Dichotomous Keys 1

Dichotomous Keys Dichotomous classification keys are used to help place organisms into the appropriate classification group. It is a tool that allows the user to determine the identity of items in the natural world, such as trees, wildflowers, mammals, reptiles, rocks and fish. These keys consist of a series of choices that lead the user to the correct name of a given organism. “Dichotomous” means “cut into 2 parts”. Therefore, these keys should always give two choices in each step.

YOUR CHALLENGE: Create a method to classify your shoes. GOAL: a stranger could identify whose shoe is whose based on your clues Instructions: each person donates 1 shoe to their group (you can also take a picture of each shoe and create a google doc) organize your shoes into groups based on similarities and differences try to get to the point where each shoe is able to be separated into smaller and smaller groups and finally a single shoe in its own category

In constructing keys, keep the following in mind: When starting the key, make sure: The first step in the key should divide the organisms in roughly half.

Other Characteristics of a Good Dichotomous Key Try and make the choice a positive one – something “is” instead of “is not”.

Both choices of a pair should be about the same attribute. If possible, start both choices of a pair with the same word, describing alternates of the same characteristic. Each pair of choices should be about a different attribute. If possible, start different pairs of choices with different words (describing different characteristics) Try and use each attribute only once.

Use measurements rather than terms like large/small, big/little or many/few. (Note: Comparisons with structures on the organism could also be used if measurements are not possible. Example: The tail is longer than the body.) Use characteristics that are generally available to the user of the key, rather than seasonal characteristics, or those seen only in the field. Use constant characteristics, rather than ones that change.

When finished: There should be one less step than there are total organisms to classify.

When using a key, keep the following in mind: Always read both choices, even if the first seems to be the logical one at first. Be sure you understand the meaning of the terms involved. Do not guess. 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.

Let’s Practice: 1) Examine the objects closely. 2) Write down some characteristics that are visible. 3) Select one that roughly divides the group in half. 4) Use the other characteristics to develop the rest of your key.

Answers: Example of the Graphical form Example of the Written form

Task 1: Make a dichotomous key create a dichotomous key for the topic of your choice Guidelines: must be school appropriate at least 5 steps (2 options per step) Recruit a student in this class or your teacher to complete your dichotomous key Task 2: Wacky People Handout Complete “Wacky Dichotomous key” handout (front and back - don’t create your own, use the wacky people on the back)

HOMEWORK: Complete “Wacky Dichotomous key” handout. Based on your understanding of what a “good” key is, list 2 good and 2 bad qualities of the key on the handout. MAKE SURE TO ASK FOR EXTRA HELP IF NEEDED. YOU WILL HAVE AN EVALUATION ON DICHOTOMOUS KEYS soon!