dichotomous keys - isaacpacheco.weebly.comisaacpacheco.weebly.com/.../dichotomous_key_2015.pdf ·...
TRANSCRIPT
Dichotomous Keys
What is a dichotomous key?• A tool that scientists can use to help
identify a particular specimen.
• Based on the organisms
characteristics (Ex. Eye color)
• Never based on the organisms
behaviors (Ex. Hibernates in winter)
Examples of the specimens
• Specimens could be trees,
wildflowers, mammals,
reptiles, rocks, and fish.
Think of three other examples of things that
could be used using a dichotomous key.
What does the prefix “di”
mean?
Two
“Dichotomous”
means “divided into
two parts”.
How to use a dichotomous key?
Dichotomous keys always give two choices in
each step. Often they are opposites
Examples Black/white; blue/green;
pointed/rounded
• The dichotomous key allows for scientists
to ask a series of questions with yes or no
answers.
Rules For Dichotomous Keys
• Read both choices in a couplet carefully.
Although the first description may seem to
fit your sample, the second may apply
even better
• 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
• When you have keyed out an organism,
do not take your effort as the final result.
Find a type specimen (if available) and
compare your unknown to the type
specimen.
• When a measurement is indicated, make
sure that you take the measurement using
a calibrated scale. Do not “eyeball” it or
take a guess
RULES IN A NUTSHELL
1.Read both choices before you make your
decision
2. If you are unsure – Follow both paths. The
correct path will become clearer once you
move on further down the couplets.
3. If possible, compare your unknown to an
actual specimen
Here are creatures we don’t know!
Lets choose one
How to use a Dichotomous Key?
Choose only one creature at a time.
How to use a Dichotomous Key?
Read steps 1a and 1b
Decide which statement is true
How to use a Dichotomous Key?
1b is true
Then follow the directions after that step.
How to use a Dichotomous Key?
At choice 5, you make another dichotomous choice
How to use a Dichotomous Key?
5a is true
Keep going until you come to a step
that gives you the creature’s name.
How to use a Dichotomous Key?
6 a. The creature has one antennae Go to Step 7.
b. The creature has two antennae Mr. Mitchell
1. a. wings covered by an exoskeleton – go to step 2
b. wings freely observed – Go to step 3
2. a. body has a round shape ……….ladybug, a red beetle with black spots
b. body has an elongated shape ……….grasshopper, a green insect that hops
3. a. wings point out from the side of the body ……….dragonfly, an insect that is 10- 15 cm long and lives in marshes
b. wings point to the posterior of the body ……….housefly, a flying insect with red eyes and an annoying buzz
Dichotomous key
1 With flower.
Without flower
Angiosperms
Go to 2
2 Seedless.
Seed bearing
Go to 3
Gymnosperms
3 Plant body do not differentiated into root,
stem and leaves.
Plant body differentiated into root, stem and
leaves
Go to 4
Go to 5
4 Without photosynthetic pigment.
With chlorophyll or other photosynthetic
pigment
Algae
Fungi
5 No vascular tissues.
With vascular tissues
Mosses
Fern
Another Example of a dichotomous key…
Dichotomous key
the dichotomous key can also be expressed in a diagram form
Non-flowering plants
Without photsyn-
thetic pigment
With chlorophyll or
other photosynthetic
pigment
Algae Fungi
No vascular
tissuesWith vascular
tissues
Mosses Gymnosperms
Plant body not
differentiated into
root, stem and
leaves
Plant body
differentiated
into stem and
leaves
Seedless Seed-bearing
Ferns
Flowering Plants
(Angiosperm)
Without flowerWith flower
Plants