these imaginary organisms represent separate species that are descended from a common ancestor. Some have remained much like the ancestral species, while others have adapted to different environments and have changed to better suit their new lifestyles. However, you will still see physical features in these species which indicate their relationship to each other. You must use those clues to determine who is most closely related to whom.

2) Cut out the creatures so that their identifying numbers remain with them. On a large surface, do some initial sorting of the creatures so you can compare them for similarities and differences. Try arranging them several different ways before you decide. When you think you have figured out the major groupings, get a blank piece of paper and draw a rough draft of your dichotomously branching tree (splitting from one branch into two) based on the similarities and differences between the organisms. Refer to the nuts and bolts tree as an example. The first branch of the tree should divide the organisms along the most distinctive lines, and subsequent branches should separate organisms based upon more and more subtle differences. Each time the tree splits (always from one branch into only two), write down the distinguishing characteristic you used. Choose your distinguishing characters so that they are unambiguous. Example: If you say "Big eye vs. little eye", you may run into problems. Big as compared to what? However if you said "Eye bigger than mouth", then you can look at the mouth and decide.

3) When your tree is complete and the creatures are all separated from each other (no further divisions are possible), glue all of the individuals down along the top of a new piece of paper according to your tree of groups and subgroups. Then redraw the branches of your relationship tree as in the nuts and bolts example.

4) Create a dichotomous key to the organisms using the example Nuts and Bolts key and your tree as a guide. The distinguishing characteristics you wrote will become the options in the key.