constructing dichotomous keys

Things to consider

  1. Individual variation

  2. Colour is subjective

  3. Need to find things that are consistent

  4. Differences at different life stage and sexes

 

Characteristics of dichotomous keys:

Statements are in pairs (often called a couplet)

Pair of statements are numbered sequentially

Each statement of a pair is exclusive

Descriptions must be accurate and unambiguous

 

Stages in key construction:

  1. Examine all members of the group to be separated, identify where variation lies

  2. Construct table of characteristics

  3. Write the couplet statements

 

Hazelnut-D

Walnut-F

Almond-C

Pecan-E

Groundnut-A

Pistachio-B

 

Texture- divots, smooth

Size

Singular nut/ double

Shape

Hard shell, soft shell

 

If its smooth=

Does it have a rough end to it or does it have a pointed edge?

Yes- D

No- does the nut have two colours or a singular one?

Yes- E

No- B

If it has divots= does it have two sections or a singular one?- AFC

Two sections=A

Singular= is the shape more round or flat? Round= F flat= C

 

Is the nut smooth or does it have divots?

If smooth= does it have a rough end to it or does it have a pointed edge?

If has rough edge= D

If pointed edge= does nut have two colours or a singular colour?

Two colours= E

Singular colour= B

 

If it has divots= does it have two sections or a singular one?

If has two sections= A

If singular section= is the shape of the nut more round or flat?

If round= F

If flat= C

 

For size-ratio

 

Worked example

Does the nut have two lobes?

  1. Yes= groundnut

  2. No= any others

Does the nut have distinct ridge running along the long axis?

  1. Yes=walnut or pistachio, go to Q3

  2. No= almond, hazelnut or pecan go Q4

Is the outer texture smooth or pitted?

  1. Smooth=pistachio

  2. Pitted= walnut

Is the outer texture smooth or pitted?

  1. Pitted= almond

  2. smooth= hazelnut or pecan

Does the nut have a rough patch at one end?

  1. Yes= hazelnut

  2. No= pecan

 

 

Potential problems when constructing a key

  • There may be variety within a species

  • Characteristics may be sex-dependent

  • Characteristics may be life-stage dependent

  • Characters may be clearly distinctive (useful) or quite subtle

  • Descriptions may prove difficult or subjective, e.g. when you are trying to describe colour

  • Some characteristics may be internal/require dissection

  • There may be other species/members of the group not covered by your key