Today's Activity:
Taxonomic Keys and Phylogenetic Trees
Part I: Using a Taxonomic Key
Your group will receive a printed copy of a Taxonomic Key to the Pasta of the United States. It
is a dichotomous (literally, "cutting in two directions") key, meaning that at each step in using
the key, you will have to make a decision between two choices about the organism you're
trying to identify.
Your group will also receive small populations of several different "species" of pasta. Use the
taxonomic key to identify each of the pasta species correctly and learn their scientific names.
Using a dichotomous key to identify species is sometimes called “keying out” the species.
Part II: Understanding a Phylogenetic Tree
When you keyed out your pasta, you used physical characteristics (= MORPHOLOGY) to
tell them apart. (Systematists may also analyze DNA, RNA, or proteins as tools to compare
species.) We can use physical characteristics (traits) to classify species and create a
branching diagram called a phylogenetic tree to represent how they evolved.
First, compare your pasta species and note:
• Your pasta species share some traits in common with each other.
• Traits shared by all the pasta were inherited from their common ancestor.
• Traits can be modified by evolutionary change.
• Closely related species will share more of these modified traits with each other.
• You can use shared traits to create a PHYLOGENETIC TREE:
a visual
representation of the evolution of your species from common ancestors.
Figure 1. Hypothetical evolutionary relationships of pasta.
Figure one shows one possible way the pasta might have evolved.
A phylogenetic tree is a hypothesis about evolutionary history.
The Hypothetical Common Ancestor of all pasta is found at the base of the tree, and all other
species branch upward from it. The branches represent evolutionary change.
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Notice the hash marks on various branches of the tree.
Each one represents a trait that is found only in the species of pasta above it on
the tree.
A primitive trait (also called a plesiomorphy) is one that has not changed much from that trait
in an ancestor. (Example: All pasta are made of wheat flour)
A derived trait (also called an apomorphy) is one that has changed from the way it looked in
an ancestor. (Example: only Ziti, Penne, and Rigatoni have a body shaped like a tube)
Study the tree to see why using shared, derived characters helps us sort species into groups
that share a recent common ancestor.
Part III: Creating a Phylogenetic Tree
Now that you're an expert in using a key, you probably won't find it difficult to create one to
identify species. The trick is to create a key that reflects evolutionary relationships.
1. Each team will receive a cup filled with specimens of different "species" of hardware.
Each “species” has different traits.
Some have traits similar to each other, while others look very different.
But all share a common ancestor.
2. Arrange your hardware species so all team members can see their physical traits.
• Is a species made of metal or plastic?
• Is it flattened or not?
• Is it cylindrical or not?
• Is it long and thin, or short and squat?
• Pick the traits that seem significant in your hardware species, and make a list.
3. Now try to determine which traits are shared by ALL (or most ) of the species.
• Is the trait exactly the same in all the species?
• If not, how has it been modified?
• Have some traits been completely lost?
4. Sort your species into groups that share traits not shown by other groups.
Species that share unique traits inherited them from a common ancestor, and are related.
5. Species showing many primitive traits are the least changed from the Hypothetical
Common Ancestor of all the hardware.
6. Create a phylogenetic tree of your hardware species.
• Species sharing new/derived traits are related to each other.
• Place them on branches sprouting from a common ancestor that also had their traits.
• Place hashmarks on the branches of your tree to show where new traits have evolved.
Use two sheets of cardboard to create a large version of your tree! Be creative!
DO NOT use actual hardware organisms on the tree. Draw them, make 3-D models, etc.
This will be our second team competition! Label your tree with a title and
your team’s name. The judges will vote on the best presentation.
Part IV: Naming and Describing Species
It’s important to identify and name species so that other scientists will be able to discuss them
without worrying about language barriers. Time to invent proper, scientific names for your
hardware. All of these are the Family Homedepotidae. Your team’s job will be to create
genus and species names for all your pasta. This is naming the species.
1.
2.
3.
4.
Choose one of your groups of related species of pasta.
Remember that related species should be grouped into the same GENUS.
Choose a trait or feature that is common to all of them.
Create a Genus (in Greek or Latinized) that reflects that feature.
(For example, since a nail, a screw, and a bolt all share an elongated body with a distinct
head, you might name the genus containing them Cephalis (cephalon is Greek for “head”).
5. Now examine each member of your genus. What makes each one different from the rest?
Use the traits unique to each species to create a unique species name for it.
For example:
• The bolt has a blunt end. Its name could be Cephalis kofos (kofos is Greek for “blunt”)
• The screw has an X-shaped mark in its head. Its name could be Cephalis stauros
(stauros is Greek for “cross”)
6. Do the same for all your hardware. You might have some species that are the only
members of their genus. That’s up to you!
7. Every species named should also be given a short description of all its characteristics. This
is called describing the species. Systematists do this they discover and name a new
species.
Part V. Creating a Taxonomic Key
A taxonomic key is used for identification, not classification. That means there’s not really a
“right” or “wrong” way to create one.
But the best taxonomic keys are arranged with evolutionary relationships in mind, using the
traits that set each group and each species apart from the others.
Using the same characteristics you used to devise your phylogenetic tree, create
a dichotomous taxonomic key that anyone could use to identify your hardware
species.
Continue making your dichotomies until you have included all species in the key.
Once you feel confident that anyone using your key could identify any of these species in the
wilds of Home Depot, let the group leader know you're finished.
When all groups have completed their keys, you will trade keys
and see how effective an identification tool you have made. If the
other team can identify all your hardware species correctly with
your key, you’ve done a good job!
Use this space to create your taxonomic key:
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