Keying Exercise

 

A taxonomic key is used to identify organism when the number of possibilities is too numerous for simply reading all the descriptions and matching the organism to the best choice. A key may be hundreds of pages long and include thousands of species!  We will learn by using short simple keys but resist the temptation to skip ahead—that won’t work when we start with longer keys.

            The type of key we will use today is called a dichotomous key because each step requires you to choose between two options, like a fork in the road.  It is important to read both chooses carefully before choosing—like when you take a multiple choice exam—because it may be a difficult choice.  Both may seem OK or neither.  Choose the best.  If the choices become inappropriate or you end up with an obvious misidentification go back through the steps until you find where you took the wrong “fork”.

            Below is an example of a dichotomous key. Like most keys it does not include every possibility, only the common ones. (How common depends on the key.)  Use this key to find the pig Latin names of coins. Assemble one each of coins worth less than $1.

 

Key to Very Common U. S. Money

 

1a. Made of paper.…illba

1b. Made of metal….2

2a. Copper colored….ennypa

2b. Silver colored….3

3a. Long-haired guy on front….4

3b. Short-haired guy on front….imeda

4a. Smooth edges….icklna

4b. Rough edges….arterqua

 

            It is important to go though a key in order.  Rough edges (4b) would misidentify a dime if you weren’t prevented from getting to the 4’s by the 3’s.  Likewise you don’t always progress to the next number.  If our key had included the Eisenhower dollar coin then 3b would have sent you to 5.  At 5 the a and b choices would have distinguished between the dime and the dollar.

            Note that “long” and “short” hair could easily be ambiguous since, having just one coin at a time to ID, you can’t compare men.  Roosevelt isn’t wearing a buzz cut or Washington and Jefferson wigs to their knees.  You could improve the key by removing relative terms, short and long, and replacing them with clearer ones.  For example “pony-tail along neck” or “hair clipped at top of neck”.  I might distinguish the dime from the dollar by a measurement.  “Smaller than 2 cm vs. Bigger than 2 cm.”  (Use metric.)

            Once you’ve reached this point show me you understand by walking through it with me.  Call me over before continuing.

 

 

CONSTRUCTING A DICHOTOMOUS KEY

 

In this exercise you will create and test a dichotomous key using shells.

 

1. Each group receives a bag of shells.  Note the group number in the bag.

 

2. Label each shell with a letter of the alphabet on a sticker.

 

3.  Make a list of the letters. Give each shell a nonsense name which contains no clue to its identity. Record the names next to the letters. This is you answer set.

 

4. Create a dichotomous key which identifies all of your shells by their nonsense names.  Don’t think of the shells as animals, just as abstract forms.  We are not utilizing biological taxonomy today, so use what you can observe as keying criteria rather than what knowledge you might have about the organisms that used to live in the shells. Make sure your key works for any single shell and is clear and legible.

 

  1. Tell me when you are done. Put your key, but NOT the answer set, in the bag with the shells. Next you will exchange shells and keys with another group.  You will try to ID their shells, one at a time, and recreate their answer list. They will be doing the same with your set.

 

  1. Exchange feedback about problem you discover with each others’ keys.  Debug your work.

 

  1. When you are done check the pairs of lists to see if they match. If the other group successfully used your key to duplicate your answer set they should let me know. Write all your group member names on one of the pages to turn in. If there were problems, identify them and solve them. Ask me for help whenever you need it.