Bio 20: Classification
Teacher Notes
How Are Living Things Classified?
Why do we need a system to classify and name organisms?
How many plants and animals can you know? 20? 50? … Hundreds? It’s hard to
believe but we share Earth with millions of different organisms.
Here are some common names:
spider monkey
sea monkey
sea horse
gray wolf
firefly
crayfish
mud puppy
horned toad
ringworm
black bear
jellyfish
These names can vary by region and can also be misleading. Is a sea horse really a
horse?
To keep track of and to distinguish these organisms, biologists classify living things
into groups. The science of classifying organisms is called taxonomy.
Carolus Linnaeus, the founder of modern taxonomy, developed a classification
system in 1735. He grouped organisms according to what they looked like. Today,
we also consider things like cell organization, chemical make-up, ancestors, and the
way an organism develops before it is born.
Classification Groups
Today’s classification system has seven major classification groups (Kingdom –
Phylum — Class — Order — Family — Genus — Species). Organisms that are classified
in the same group are similar in some ways. The more alike organisms are, the
more groups they share.
Think of the classification system as an upside-down pyramid.
 The kingdom is the largest part. It has the most room so it can hold the
largest number of organisms (e.g. all plants, all animals, all protists).
 As you move down the pyramid, each room gets smaller. It can hold fewer
and fewer members, however these members have more traits in common.
They begin to become more alike.
Bio 20: Classification
Teacher Notes
Here’s a saying to help you remember the order:
King Philip Crossed Over For Gold and Silver
which stands for : Kingdom – Phylum — Class — Order — Family — Genus — Species
Naming Organisms
Linneaus also devised a system for naming organisms that is still used today. This
two-word naming system is called Binomial Nomenclature.
In this system:
 Each organism is given a two-part scientific name
 Usually Latin or Greek
 -1st word is Capitalized –Genus
-2nd word is lowercase —species
 The genus name is italicized (or underlined)
Examples: Felis concolor, Ursus arctos, Homo sapiens, Panthera leo , Panthera
tigris. These can also be abbreviated as (P. tigris or P. leo).
Bio 20: Classification
Teacher Notes
Current Classification System
Originally all living things on earth were classified into either the plant or animal
kingdom. As the microscope improved, the classification of microorganisms became
difficult. For example, was a microorganism that contained chlorophyll but also
moved considered a plant or animal?
Today we use six kingdoms: Eubacteria, Archaeabacteria, Protista, Fungi, Plantae,
and Animalia. Organisms are classified into based on the number and types of
cells they have and their nutritional needs.
The Six Kingdoms
Number of
Cells
Energy
Cell Type
Examples
Archaebacteria unicellular
some autotrophic,
most
chemotrophic
prokaryote "extremophiles"
Eubacteria
unicellular
autotrophic and
heterotrophic
prokaryote bacteria, E. coli
Protista
most
unicellular
heterotrophic or
autotrophic
eukaryote
ameba,
paramecium,
algae
Fungae
most
multicellular
heterotrophic
eukaryote
mushrooms, yeast
Plantae
multicellular
autotrophic
eukaryote
trees, grass
Animalia
multicellular
heterotrophic
eukaryote
humans, insects,
worms
Prokaryote – without a nucleus or other membrane-bound organelles
Eukaryote – contains membrane- bound nucleus and organelles
Bio 20: Classification
Teacher Notes
Phylogeny
The relationship between the six kingdoms is illustrated in a phylogenetic tree, a
branching diagram that shows relationships between species from an evolutionary
perspective.
The term phylogeny refers to the evolution or historical development of any
species. This history is established using radioactive dating of fossils and rocks
and comparative biochemical studies.
Example: Phylogenetic Tree of Life
A phylogenetic tree is a specific type of cladogram where the branch lengths are
proportional to the predicted evolutionary time between organisms.
Cladograms are branched diagrams that illustrate the probability that two
organisms are closely related, although it does not show how they are related.
Cladograms should not be seen as evolutionary fact, but only as a possible path for
speciation.
Bio 20: Classification
Teacher Notes
Example: Cladogram for the Phylum Chordata
Dichotomous Key
A dichotomous key is a device that can be used to easily identify an unknown
organism. The word dichotomous comes from two Greek words that together
mean, "divided in two parts".
A dichotomous key consists of a series of two part statements that describe
characteristic of organisms. At each step of a dichotomous key the user is
presented with two choices. As the user makes a choice about a particular
characteristic of an organism they are led to a new branch of the key. Eventually
the user will be led to the name of the organism that they are trying to identify.
Bio 20: Classification
Teacher Notes
Example: Here are two examples of keys to show how to key out
organisms, such as a kangaroo, a cow, a bee and a spider. Shown are a
branching key and a numbered key.
1a. Internal skeleton
1b. No internal
skeleton
2a. Pouch present
2b. Pouch absent
3a. Six legs
3b. Not six legs
……………………
go to 2
……………………
go to 3
……………………
……………………
……………………
……………………
kangaroo
cow
bee
spider