Dichotomous Keys A dichotomous key = a system of identifying organisms using a two-choice key a.k.a. Taxonomy key "Dichotomous" means “divided into two.

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Direct Instruction: Dichotomous Keys

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But what if we find things we don't know and we want to know what they are?

Dichotomous Keys A dichotomous key = a system of identifying organisms using a two-choice key a.k.a. Taxonomy key "Dichotomous" means “divided into two parts” (Greek origin) dichotomous keys always give two distinct choices in each step, often they are opposites Black/white; good/evil; pointed/rounded

Virtual Dichotomous Key Activity http://coolsciencelab.com/walk4_broadband.swf

Two ways to write a dichotomous key: Graphic Form Written Form

Try this example: EXPLORE: Introduce your students to the dichotomous key. Together, use the key to identify the group to which each organism belongs. Assign the student reading. You may also wish to direct your students’ attention to the “Kingdoms” animation which is accessible from the multimedia lesson home page.

MAKING A DICHOTOMOUS CLASSIFICATION KEY http://www.qacps.k12.md.us/ces/Staff2.htm

Making a Dichotomous Key

POSSIBLE “DERIVED CHARACTERISTICS/TRAITS” Derived trait = a trait that the current organism has, & previous one didn't. For example: Hammer-like head All made of one material Hinged Has a sharp edge Has holes

Hammer-like head Other types of head

Hammer-like head Other types of head

Hammer-like head Other types of head ALL Made of Wood Made of Wood and other materials

Hammer-like head Other types of head Sides of head are different Sides of head are same

Hammer-like head Other types of head Sides of head are different Sides of head are same All wooden NOT all wooden

? ? Hammer-like head Other types of head Sides of head are different Sides of head are same ? ? All wooden Wooden and rubber

Things to consider when making a key: - Use constant characteristics rather than variable ones. (Flowers change with the seasons) - Use measurements rather than terms like "large" and "small". - Make the choice a positive one - something "is" instead of "is not". - Ex: “snake ears are internal only” If possible, start both choices of a pair with the same word or item. - Ex: “the body is round” vs “the body is square”