The Diversity of Living Things An Introduction

1. The Diversity of Living ThingsAn Introduction

2. 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

3. Class Challenge! • Look at the type of footwear you have on today. • Look around at other peoples footwear. • How could the class be split into two different groups based on footwear? • Can the two newly formed groups be further separated? • Can the groups split yet again?

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

5. Dichotomous Classification Keys

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.

7. 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.

8. Example: • Suppose you have four insects a ladybug, a housefly, a dragonfly and a grasshopper. After studying the insects, you might use: • wing covering • body shape • where the wings point towards.

9. 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:

10. 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.