Transcript (.ppt)

Dichotomous Keys
Tools to help people identify
and classify organisms
What is a dichotomy?
Two
Cut
Although I’m not going to quiz you on this word’s definition, it is
an important concept in understanding how dichotomous
keys work.
di·chot·o·my [dahy-kot-uh-mee] –noun, plural -mies.
1. division into two parts, kinds, etc.;
subdivision into halves or pairs.
2. division into two mutually exclusive,
opposed, or contradictory groups.
e.g.
(means “exemplar gratis” or “for example”)
There are no “in betweens”
Fish either have bones made of calcium or lack
calcified bones and have only cartilage.
Dichotomous Keys
A sequence of dichotomies that are used to
identify and classify organisms.
Each dichotomy in the key is referred to as a
“couplet” (just like the last two rhyming lines in a sonnet)
When using a dichotomous key, ALWAYS
START WITH THE FIRST COUPLET!
Good Dichotomous Keys
In a good key, each couplet is:
Jointly exhaustive: everything must belong to one part or the other.
Mutually exclusive: nothing can belong simultaneously to both parts.
Objective: factors in the dichotomy are not “open to interpretation”.
Balanced: each couplet line should deal with the same characteristic.
When making a key, make sure your
couplets follow these rules!