How Do I Construct A Diagram That Uses "Shared Derived Characteristics?"

The Jason Project has asked you to construct a diagram that will graphically illustrate the relationships between six new critters they discovered in the Rain Forest. How do you do that? What are the steps you should take?

So you've filled in your data matrix and you think you can construct an evolutionary tree with it! Let's see what happens.

Let's recall what the geometric shapes look like.

 A B C D E

Here's the data matrix again, with more of the fields filled in. You may have different characters and character states.

Data Matrix
 Name Color Shape Size Character A Blue Circle large B Blue Square large C Pink Triangle large D Red Square large E Red Triangle large

Now you will have to make some decisions. The idea behind constructing evolutionary trees (what scientists call "cladograms") is that evolutionary changes are rare, and tend to only occur once. The fewer changes your tree has, the more likely it is to reflect actual evolutionary history.

The data matrix above has one character that is considered "uninformative." That is the character of "size." Although the character states are "large" and "small," since all the geometric shapes have the character state of "large," there is no way to use the character of "size" to help make the cladogram. One thing that will help you is knowing what the common ancestor of your geometric shape looked like. Fortunately, in this case, we have an ancestor!

 "Our Ancestor"

The ancestor is small relative to the other shapes. Now you have a character that shows there must have been an evolutionary change from "small" to "large" at some time. When? That's something our cladogram may help with.

Remember, although in this instance we have an ancestor, in many cases scientists do not have sufficient fossil evidence to know exactly what the ancestor looked like.

Steps to constructing a cladogram

1. Draw lines to each of the critters (in this case, the geometric shapes).

2. Determine which of the critters require the LEAST NUMBER of steps to change from the ancestor. In this example, the big pink triangle requires the least number of steps to change from the ancestor. The big pink triangle is the ancestor's closest relative, and therefore is the first branch of the cladogram. The switch from "little" to "big" is the "derived character" that marks the evolutionary change.

That's all there is to it!

Task: Can you finish the cladogram with the geometric objects, showing the derived characters in each step?

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