Sea squirt - Dave Wells. 3

Do It Yourself Cladogram

So as promised, we will be constructing our own evolutionary trees. Please download the file.

As you can see we have included 6 taxa in our phylogenetic analysis. We can identify a number of variable characters which can be used to distinguish between each taxon. Table 1 lists these characters and also states whether each character is primitive or derived. When dealing with fossil taxa it is often difficult to discern whether the character you are examining is a primitive or derived state (in other words whether a character appeared early or late in the evolutionary history of the group of organisms you are examining). For example, within reptiles, snakes lack limbs. This is a derived character within reptiles because reptiles primitively possessed limbs (reptiles evolved from amphibian-like ancestors which possessed 4 limbs).

Within this example, we will assume that the polarity of the characters (that is whether a character is primitive or derived) has been determined in a previous study.

So let’s get building our tree!

STEP 1

Table 2 lists each taxon and each character. Access whether each character exhibits a primitive or derived state within each taxon. Mark a primitive state with 0 and a derived state with 1. If the character is not applicable for a certain taxon, mark with a ?. In the last column, add up every 1 for each taxon. This will give you the derivation state for that taxon.

STEP 2

In table 3, rank the taxa in order of their derivation state (1st=highest etc.).

STEP 3

For each of the derived character states in the first column of table 1, list all taxa which possess that character.

STEP 4

Using both tables 3 & 4, draw a cladogram to show the interrelations of the taxa.
Tips:

- The derivation state should increase as you move up the cladogram.

- A cladogram should divide from 1 branch to 2 branches.

- Try to group the most exclusive groups first (that is the groups from table 4 that have the fewest members).

There are two cladograms that fit the available data fairly well. Once you have completed the cladogram, compare yours with the examples below.

Answers