Page 95 - Biology_F5

Biology for Advanced Level Secondary Schools

2.5.1 Procedure for construction of 2.5.2 Types of taxonomic keys
taxonomic keys There are various types of diagnostic keys
(a) The organisms to be identified used in taxonomy. The most common
are collected and displayed for a and simple key used for identification of
thorough observation of identifiable organisms is called dichotomous key. This
features. The collected organisms, type of key was pioneered by a French
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such as, animals and plants are called
specimens. taxonomist, Jean Baptiste Lamark in 1778.
Dichotomous is a Greek word which
(b) Each of the collected specimens is literally means ‘divided into 2 branches’ or
carefully examined, and its easily
observable features are noted and ‘cut in half.’ This kind of key is constructed
recorded in a notebook. using two statements with contrasting

(c) A table listing the specimens in one features and mutually exclusive choices at
column and the characters to be a branching point. The pair of statements is
studied in the other is created. called leads or couplet, and they provide two

(d) From the table, study the recorded alternatives at each branching, each leading
specimen features carefully and to a subsequent statement until an organism
determine a general pair of exclusion is identified. As one moves down the key,
character, which can be used to a large group of organisms with diverse
divide the entire group of collected
specimens into two major groups. features is reduced to a smaller group and
finally to an individual organism or taxon.
(e) Subdivide each of the obtained
groups into two more groups using
shared or common features. The couplets can be organised using

(f) The second group is subdivided numbers, as in numeric keys or letters. It
further into two other smaller groups is worth noting that the characteristics used
based on their common features. in dichotomous keys may be quantitative or

(g) The smaller groups are continuously qualitative. The former can be examplified
subdivided into two groups using by features such as number of petals, leaf
contrasting pairs of statements or size, and petiole size, while the latter could
couplets until all of the specimens are be leaf shape, stem texture and flower-
identified.
colour. In constructing dichotomous keys,
(h) If the constructed key is numbered, features used in describing organisms
allocate the number of couplet to be
considered next after each step to the must be permanent, and not temporary
last step in which the specimen is or transitional features, such as those
identified. emanating from seasonal variations or

(i) Write down the couplets in a special growth of an organism. Two types of
pattern to get a desired framework for dichotomous keys are recognised. These
a particular key. are indented key and bracketed or simple
numbered key.

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