Dichotomous Keying

Introduction:

The identification of biological organisms can be greatly simplified using tools such as dichotomous keys.  A dichotomous key is an organized set of couplets of mutually exclusive characteristics of biological organisms.  You simply compare the characteristics of an unknown organism against an appropriate dichotomous key.  These keys will begin with general characteristics and lead to couplets indicating progressively specific characteristics. If the organism falls into one category, you go to the next indicated couplet.  By following the key and making the correct choices, you should be able to identify your specimen to the indicated taxonomic level. 

Couplets can be organized in several forms.  The couplets can be presented using numbers (numeric) or using letters (alphabetical).  The couplets can be presented together or grouped by relationships.  There is no apparent uniformity in presentation for dichotomous keys.

Sample keys to some common beans used in the kitchen:

Numeric key with couplets presented together.  The major advantage of this method of presentation is that both characteristics in a couple can be evaluated and compared very easily.

 

1a.

Bean round

Garbanzo bean

1b.

Bean elliptical or oblong

Go to 2

 

 

 

2a.

Bean white

White northern

2b.

Bean has dark pigments

Go to 3

 

 

 

3a.

Bean evenly pigmented

Go to 4

3b.

Bean pigmentation mottled

Pinto bean

 

 

 

4a.

Bean black

Black bean

4b.

Bean reddish-brown

Kidney bean

Alphabetical key with couplets grouped by relationship.  This key uses the same couplet choices as the key above.  The choices within the first and succeeding couplets are separated to preserve the relationships between the characteristics.

 

A.

Bean elliptical or oblong

Go to B

 

   B.  Bean has dark pigments

Go to C

 

            C.  Bean color is solid

Go to D

 

            C.  Bean color is mottled

Pinto bean

 

                     D.  Bean is black

Black bean

 

                     D.  Bean is reddish-brown

Kidney bean

 

   B.  Bean is white

White northern

A.

Bean is round

Garbanzo bean

Rules for Using Dichotomous Keys: 

When you follow a dichotomous key, your task becomes simpler if you adhere to a few simple rules of thumb: 

  1. Read both choices in a couplet carefully.  Although the first description may seem to fit your sample, the second may apply even better.

  2. Keep notes telling what sequence of identification steps you took.  This will allow you to double-check your work later and indicate sources of mistakes, if they have been made.

  3. If you are unsure of which choice to make in a couplet, follow both forks (one at a time).  After working through a couple of more couplets, it may become apparent that one fork does not fit your sample at all.

  4. Work with more than one sample if at all possible.  This will allow you to tell whether the one you are looking at is typical or atypical.  This is especially true when working with plants – examine more than one leaf, branch, cone, seed, flower,…etc.

  5. When you have keyed out an organism, do not take your effort as the final result.  Double check your identification scheme, using your notes.  Find a type specimen (if available) and compare your unknown to the type specimen.  If a type specimen is unavailable, find a good description of the indicated taxonomic group and see if your unknown reflects this description.

  6. When reading a couplet, make sure you understand all of the terms used.  The best keys will have a glossary of technical terms used in the key.  If a glossary is unavailable, find a good reference work for the field (textbook, biological dictionary,…etc.) to help you understand the term.

  7. When a measurement is indicated, make sure that you take the measurement using a calibrated scale.  Do not “eyeball” it or take a guess.

Exercise 1:

Using a container of beans, use one of the dichotomous keys above to identify the beans.  Glue the beans to the card provided and label them with their common name. Indicate what steps you followed to arrive at your answer.  Turn the card in to your instructor.  Compare your answers to the instructor’s descriptions and type specimen.

Exercise 2:

Obtain samples of the snack chips provided.  Develop a dichotomous key to identify the snacks.  In your notebook, keep track of the characteristics you used to differentiate between the different snack families.  What are the values of the characteristic for each snack food? 

Exercise 3:

Use the dichotomous key to conifers provided below to identify conifers.

A Key to Selected North American Native and Introduced Conifers

 

01a

Leaves needle-like

Go to 02

01b

Leaves flattened and scale-like

Go to 27

 

 

 

02a

Leaves are in clusters

Go to 03

02b

Leaves are borne singly

Go to 15

 

 

 

03a

Two to five leaves in a cluster

Go to 04  Genus Pinus

03b

More than five leaves in a cluster

Go to 14

 

 

 

04a

Leaves mostly 5 in a cluster

White Pine (Pinus strobus)

04b

Leaves 2 or 3 in a cluster

Go to 05

 

 

 

05a

Leaves mostly 3 in a cluster                      

Go to 06

05b

Leaves mostly 2 in a cluster

Go to 08

 

 

 

06a

Leaves twisted, less than 5 inches long

Pitch Pine (Pinus rigida)

06b

Leaves straight, more than 5 inches long

Go to 07

 

 

 

07a

Leaves 5-10 inches long, cones very thorny

Loblolly pine (Pinus taeda)

07b

Leaves mostly over 10 inches long, cones unthorned

Longleaf pine (Pinus palustris)

 

 

 

08a

Leaves mostly longer than 3 inches

Go to 09

08b

Leaves mostly shorter than 3 inches

Go to 11

 

 

 

09a

Leaves rigid, bark grayish

Black pine (Pinus nigra)

09b

Leaves narrower than 1.6mm; bark reddish brown or brown

Go to 10

 

 

 

10a

Cones thornless, twigs brown

Norway pine (Pinus resinosa)

10b

Cones thorny, twigs whitish

Shortleaf pine (Pinus echinata)

 

 

 

11a

Leaves mostly wider than 1.5 mm

Go to 12

11b

Leaves mostly narrower than 1.5 mm

Go to 13

 

 

 

12a

Leaves mostly longer than 35 mm

Mugho pine (Pinus mugo)

12b

Leaves mostly shorter than 35 mm

Jack pine (Pinus banksiana)

 

 

 

13a

Twigs whitened

Virginia pine (Pinus virginiana)

13b

Twigs not whitened

Scotch pine (Pinus sylvestris)

 

 

 

14a

Leaves deciduous, clusters of 20-40

Larch (Larix sp.)

14b

Leaves persistent, stiff, and four sided

True cedar (Cedrus sp.)

 

 

 

15a

Needles short and sharp

Giant Sequioa  (Sequioadendron giganteum)

15b

Needles longer than 12 mm

Go to 16

 

 

 

16a

Tiny pegs on twigs

Go to 17

16b

No pegs on twigs

Go to 22

 

 

 

17a

Pegs square, needles sharp

Go to 18 Genus Picea

17b

Pegs round, needles flat and blunt

Hemlock (Tsuga sp.)

 

 

 

18a

Leaves dark green or yellow green

Go to 19

18b

Leaves blue-green

Go to 20

 

 

 

19a

Branchlets droop

Norway spruce (Picea abies)

19b

Branchlets do not droop

Red spruce (Picea rubens)

 

 

 

20a

Leaves at right angles to stems

Blue spruce (Picea pungens)

20b

Leaves point forward

Go to 21

 

 

 

21a

Leaves about 12 mm long, seed cones 15-32 mm in length, crown narrow and pointed

Black spruce (Picea mariana)

21b

Leaves about 19 mm long, seed cones 50 mm in length, spire-like crown

White spruce (Picea glauca)

 

 

 

22a

Buds large and pointed

Douglas fir (Pseudotsuga sp.)

22b

Buds small and rounded

Go to 23

 

 

 

23a

Terminal buds round and clustered

True fir (Abies sp.)

23b

Terminal buds not clustered

Go to 24

 

 

 

24a

Needles white underneath

Go to 25

24b

Needles green underneath

Go to 26  Genus Taxus

 

 

 

25a

Needles pointed

Redwood (Sequoia sempervirens)

25b

Needles blunt

Hemlock (Tsuga sp.)

 

 

 

26a

Leaves 18 mm long or less with inconspicuous midrib

American Yew (Taxus canadensis)

26b

Leaves 25 mm long or more with conspicuous midrib

Japanese Yew (Taxus cuspidata)

 

 

 

27a

All leaves short and sharp

Giant Sequioa  (Sequioadendron giganteum)

27b

Some leaves not sharp

Go to 28

 

 

 

28a

Cones round

Go to 29

28b

Cones not round

Go to 31

 

 

 

29a

Cones soft and leathery

Juniper (Juniperus sp.)

29b

Cones woody

Go to 30

 

 

 

30a

Cones under 12 mm in diameter

False cypress  (Chamaecyparis)

30b

Cones over 12 mm in diameter

Cypress (Cuppressus)

 

 

 

31a

Cones resemble rosebuds

White cedar or arbor vitae (Thuja)

31b

Cones resemble duck bills

Incense cedar (Calocedrus)

Conifers to Identify:

1. Name:

2. Name:

3. Name:

4. Name:



5. Name:

6. Name:



7. Name:

8. Name:



9. Name:

10. Name:



11. Name:

12. Name:



13. Name:

14. Name:



15. Name:

16. Name:

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