Systematics and cladograms
Systematics is the study of the diversity of living things and their evolutionary (or phylogenetic) relationships. You'll find a detailed discussion of systematics in Chapter 25 of Campbell; you should look over that chapter before doing the systematics lab.
I won't try to give a full explanation of systematics on this page. Instead, I will focus on cladograms, which are tree-like graphic representations of phylogenetic relationships.
A simple cladogram
Here is a cladogram showing relationships among the various groups of green algae and plants. (The cladogram is from the Tree of Life Web Project.) You don't need to know the names on this diagram; just focus on how the cladogram expresses information.
The key point of a cladogram like this is that taxa (groups of organisms) that are separated only by a short branch on the diagram (for example, the Chlorophyceae and the Trebouxiophyceae) are closely related. This means that these groups had a common ancestor fairly recently, and so they are likely to be similar to one another in many ways.
On the other hand, neither of these groups is closely related to the Prasinophytes. Chlorophyceae and Prasinophytes may be right next to each other on the tree, but the length of the branches between them shows that you would have to go far back in time to find the now-extinct common ancestor of these two groups.
Common ancestors
Each branching point on the cladogram represents a species that lived in the past and gave rise to two or more species that came later. In other words, it's a common ancestor to the species that come later. The cladogram at right shows the most recent common ancestor to the Chlorophyceae and the Trebouxiophyceae.
Line length on the cladogram
On this cladogram, the horizontal axis represents degree of relatedness. How would you express that? Two different approaches are commonly used:
- Time. The distance along a line in the cladogram would represent the passage of time. You'd probably use time to define the axis of your cladogram if you had a detailed fossil record for the organisms you were studying.
- Divergence. The distance along the line would represent some quantitative measure of how different the organisms are from one another. This is usually expressed in terms of the number of DNA differences. This kind of divergence scale is widely used; you don't need fossils to figure it out.
So which type of scale is used in the cladogram above -- time or DNA divergence? The axis isn't labeled, so it's a little vague. In practice, this is fine; DNA divergence is usually closely correlated with the passage of time, so the two kinds of scales turn out to be more or less the same.
The vertical access on this cladogram doesn't represent anything; if you stretched the diagram taller, it would still say the same thing.
Thus, this simple cladogram shows two closely related kinds of information: how recently the various taxa have shared a common ancestor, and how similar they are to one another.
A more elaborate cladogram:
This diagram is from the following article: Wedmann S, Bradler S, Rust, 2007. The first fossil leaf insect: 47 million years of specialized cryptic morphology and behavior. Proc Natl Acad Sci U S A. 2007 Jan 9;104(2):565-9. This article can be accessed online at:
http://www.pnas.org/cgi/content/full/104/2/565
This cladogram shows relationships in the order Phasmatodea, which includes stick insects and leaf insects. Notice that this cladogram is pointing upwards, instead of to the side. It also includes a few features not shown in the simpler cladogram for the green plants:
- Time scale. The scale on the left is in terms of mya --millions of years ago. At the top is the present: 0 mya. The scale goes back to 50 mya; no fossils of Phasmatodea have been found that are older than that, although the authors think the Phasmatodea must have existed even earlier.
- Pictures of the organisms so we can have some idea what they are. The leaf insects (phyliinae) on the right look like leaves; the others are all stick insects.
- The names of some larger taxa, such as the order Phasmatodea.
- Defining characteristics for the branching points on the cladogram.
Also, below the cladogram, there is a map showing the distribution of the Phyliinae (leaf insects).
You can get a lot of information from this cladogram. For example, look at Eophyllium in the middle of the diagram. This genus of stick insects is only known from fossils; that's why its branch ends at about 40 mya. It's considered to be a member of the Phyliinae, because (like all the Phyliinae) it has "abdominal sterna and terga dilated" (if you were an entomologist, you might know what that means; for now, it's not important). However, Eophyllium is in a separate group from the other members of the Phyliinae because the others all have certain characteristics (like "tergal thorn pads reduced") that Eophyllium doesn't have.
The length of the vertical lines represents time on this diagram. The diagram says that the Timematodea split apart from the other stick insects a very long time ago -- much more than 50 million years ago, but the exact time is unknown. In contrast, the three genera at the top right shared a common ancestor much more recently.
Each branching point represents a characteristic that is found in all the species on one branch and not in the species on the other branch. For example, at the bottom of the tree, the Phasmatodea are split into two groups. All the taxa on the right have "thorn pads on male tergum 10, fore femora basally curved," while the one group on the left, while the one group on the left (Timematodea) doesn't have these characteristics. Most of the characteristics in the cladogram are specialized features of male reproductive organs, which are the only way to tell the species apart.
The cladogram represents both shared characteristics and shared ancestry. For example, the three taxa at the top right all show "complete loss of tergal thorn pads." This cladogram says not only that these three groups of insects don't have tergal thorn pads, but that they all inherited this characteristic from a common ancestor that lost its tergal thorn pads about 15 million years ago.
This page updated September 17, 2011