Constructing a Dichotomous Key

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Constructing a Dichotomous Key
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What is a Dichotomous Key?

• A system of identifying objects or organisms in
  which a succession of paired choices leads
  progressively to identification of the object
  (the only remaining correct choice.)

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Using a Dichotomous Key

• Study the organisms that will be classified.
  Choose one
• Read the first set of choices in the dichotomous
  key
• Decide which characteristic the organism has
• Follow the numbered items in the key and name the
  organism

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Finch Identification
1. a. The beak is relatively long and slender. Certhdea sp.
b. The beak is relatively stout and heavy. Go to set 2

2. a. The bottom surface of the lower bill is flat and straight. Geospiza sp.
b. The bottom surface of the lower bill has a bend Go to set 3

3. a. The lower edge of the upper bill has a distinct bend. Camarhynchus sp.
b. The lower edge of the upper bill is mostly flat. Platyspiza sp.
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Seatwork

• Complete the Using a Dichotomous Key assignment

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Constructing a Dichotomous Key

• The first step is to look at the group of objects
  or organisms and separate them into two groups
  based on a single distinguishing characteristic.
• Then continue to separate each of the groups
  until each object has its own separate set of
  characteristics.

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An Example

• As an example, we could construct a key using the
  following stationary supplies taken from a
  student's pencil case.
• pen, highlighter, pencil sharpener, paper clip

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A Spider Key

• Some students find it easier to arrange their
  information in a spider key before setting it up
  in the traditional dichotomous key format.

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A Traditional Dichotomous Key

• The same information presented in a traditional
  dichotomous key format would look like this
• a. long tubular objectsgo to 2
• b. short, non-tubular objects . ..go to 4
• 2. a. constructed from plastic go to 3
• b. not constructed of plastic .pencil
• 3. a. green and grey ......highlighter
• b. blue and clear .pen
• 4. a. black and silver ..pencil sharpener
• b. silver .paper clip

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Homework

• Create a dichotomous key for the insects on page
  26 16
• Number them from 1 7 starting on the top row
  left to right (1 3)
• Second row left to right (4 7 )