CLASSIFICATION
Classification is a way of dividing a large group of anything into smaller groups where all the
members of the group share certain characteristics. It makes it easier for scientists to study
organisms.
Organisms may be made up of two basic types of cells.
1. Prokaryotes - Chromosomes not enclosed in a nucleus; no membrane-bound organelles;
simple unicellular organisms like bacteria.
2. Eukaryotes - Chromosomes enclosed in a nucleus; membrane-bound organelles; may be
unicellular, but most are multicellular organisms like plants, animals, yeast and fungi.
Simple classifications can be done based on visible characteristics such as hairiness, colour,
shape, venation (arrangement of veins), number of legs and wings, and body segmentation or
organs found. The classification system has 7 levels. You can remember them using the
acronym shown below.
A species is a group of organisms of common ancestry that closely resemble each other and are
normally capable of interbreeding to produce fertile offspring. The scientific name of an
organism is based on the genus and species of an organism e.g. Homo sapien is the scientific
name for humans.
Viruses are acellular particles (meaning they aren't made up of living cells), consisting instead of
a central core of either DNA or RNA, surrounded by a coating of protein. Viruses cannot carry
out all the life processes. They do not respire, grow, excrete, and they do not respond to
stimuli. They also don't reproduce independently but must replicate by invading living cells.
Flowcharts from CSEC Biology by Anne Tindale
DICHOTOMOUS KEYS
A dichotomous key is a set of paired statements that act as clues leading to the identification
of an organism. The word "dichotomous" means branching into two. A single observable
characteristic is considered at a time, but two contrasting statements are put forward to
describe each characteristic in such a way as to separate the organisms. This continues until all
the organisms have been identified.
Constructing a Dichotomous Key
- Use pencil.
- Use visible characteristics, e.g. type of leaf or number of legs.
- Start with one major characteristic that divides the organisms into two large
groups, then proceed to less noticeable characteristics that would separate
the organisms further into smaller groups. Use two contrasting statements to
describe the characteristic chosen, e.g. "simple leaves versus compound
leaves" or “2 pairs of legs versus more than 2 pairs of legs."
- Avoid generalizations, e.g. long body versus short body.
Be specific in your description, e.g. “body more than 10 cm long versus body less than
10 cm long."
- Avoid repeating the same characteristic.
- Place all of the names or groups of organisms identified at the bottom of the key.
- Give the key a title.
Example
Construct a dichotomous key that may be used to identify the following animals found in a
garden. (bee, lizard, snake, spider)