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Creating Cladograms

Why Classify Organisms?

There is an amazing amount of biodiversity - from fungi and bacteria to humans. There are many reasons why it helps scientists and non-scientists alike to classify organisms. We couldn't possibly remember all of the characteristics of living things if we didn't group them into categories.

For example, any animal we call a "bird" has some traits in common - it has feathers, it has wings, and two legs. And if you're lucky enough to see a beautiful bird, you may tell your friend about the bird's size and color, but you don't have to tell that person that this bird had feathers, wings and two legs. Your friend already knows this, because all birds share these characteristics.

Classification helps scientists in these ways:

Classification Terms

Cladogram - a dichotomous branching tree that describes how evolutionary changed occurred among the species over time. The terms "cladogram" and "phylogenetic tree" have the same meaning.

Phylogeny -the evolutionary history of species or a group of related species.

A cladogram describes how organisms descended from their common ancestors. A cladogram shows the chronological sequence in which these organisms split from their common ancestors. Cladograms are constructed from a series of two way branch points. Each branch point (or node) represents the divergence of two species from a common ancestor. ("Clados" is Greek for "branch".)

Systematists study fossils as well as organisms living today. They look for homologies—traits that are similar among different species. For living organisms, researchers compare morphological, molecular and behavioral traits. Morphological traits include the size and shape of the organism and molecular traits include protein and DNA sequences.

In this activity, you'll use morphological traits to create a cladogram.

Creating a Cladogram

First, I'll show you the steps to create a cladogram along with an example (in the blue boxes). Next, you'll follow the steps to create your own cladogram.

Step 1: Choose a group of organisms to classify - make sure this group of organisms is a "clade". You'll also need to choose an outgroup.

Our clade consists of a shark, a salamander, a lizard, a tiger, a gorilla and a human. The lamprey is the outgroup.

Step 2: Determine the traits (also called characters) you'll compare in your clade. In this example, we'll use morphological traits.

Our traits to examine include the presence of jaws, lungs, an amniotic membrane, hair, lack of a tail, and bipedalism.

(An amniotic membrane holds amniotic fluid around the embryo. It may or may not be in an eggshell. Bipedalism = walking upright. )

Step 3: Determine whether the traits you're comparing are ancestral or derived.

In the table below, "1" means it has the trait and "0" means the organism lacks the trait. The traits examined in the table below are derived - they are not shared by the outgroup.

http://dtc.pima.edu/blc/182/lesson3/3step3/3step3images/charactertable.gif

Step 4: Group the organisms by shared derived characters. Use a nested diagram (Venn diagram).

In this diagram, the lamprey has ancestral traits only. The lamprey is the outgroup - it branched from this lineage before any derived traits appeared. The other organisms have derived traits (listed in purple). The organism in the smallest rectangle has the most derived traits.

http://dtc.pima.edu/blc/182/lesson3/3step3/3step3images/venn.gif

Step 5: Using the Venn diagram, create the cladogram.

Shared derived traits are listed after branch points. Species names go at the top of the branches.

http://dtc.pima.edu/blc/182/lesson3/3step3/3step3images/cladogram.jpg

 

Your turn

Now it's your turn to make a cladogram. The first two steps are given to you.

Step 1: Choose a group of organisms to classify - make sure this group of organisms is a "clade". A clade contains an ancestral species plus a group of descendents from that species.

Your clade consists of a kangaroo, a rhesus monkey, a bullfrog, a human, a snapping turtle, and a tuna. Use a lamprey as your outgroup.

Step 2: Determine the traits (also called characters) you'll compare in your clade. In this example, we'll use morphological traits.

Traits to compare:

  • Paired appendages (legs, arms, wings, flippers, fins)
  • Paired legs
  • Amniotic membrane (holds amniotic fluid surrounding the embryo; may or may not be inside of an eggshell)
  • Mammary glands (milk-secreting glands to nourish the young)
  • Placenta (structure attached to inside of uterus of mother, and joined to embryo by umbilical cord; provides nourishment and oxygen to the embryo)
  • Foramen magnum forward (spinal cord opening located forward, under skull)

 

 

 

Step 3: Determine the outgroup, and determine the number of derived traits for each member of the clade.

 

Paired appendages

Paired legs

Amniotic membrane

Mammary glands

Placenta

Foramen magnum forward

Total # of "1"s

Kangaroo

1

1

1

1

0

0

4

Rhesus monkey

1

1

1

1

1

0

5

Bullfrog

1

1

0

0

0

0

2

Human

1

1

1

1

1

1

6

Snapping turtle

1

1

1

0

0

0

3

Tuna

1

0

0

0

0

0

1

Lamprey

0

0

0

0

0

0

0

Step 4: Group the organisms by shared derived characters. Use a nested diagram (Venn diagram).

Complete the Venn diagram by adding the organisms and the derived traits.

http://dtc.pima.edu/blc/182/lesson3/3step3/3step3images/vennempty.gif

 

Step 5: Using the Venn diagram, create the cladogram.

Be sure to indicate shared derived traits after branch points. Names of species must go at the top of the branches.