The Diversity of Living ThingsAn Introduction

Why Classify? Scientists have determined that the Earth has

8.7 million species of living things. Biologists have described only 1.75 million of

these Only a small percentage (20%) of the estimated

life on earth

To understand the diversity of life a system of biological classification that names and groups organisms into meaningful categories was needed.

Helps us understand how things are similar and different from one another

Class Challenge!1. Look at the type of footwear you have on

today.2. Look around at other peoples footwear.3. How could the class be split into two

different groups based on footwear?4. Can the two newly formed groups be further

separated?5. Can the groups split yet again?

In biology, what do we use to help us with classifications? Dichotomous Keys

Dichotomous Classification Keys

Specimen

Does Your Specimen Have?

Yes (Subgroup 1)

Does Your Specimen

Have?

Yes (Subgroup 3)

No (Subgroup 4)

NO (Subgroup 2)

Does Your Specimen

Have?

Yes (Subgroup 5)

No (Subgroup 6)

Dichotomous Keys: Means – divided into two parts. Tool that allows the user to determine the

identity of items in the natural world such as trees, wildflowers, mammals, reptiles, rocks, and fish.

Keys consist of a series of choices that lead the user to the correct name of a given item.

Always give two choices in each step.

When Constructing Keys: 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.

Example: 

Suppose you have four insects a ladybug, a housefly, a dragonfly and a grasshopper. After studying the insects, you might use:o wing coveringo body shapeo where the wings point towards.

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."

A sample dichotomous key might look like:

Step 1. a. wings covered by an exoskeleton……go to step 2

b. wings not covered by exoskeleton….go to step 3

 

Step 2. a. body has a round shape .………………..ladybug

b. body has an elongated shape .…….…grasshopper

 

Step 3. a. wings point out from side of body …dragonfly

b. wings point to posterior of body.……housefly

Notice that there were four organisms to be identified and it only took three steps. Usually, there should be one less step than the total number of organisms to be identified in your dichotomous key.