Topic
Biological Keys
Contents
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What is biological Key?
Types of keys.
Dichotomous Keys
Polytomous Keys
Example of Classification of Elephant
Common problems in key usage
What is A Biological Key?
• A biological key is a list or a series of questions
that asks questions about physical
characteristics.
• A biological key is used to determine the
classification of any living object.
Classifications include kingdom, phylum, class,
order & so on.
Types of keys.
• There are 2 major types of Biological keys
1. Dichotomous Keys
2. Polytomous Keys
Dichotomous Keys
• Dichotomous means divided in 2 parts.
• Such keys in which each following step or
question has only 2 choices or answers are called
Dichotomous Keys. These are most common
biological keys.
• For example, an organism may be:
• Microscopic or Macroscopic.
• If it is microscopic then it may be:
• Prokaryote or eukaryote.
Organism
Microscopic
Prokaryotic
(Kingdom Monera)
Eukaryotic
Macroscopic
Autotrophic
Hetrotrophic
(Kingdom Protista)
Decomposers
Consumers
( Kingdom Fungi)
( Kingdom Animalia)
Dichotomous Keys Given By Aristotle
• Aristotle was one of the great thinkers of his
time. One of his achievements was to outline
a scientific classification of animals. He
observed many animals to find features by
which to classify them.
Polytomous Keys
• Such keys in which each following step or
question has more than 2 choices or answers
are called Polytomous Keys.
Example of Classification of Elephant
• Key1: What is the size of elephant?
• Since we don't need to magnify an elephant in
order to see it, the elephant is macroscopic.
• Key2: We are asked whether or not the
elephant is autotrophic or heterotrophic?
• Clearly, the elephant is heterotrophic; it eats
plants in order to live.
• Key 3: we need to determine whether it is a
decomposer or a consumer.?
• Since the elephant eats plants, it is a
consumer.
• That tells us that our first classification is
kingdom Animalia.
• Key 4: We determine whether or not it has a
backbone?
• people ride on elephants' backs or elephants
carry heavy loads on their backs. So, elephants
have backbones.
• So we learn that the elephant is in
• Phylum Chordata
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Key 5: Whether the animal has a jaw or beak?
It has a jaw.
Key 6: If there are scales on the skin?
There are no scales.
Key 7: This key asks about hair or feathers?
The picture shows hair on the head.
Thus, we learn that the elephant is in
Class Mammalia
• Key 8: whether the elephant is a herbivore or
carnivore?
• We already know that elephants eat plants,
making them herbivores.
• Key 9: we must decide whether or not the
elephant has hooves?
• The feet have skin all the way to the bottom,
so there are no hooves.
• Key 10: we are asked whether there is an
enlarged trunk?
• Yes, there is. Thus, we know that the elephant
is in
• order Proboscidea
• This is as detailed a classification as we can
make with this key.
• Then, the elephant classification is:
Kingdom: Animalia Phylum: Chordata Class:
Mammalia Order: Proboscidea
Common problems in key usage
• Key users must overcome many practical
problems, such as:
• Variant forms: The key may identify only some
forms of the species, such as adult males (or,
more rarely, females). Keys for larvae
identification may consider only the final instar.
(This is not the case, however, of keys used in
forensic identification of fly larvae.)
• Incomplete coverage: Species and groups that
are difficult to identify or that have been poorly
characterized may have been left out of the key,
or may be mentioned only in introductory text.
Cont….
• Lighting and magnification: Very few keys give details
of how the specimen was viewed (the magnification,
lighting system, angle of view etc.). This can cause
problems. The author may, for instance refer to tiny
bristles, hairs or chaetae--but how tiny?
• Language: Very few keys are multilingual. Translations
of a key may be incorrect or misleading. Many keys
contain vague words that do not translate.
• Obsolescence: Older keys may not include more
recently described species. They may also use outdated
species names, which must then be mapped to the
current ones