2 What is a Classification Key? Classification keys are used to identify an unknown biological specimen.One of the most frequently used types of keys is the “Dichotomous Key”What does “Dichotomous” mean?
4 A dichotomous key always presents you with two choices Two choices of what?The choices are just different characteristics, or features, of the specimen. For example, is the shape of a leaf tip pointed or round?
5 Or, for the characteristic “leaf type”, is the leaf... A broadleaf typeOr needlelike?
6 It’s a little tricky to make a dichotomous key for another person to use, but there is a process you can follow to develop a key that works well.
8 You will create a key for a group of seven specimens You will create a key for a group of seven specimens. They are all broadleafed deciduous trees (they lose their leaves at the end of each growing season).
9 Look at the sample specimens shown on the next slide Look at the sample specimens shown on the next slide. Study them carefully, then write a list of some of the general leaf characteristics that could be used to discuss differences between the specimens.Note:These should just be general features, not the specific descriptions you will use in your dichotomous key.
20 Now it’s time to create a key for the seven tree specimens Now it’s time to create a key for the seven tree specimens. Remember that the key must be dichotomous, and we’ll start by using a spider key format to organize the trees into successively smaller and smaller groups.
21 For example, for the animal group - person, fish, horse, bird, and snake - a dichotomous spider key might look like this:Animal GroupHas no legsHas legsHas two legsMore than two legsHas finsand scalesDoesn’thave finsand scalesHorseNo feathersFishHas feathersSnakePersonBird
22 Once you have created the spider key, you must number the choices - each group of two has the same number, and either an a) or a b),for example:Animal Group1 a)Has no legs1 b)Has legs2 a)3 a)3 b)2 b)Has two legsMore than two legsHas finsand scalesDoesn’thave finsand scalesHorse4 a)4 b)No feathersFishHas feathersSnakePersonBird
23 The first set of choices is always 1a) and b), but notice that you can number differently from there. Compare the numbering onthis slide to the previous slide. Both are equally correct.Animal Group1 a)Has no legs1 b)Has legs4 a)2 a)2 b)4 b)Has two legsMore than two legsHas finsand scalesDoesn’thave finsand scalesHorse3 a)3 b)No feathersFishHas feathersSnakePersonBird
24 … and so on Now that you have created a numbered spider key, rewrite the key by arranging the choices in a listformat so that they fit nicely one below the otheron the page. The list key puts the numbers insequential order, and gives “go to” directions oridentifications after each choice:1a) Has no legs …………………………..go to 21b) Has legs………………………………go to 32a) Has fins and scales……………………Fish2b) Doesn’t have fins and scales………… Snake3a) Has two legs………………………….go to 43b) More than two legs…………………..Horse… and so on
25 So that’s it!Your teacher will now give you thematerials to makea dichotomous key of your own.Good Luck!