List of characteristics of a group of animals that can be used to construct a cladogram.
Which of these characters is common to all of these organisms? Obviously, it is the notochord. Assume that is the ancestral condition, and thus it should characterize the oldest ancestor at the base of any cladogram that you construct. Now you need to determine what the first branch in the tree is based on. That would be the character that is shared by all but one of these organisms, the presence of jaws. If you continue this process, adding a branch to the tree to depict groups that have, or do not have, a character, you will generate this cladogram (
[link] ).
A cladogram constructed from the data in the table above. Using the character list in the table above, and recognizing that the notochord is common to all of these organisms, a cladogram was constructed, using the other characteristics in that list. Each shaded ellipse represents an ancestor possessing that trait, wile the organism(s) on the opposite branch lack that trait.
Now that you know how to construct one, there are a few other things about cladograms that you need to know.
The organisms along the top, or end, of a cladogram are not ordered in any particular way, and are not ancestors of each other. The order does not imply any sort of ranking from “primitive” to “advanced”. As discussed in the previous chapter, evolution is not synonymous with progress. As discussed above, the branches, and not the sequence of organisms across the top, are the important information in these diagrams.
The most closely related organisms have the most recent common ancestor, which is determined by counting the branch points backward from the organisms aligned across the top. For example , the bald Eagle and the Alligator have a more recent common ancestor than the Alligator and the Antelope; the Bald Eagle and the Alligator are more closely related than the Alligator and the Antelope.
The lines between branches represents the arrow of time, but in most cases the length of these lines is not directly related to the length of time involved between the branch points. Some representations of cladograms include an axis where time can be deduced, but if there is no such axis, do not assume that a longer line represents a longer length of time.
There are many ways to represent a cladogram, and many of these may look different, but will actually be identical. For example, the two cladograms shown in figure 4.2 depict the same set of relationships, even though one is vertically and horizontally aligned, and the other is diagonal in nature. In both cases organism C is more closely related to D (they share a relatively recent common ancestor) than A is related to B (which share a less recent common ancestor).
Cladograms can be generated from any set of inherited characters. But they become powerful tools, both for understanding the world and for generating predictive hypotheses, when they include characters that we know are quantitatively representative of relatedness, such as amino acid or nucleic acid sequences. But the single most important character is one that most directly reflects the degree of common ancestry – the DNA sequence. Since (as you will learn in detail later) DNA is the molecule of “descent with modification”, similarities and differences in DNA sequences from different organisms are thus very useful in determining phylogenetic relationships. The development of DNA sequencing technology has allowed scientists to apply this tool to taxonomy, and this has resulted in an explosion of new and exciting discoveries about phylogeny.
